2. C# Language Basics – C# 8.0 in a Nutshell

Chapter 2. C# Language Basics

In this chapter, we introduce the basics of the C# language.


All programs and code snippets in this and the following two chapters are available as interactive samples in LINQPad. Working through these samples in conjunction with the book accelerates learning in that you can edit the samples and instantly see the results without needing to set up projects and solutions in Visual Studio.

To download them in LINQPad, click the Samples tab, and then click “Download more samples.”

A First C# Program

Following is a program that multiplies 12 by 30 and prints the result, 360, to the screen. The double forward slash indicates that the remainder of a line is a comment:

using System;                     // Importing namespace

class Test                        // Class declaration
  static void Main()              //   Method declaration
    int x = 12 * 30;              //     Statement 1
    Console.WriteLine (x);        //     Statement 2
  }                               //   End of method
}                                 // End of class

At the heart of this program lie two statements:

    int x = 12 * 30;
    Console.WriteLine (x);

Statements in C# execute sequentially and are terminated by a semicolon (or a code block, as you’ll see later). The first statement computes the expression 12 * 30 and stores the result in a local variable, named x, which is an integer type. The second statement calls the Console class’s WriteLine method, to print the variable x to a text window on the screen.

A method performs an action in a series of statements, called a statement block—a pair of braces containing zero or more statements. We defined a single method named Main:

  static void Main()

Writing higher-level functions that call upon lower-level functions simplifies a program. We can refactor our program with a reusable method that multiplies an integer by 12, as follows:

using System;

class Test
  static void Main()
    Console.WriteLine (FeetToInches (30));      // 360
    Console.WriteLine (FeetToInches (100));     // 1200

  static int FeetToInches (int feet)
    int inches = feet * 12;
    return inches;

A method can receive input data from the caller by specifying parameters and output data back to the caller by specifying a return type. We defined a method called FeetToInches that has a parameter for inputting feet, and a return type for outputting inches:

static int FeetToInches (int feet ) {...}

The literals 30 and 100 are the arguments passed to the FeetToInches method. The Main method in our example has empty parentheses because it has no parameters; it is void because it doesn’t return any value to its caller:

static void Main()

C# recognizes a method called Main as signaling the default entry point of execution. The Main method can optionally return an integer (rather than void) in order to return a value to the execution environment (where a nonzero value typically indicates an error). The Main method can also optionally accept an array of strings as a parameter (that will be populated with any arguments passed to the executable); for example:

static int Main (string[] args) {...}

An array (such as string[]) represents a fixed number of elements of a particular type. Arrays are specified by placing square brackets after the element type. We describe them in “Arrays”.

(The Main method can also be declared async and return a Task or Task<int> in support of asynchronous programming, which we cover in Chapter 14.)

Methods are one of several kinds of functions in C#. Another kind of function we used in our example program was the * operator, which performs multiplication. There are also constructors, properties, events, indexers, and finalizers.

In our example, the two methods are grouped into a class. A class groups function members and data members to form an object-oriented building block. The Console class groups members that handle command-line input/output (I/O) functionality, such as the WriteLine method. Our Test class groups two methods—the Main method and the FeetToInches method. A class is a kind of type, which we examine in “Type Basics”.

At the outermost level of a program, types are organized into namespaces. The using directive makes the System namespace available to our application, to use the Console class. We could define all of our classes within the TestPrograms namespace, as follows:

using System;

namespace TestPrograms
  class Test  {...}
  class Test2 {...}

The .NET Core libraries are organized into nested namespaces. For example, this is the namespace that contains types for handling text:

using System.Text;

The using directive is there for convenience; you can also refer to a type by its fully qualified name, which is the type name prefixed with its namespace, such as System.Text.StringBuilder.


The C# compiler compiles source code (as a set of files with the .cs extension) into an assembly. An assembly is the unit of packaging and deployment in .NET. An assembly can be either an application or a library. A normal console or Windows application has a Main method (the entry point), whereas a library does not. The purpose of a library is to be called upon (referenced) by an application or by other libraries. .NET Core itself is a set of assemblies (as well as a runtime environment).


Unlike .NET Framework, .NET Core assemblies never have a .exe extension. The .exe you might see after building a .NET Core application is a platform-specific native loader responsible for starting your application’s .dll assembly.

.NET Core also allows you to create a self-contained deployment that includes the loader, your assemblies, and the .NET Core Framework—all in a single .exe file.

The dotnet tool (dotnet.exe on Windows) helps you to manage .NET source code and binaries from the command line. You can use it to both build and run your program, as an alternative to using an Integrated Development Environment (IDE) such as Visual Studio or Visual Studio Code.

You can obtain the dotnet tool either by installing the .NET Core SDK or by installing Visual Studio. Its default location is %ProgramFiles%\dotnet on Windows or /usr/bin/dotnet on Ubuntu Linux.

To compile an application, the dotnet tool requires a project file as well as one or more C# files. The following command scaffolds a new console project (creates its basic structure):

dotnet new Console -n MyFirstProgram

This creates a subfolder called MyFirstProgram containing a project file called MyFirstProgram.csproj and a C# file called Program.cs with a Main method that prints “Hello, world”.

To build and run your program, run this command from the MyFirstProgram folder:

dotnet run MyFirstProgram

Or, if you just want to build without running:

dotnet build MyFirstProgram.csproj

The output assembly will be written to a subdirectory under bin\debug.

We explain assemblies in detail in Chapter 18.


C# syntax is inspired by C and C++ syntax. In this section, we describe C#’s elements of syntax, using the following program:

using System;

class Test
  static void Main()
    int x = 12 * 30;
    Console.WriteLine (x);

Identifiers and Keywords

Identifiers are names that programmers choose for their classes, methods, variables, and so on. Here are the identifiers in our example program, in the order in which they appear:

System   Test   Main   x   Console   WriteLine

An identifier must be a whole word, essentially made up of Unicode characters starting with a letter or underscore. C# identifiers are case sensitive. By convention, parameters, local variables, and private fields should be in camel case (e.g., my​Variable), and all other identifiers should be in Pascal case (e.g., MyMethod).

Keywords are names that mean something special to the compiler. These are the keywords in our example program:

using   class   static   void   int

Most keywords are reserved, which means that you can’t use them as identifiers. Here is the full list of C# reserved keywords:


Avoiding conflicts

If you really want to use an identifier that clashes with a reserved keyword, you can do so by qualifying it with the @ prefix. For instance:

class class  {...}      // Illegal
class @class {...}      // Legal

The @ symbol doesn’t form part of the identifier itself. So, @myVariable is the same as myVariable.


The @ prefix can be useful when consuming libraries written in other .NET languages that have different keywords.

Contextual keywords

Some keywords are contextual, meaning that you also can use them as identifiers—without an @ symbol:


With contextual keywords, ambiguity cannot arise within the context in which they are used.

Literals, Punctuators, and Operators

Literals are primitive pieces of data lexically embedded into the program. The literals we used in our example program are 12 and 30.

Punctuators help demarcate the structure of the program. These are the punctuators we used in our example program:

{   }   ;

The braces group multiple statements into a statement block.

The semicolon terminates a statement. (Statement blocks, however, do not require a semicolon.) Statements can wrap multiple lines:

  (1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10);

An operator transforms and combines expressions. Most operators in C# are denoted with a symbol, such as the multiplication operator, *. We discuss operators in more detail later in this chapter. These are the operators we used in our example program:

.  ()   *   =

A period denotes a member of something (or a decimal point with numeric literals). Parentheses are used when declaring or calling a method; empty parentheses are used when the method accepts no arguments. (Parentheses also have other purposes that you’ll see later in this chapter.) An equals sign performs assignment. (The double equals sign, ==, performs equality comparison, as you’ll see later.)


C# offers two different styles of source-code documentation: single-line comments and multiline comments. A single-line comment begins with a double forward slash and continues until the end of the line; for example:

int x = 3;   // Comment about assigning 3 to x

A multiline comment begins with /* and ends with */; for example:

int x = 3;   /* This is a comment that
                spans two lines */

Comments can embed XML documentation tags, which we explain in “XML Documentation” in Chapter 4.

Type Basics

A type defines the blueprint for a value. In our example, we used two literals of type int with values 12 and 30. We also declared a variable of type int whose name was x:

static void Main()
  int x = 12 * 30;
  Console.WriteLine (x);

A variable denotes a storage location that can contain different values over time. In contrast, a constant always represents the same value (more on this later):

const int y = 360;

All values in C# are instances of a type. The meaning of a value and the set of possible values a variable can have are determined by its type.

Predefined Type Examples

Predefined types are types that are specially supported by the compiler. The int type is a predefined type for representing the set of integers that fit into 32 bits of memory, from −231 to 231−1, and is the default type for numeric literals within this range. We can perform functions such as arithmetic with instances of the int type, as follows:

int x = 12 * 30;

Another predefined C# type is string. The string type represents a sequence of characters, such as “.NET” or “http://oreilly.com”. We can work with strings by calling functions on them, as follows:

string message = "Hello world";
string upperMessage = message.ToUpper();
Console.WriteLine (upperMessage);               // HELLO WORLD

int x = 2015;
message = message + x.ToString();
Console.WriteLine (message);                    // Hello world2015

The predefined bool type has exactly two possible values: true and false. The bool type is commonly used with an if statement to conditionally branch execution flow:

bool simpleVar = false;
if (simpleVar)
  Console.WriteLine ("This will not print");

int x = 5000;
bool lessThanAMile = x < 5280;
if (lessThanAMile)
  Console.WriteLine ("This will print");

In C#, predefined types (also referred to as built-in types) are recognized with a C# keyword. The System namespace in .NET Core contains many important types that are not predefined by C# (e.g., DateTime).

Custom Type Examples

Just as we can build complex functions from simple functions, we can build complex types from primitive types. In this next example, we define a custom type named UnitConverter—a class that serves as a blueprint for unit conversions:

using System;

public class UnitConverter
  int ratio;                                                 // Field
  public UnitConverter (int unitRatio) {ratio = unitRatio; } // Constructor
  public int Convert   (int unit)    {return unit * ratio; } // Method

class Test
  static void Main()
    UnitConverter feetToInchesConverter = new UnitConverter (12);
    UnitConverter milesToFeetConverter  = new UnitConverter (5280);

    Console.WriteLine (feetToInchesConverter.Convert(30));    // 360
    Console.WriteLine (feetToInchesConverter.Convert(100));   // 1200
    Console.WriteLine (feetToInchesConverter.Convert(
                         milesToFeetConverter.Convert(1)));   // 63360

Members of a type

A type contains data members and function members. The data member of UnitConverter is the field called ratio. The function members of UnitConverter are the Convert method and the UnitConverter’s constructor.

Symmetry of predefined types and custom types

A beautiful aspect of C# is that predefined types and custom types have few differences. The predefined int type serves as a blueprint for integers. It holds data—32 bits—and provides function members that use that data, such as ToString. Similarly, our custom UnitConverter type acts as a blueprint for unit conversions. It holds data—the ratio—and provides function members to use that data.

Constructors and instantiation

Data is created by instantiating a type. Predefined types can be instantiated simply by using a literal such as 12 or "Hello world". The new operator creates instances of a custom type. We created and declared an instance of the UnitConverter type with this statement:

UnitConverter feetToInchesConverter = new UnitConverter (12);

Immediately after the new operator instantiates an object, the object’s constructor is called to perform initialization. A constructor is defined like a method, except that the method name and return type are reduced to the name of the enclosing type:

public class UnitConverter
  public UnitConverter (int unitRatio) { ratio = unitRatio; }

Instance versus static members

The data members and function members that operate on the instance of the type are called instance members. The UnitConverter’s Convert method and the int’s ToString method are examples of instance members. By default, members are instance members.

Data members and function members that don’t operate on the instance of the type but rather on the type itself must be marked as static. The Test.Main and Console.WriteLine methods are static methods. The Console class is actually a static class, which means that all of its members are static. You never actually create instances of a Console—one console is shared across the entire application.

Let’s contrast instance from static members. In the following code, the instance field Name pertains to an instance of a particular Panda, whereas Population pertains to the set of all Panda instances:

public class Panda
  public string Name;             // Instance field
  public static int Population;   // Static field

  public Panda (string n)         // Constructor
    Name = n;                     // Assign the instance field
    Population = Population + 1;  // Increment the static Population field

The following code creates two instances of the Panda, prints their names, and then prints the total population:

using System;

class Test
  static void Main()
    Panda p1 = new Panda ("Pan Dee");
    Panda p2 = new Panda ("Pan Dah");

    Console.WriteLine (p1.Name);      // Pan Dee
    Console.WriteLine (p2.Name);      // Pan Dah

    Console.WriteLine (Panda.Population);   // 2

Attempting to evaluate p1.Population or Panda.Name will generate a compile-time error.

The public keyword

The public keyword exposes members to other classes. In this example, if the Name field in Panda was not marked as public, it would be private and the Test class could not access it. Marking a member public is how a type communicates: “Here is what I want other types to see—everything else is my own private implementation details.” In object-oriented terms, we say that the public members encapsulate the private members of the class.


C# can convert between instances of compatible types. A conversion always creates a new value from an existing one. Conversions can be either implicit or explicit: implicit conversions happen automatically, and explicit conversions require a cast. In the following example, we implicitly convert an int to a long type (which has twice the bit capacity of an int), and we explicitly cast an int to a short type (which has half the bit capacity of an int):

int x = 12345;       // int is a 32-bit integer
long y = x;          // Implicit conversion to 64-bit integer
short z = (short)x;  // Explicit conversion to 16-bit integer

Implicit conversions are allowed when both of the following are true:

  • The compiler can guarantee that they will always succeed.

  • No information is lost in conversion.1

Conversely, explicit conversions are required when one of the following is true:

  • The compiler cannot guarantee that they will always succeed.

  • Information might be lost during conversion.

(If the compiler can determine that a conversion will always fail, both kinds of conversion are prohibited. Conversions that involve generics can also fail in certain conditions—see “Type Parameters and Conversions” in Chapter 3.)


The numeric conversions that we just saw are built into the language. C# also supports reference conversions and boxing conversions (see Chapter 3) as well as custom conversions (see “Operator Overloading” in Chapter 4). The compiler doesn’t enforce the aforementioned rules with custom conversions, so it’s possible for badly designed types to behave otherwise.

Value Types versus Reference Types

All C# types fall into the following categories:

  • Value types

  • Reference types

  • Generic type parameters

  • Pointer types


In this section, we describe value types and reference types. We cover generic type parameters in “Generics” in Chapter 3, and pointer types in “Unsafe Code and Pointers” in Chapter 4.

Value types comprise most built-in types (specifically, all numeric types, the char type, and the bool type) as well as custom struct and enum types.

Reference types comprise all class, array, delegate, and interface types. (This includes the predefined string type.)

The fundamental difference between value types and reference types is how they are handled in memory.

Value types

The content of a value-type variable or constant is simply a value. For example, the content of the built-in value type, int, is 32 bits of data.

You can define a custom value type with the struct keyword (see Figure 2-1):

public struct Point { public int X; public int Y; }

or more tersely:

public struct Point { public int X, Y; }
Figure 2-1. A value-type instance in memory

The assignment of a value-type instance always copies the instance; for example:

static void Main()
  Point p1 = new Point();
  p1.X = 7;

  Point p2 = p1;             // Assignment causes copy

  Console.WriteLine (p1.X);  // 7
  Console.WriteLine (p2.X);  // 7

  p1.X = 9;                  // Change p1.X

  Console.WriteLine (p1.X);  // 9
  Console.WriteLine (p2.X);  // 7

Figure 2-2 shows that p1 and p2 have independent storage.

Figure 2-2. Assignment copies a value-type instance

Reference types

A reference type is more complex than a value type, having two parts: an object and the reference to that object. The content of a reference-type variable or constant is a reference to an object that contains the value. Here is the Point type from our previous example rewritten as a class rather than a struct (shown in Figure 2-3):

public class Point { public int X, Y; }
Figure 2-3. A reference-type instance in memory

Assigning a reference-type variable copies the reference, not the object instance. This allows multiple variables to refer to the same object—something not ordinarily possible with value types. If we repeat the previous example, but with Point now a class, an operation to p1 affects p2:

static void Main()
  Point p1 = new Point();
  p1.X = 7;

  Point p2 = p1;             // Copies p1 reference

  Console.WriteLine (p1.X);  // 7
  Console.WriteLine (p2.X);  // 7

  p1.X = 9;                  // Change p1.X

  Console.WriteLine (p1.X);  // 9
  Console.WriteLine (p2.X);  // 9

Figure 2-4 shows that p1 and p2 are two references that point to the same object.

Figure 2-4. Assignment copies a reference


A reference can be assigned the literal null, indicating that the reference points to no object:

class Point {...}

Point p = null;
Console.WriteLine (p == null);   // True

// The following line generates a runtime error
// (a NullReferenceException is thrown):
Console.WriteLine (p.X);

C# 8 introduces a new feature to reduce accidental Null​ReferenceException errors. For more on this, see “Nullable Reference Types (C# 8)” in Chapter 4.

In contrast, a value type cannot ordinarily have a null value:

struct Point {...}

Point p = null;  // Compile-time error
int x = null;    // Compile-time error

C# also has a construct called nullable value types for representing value-type nulls. For more information, see “Nullable Reference Types (C# 8)” in Chapter 4.

Storage overhead

Value-type instances occupy precisely the memory required to store their fields. In this example, Point takes eight bytes of memory:

struct Point
  int x;  // 4 bytes
  int y;  // 4 bytes

Technically, the CLR positions fields within the type at an address that’s a multiple of the fields’ size (up to a maximum of eight bytes). Thus, the following actually consumes 16 bytes of memory (with the seven bytes following the first field “wasted”):

struct A { byte b; long l; }

You can override this behavior by applying the StructLayout attribute (see “Mapping a Struct to Unmanaged Memory” in Chapter 25).

Reference types require separate allocations of memory for the reference and object. The object consumes as many bytes as its fields, plus additional administrative overhead. The precise overhead is intrinsically private to the implementation of the .NET runtime, but at minimum, the overhead is eight bytes, used to store a key to the object’s type as well as temporary information such as its lock state for multithreading and a flag to indicate whether it has been fixed from movement by the garbage collector. Each reference to an object requires an extra four or eight bytes, depending on whether the .NET runtime is running on a 32- or 64-bit platform.

Predefined Type Taxonomy

The predefined types in C# are as follows:

Value types
  • Numeric

    • Signed integer (sbyte, short, int, long)
    • Unsigned integer (byte, ushort, uint, ulong)
    • Real number (float, double, decimal)
  • Logical (bool)

  • Character (char)

Reference types
  • String (string)

  • Object (object)

Predefined types in C# alias .NET Core types in the System namespace. There is only a syntactic difference between these two statements:

int i = 5;
System.Int32 i = 5;

The set of predefined value types excluding decimal are known as primitive types in the CLR. Primitive types are so called because they are supported directly via instructions in compiled code, and this usually translates to direct support on the underlying processor; for example:

                   // Underlying hexadecimal representation
int i = 7;         // 0x7
bool b = true;     // 0x1
char c = 'A';      // 0x41
float f = 0.5f;    // uses IEEE floating-point encoding

The System.IntPtr and System.UIntPtr types are also primitive (see Chapter 25).

Numeric Types

C# has the predefined numeric types shown in Table 2-1.

Table 2-1. Predefined numeric types in C#
C# type System type Suffix Size Range
sbyte SByte   8 bits –27 to 27–1
short Int16   16 bits –215 to 215–1
int Int32   32 bits –231 to 231–1
long Int64 L 64 bits –263 to 263–1
byte Byte   8 bits 0 to 28–1
ushort UInt16   16 bits 0 to 216–1
uint UInt32 U 32 bits 0 to 232–1
ulong UInt64 UL 64 bits 0 to 264–1
float Single F 32 bits ± (~10–45 to 1038)
double Double D 64 bits ± (~10–324 to 10308)
decimal Decimal M 128 bits ± (~10–28 to 1028)

Of the integral types, int and long are first-class citizens and are favored by both C# and the runtime. The other integral types are typically used for interoperability or when space efficiency is paramount.

Of the real number types, float and double are called floating-point types2 and are typically used for scientific and graphical calculations. The decimal type is typically used for financial calculations, for which base-10-accurate arithmetic and high precision are required.

Numeric Literals

Integral-type literals can use decimal or hexadecimal notation; hexadecimal is denoted with the 0x prefix; for example:

int x = 127;
long y = 0x7F;

From C# 7, you can insert an underscore anywhere within a numeric literal to make it more readable:

int million = 1_000_000;

C# 7 and above also lets you specify numbers in binary with the 0b prefix:

var b = 0b1010_1011_1100_1101_1110_1111;

Real literals can use decimal and/or exponential notation:

double d = 1.5;
double million = 1E06;

Numeric literal type inference

By default, the compiler infers a numeric literal to be either a double or an integral type:

  • If the literal contains a decimal point or the exponential symbol (E), it is a double.

  • Otherwise, the literal’s type is the first type in this list that can fit the literal’s value: int, uint, long, and ulong.

For example:

Console.WriteLine (        1.0.GetType());  // Double  (double)
Console.WriteLine (       1E06.GetType());  // Double  (double)
Console.WriteLine (          1.GetType());  // Int32   (int)
Console.WriteLine ( 0xF0000000.GetType());  // UInt32  (uint)
Console.WriteLine (0x100000000.GetType());  // Int64   (long)

Numeric suffixes

Numeric suffixes explicitly define the type of a literal. Suffixes can be either lowercase or uppercase, and are as follows:

Category C# type Example
F float float f = 1.0F;
D double double d = 1D;
M decimal decimal d = 1.0M;
U uint uint i = 1U;
L long long i = 1L;
UL ulong ulong i = 1UL;

The suffixes U and L are rarely necessary because the uint, long, and ulong types can nearly always be either inferred or implicitly converted from int:

long i = 5;     // Implicit lossless conversion from int literal to long

The D suffix is technically redundant in that all literals with a decimal point are inferred to be double. And you can always add a decimal point to a numeric literal:

double x = 4.0;

The F and M suffixes are the most useful and should always be applied when specifying float or decimal literals. Without the F suffix, the following line would not compile, because 4.5 would be inferred to be of type double, which has no implicit conversion to float:

float f = 4.5F;

The same principle is true for a decimal literal:

decimal d = -1.23M;     // Will not compile without the M suffix.

We describe the semantics of numeric conversions in detail in the following section.

Numeric Conversions

Converting between integral types

Integral type conversions are implicit when the destination type can represent every possible value of the source type. Otherwise, an explicit conversion is required; for example:

int x = 12345;       // int is a 32-bit integer
long y = x;          // Implicit conversion to 64-bit integral type
short z = (short)x;  // Explicit conversion to 16-bit integral type

Converting between floating-point types

A float can be implicitly converted to a double given that a double can represent every possible value of a float. The reverse conversion must be explicit.

Converting between floating-point and integral types

All integral types can be implicitly converted to all floating-point types:

int i = 1;
float f = i;

The reverse conversion must be explicit:

int i2 = (int)f;

When you cast from a floating-point number to an integral type, any fractional portion is truncated; no rounding is performed. The static class System.Convert provides methods that round while converting between various numeric types (see Chapter 6).

Implicitly converting a large integral type to a floating-point type preserves magnitude but can occasionally lose precision. This is because floating-point types always have more magnitude than integral types, but can have less precision. Rewriting our example with a larger number demonstrates this:

int i1 = 100000001;
float f = i1;          // Magnitude preserved, precision lost
int i2 = (int)f;       // 100000000

Decimal conversions

All integral types can be implicitly converted to the decimal type given that a decimal can represent every possible C# integral-type value. All other numeric conversions to and from a decimal type must be explicit because they introduce the possibility of either a value being out of range or precision being lost.

Arithmetic Operators

The arithmetic operators (+, -, *, /, %) are defined for all numeric types except the 8- and 16-bit integral types:

+    Addition
-    Subtraction
*    Multiplication
/    Division
%    Remainder after division

Increment and Decrement Operators

The increment and decrement operators (++, --, respectively) increment and decrement numeric types by 1. The operator can either follow or precede the variable, depending on whether you want its value before or after the increment/decrement; for example:

int x = 0, y = 0;
Console.WriteLine (x++);   // Outputs 0; x is now 1
Console.WriteLine (++y);   // Outputs 1; y is now 1

Specialized Operations on Integral Types

The integral types are int, uint, long, ulong, short, ushort, byte, and sbyte.


Division operations on integral types always truncate remainders (round toward zero). Dividing by a variable whose value is zero generates a runtime error (a Divide​ByZeroException):

int a = 2 / 3;      // 0

int b = 0;
int c = 5 / b;      // throws DivideByZeroException

Dividing by the literal or constant 0 generates a compile-time error.


At runtime, arithmetic operations on integral types can overflow. By default, this happens silently—no exception is thrown, and the result exhibits “wraparound” behavior, as though the computation were done on a larger integer type and the extra significant bits discarded. For example, decrementing the minimum possible int value results in the maximum possible int value:

int a = int.MinValue;
Console.WriteLine (a == int.MaxValue); // True

Overflow check operators

The checked operator instructs the runtime to generate an OverflowException rather than overflowing silently when an integral-type expression or statement exceeds the arithmetic limits of that type. The checked operator affects expressions with the ++, −−, +, (binary and unary), *, /, and explicit conversion operators between integral types. Overflow checking incurs a small performance cost.


The checked operator has no effect on the double and float types (which overflow to special “infinite” values, as you’ll see soon) and no effect on the decimal type (which is always checked).

You can use checked around either an expression or a statement block:

int a = 1000000;
int b = 1000000;

int c = checked (a * b);      // Checks just the expression.

checked                       // Checks all expressions
{                             // in statement block.
   c = a * b;

You can make arithmetic overflow checking the default for all expressions in a program by selecting the checked option at the project level (in Visual Studio, go to Advanced Build Settings). If you then need to disable overflow checking just for specific expressions or statements, you can do so with the unchecked operator. For example, the following code will not throw exceptions—even if the project’s checked option is selected:

int x = int.MaxValue;
int y = unchecked (x + 1);
unchecked { int z = x + 1; }

Overflow checking for constant expressions

Regardless of the “checked” project setting, expressions evaluated at compile time are always overflow-checked—unless you apply the unchecked operator:

int x = int.MaxValue + 1;               // Compile-time error
int y = unchecked (int.MaxValue + 1);   // No errors

Bitwise operators

C# supports the following bitwise operators:

Operator Meaning Sample expression Result
~ Complement ~0xfU 0xfffffff0U
& And 0xf0 & 0x33 0x30
| Or 0xf0 | 0x33 0xf3
^ Exclusive Or 0xff00 ^ 0x0ff0 0xf0f0
<< Shift left 0x20 << 2 0x80
>> Shift right 0x20 >> 1 0x10

8- and 16-Bit Integral Types

The 8- and 16-bit integral types are byte, sbyte, short, and ushort. These types lack their own arithmetic operators, so C# implicitly converts them to larger types as required. This can cause a compile-time error when trying to assign the result back to a small integral type:

short x = 1, y = 1;
short z = x + y;          // Compile-time error

In this case, x and y are implicitly converted to int so that the addition can be performed. This means that the result is also an int, which cannot be implicitly cast back to a short (because it could cause loss of data). To make this compile, we must add an explicit cast:

short z = (short) (x + y);   // OK

Special Float and Double Values

Unlike integral types, floating-point types have values that certain operations treat specially. These special values are NaN (Not a Number), +∞, −∞, and −0. The float and double classes have constants for NaN, +∞, and −∞, as well as other values (MaxValue, MinValue, and Epsilon); for example:

Console.WriteLine (double.NegativeInfinity);   // -Infinity

The constants that represent special values for double and float are as follows:

Special value Double constant Float constant
NaN double.NaN float.NaN
+∞ double.PositiveInfinity float.PositiveInfinity
−∞ double.NegativeInfinity float.NegativeInfinity
−0 −0.0 −0.0f

Dividing a nonzero number by zero results in an infinite value:

Console.WriteLine ( 1.0 /  0.0);                  //  Infinity
Console.WriteLine (−1.0 /  0.0);                  // -Infinity
Console.WriteLine ( 1.0 / −0.0);                  // -Infinity
Console.WriteLine (−1.0 / −0.0);                  //  Infinity

Dividing zero by zero, or subtracting infinity from infinity, results in a NaN:

Console.WriteLine ( 0.0 /  0.0);                  //  NaN
Console.WriteLine ((1.0 /  0.0) − (1.0 / 0.0));   //  NaN

When using ==, a NaN value is never equal to another value, even another NaN value:

Console.WriteLine (0.0 / 0.0 == double.NaN);    // False

To test whether a value is NaN, you must use the float.IsNaN or double.IsNaN method:

Console.WriteLine (double.IsNaN (0.0 / 0.0));   // True

When using object.Equals, however, two NaN values are equal:

Console.WriteLine (object.Equals (0.0 / 0.0, double.NaN));   // True

NaNs are sometimes useful in representing special values. In Windows Presentation Foundation (WPF), double.NaN represents a measurement whose value is “Automatic”. Another way to represent such a value is with a nullable type (Chapter 4); another is with a custom struct that wraps a numeric type and adds an additional field (Chapter 3).

float and double follow the specification of the IEEE 754 format types, supported natively by almost all processors. You can find detailed information on the behavior of these types on the IEEE website.

double Versus decimal

double is useful for scientific computations (such as computing spatial coordinates). decimal is useful for financial computations and values that are man-made rather than the result of real-world measurements. Here’s a summary of the differences.

Category double decimal
Internal representation Base 2 Base 10
Decimal precision 15–16 significant figures 28–29 significant figures
Range ±(~10−324 to ~10308) ±(~10−28 to ~1028)
Special values +0, −0, +∞, −∞, and NaN None
Speed Native to processor Non-native to processor (about 10 times slower than double)

Real-Number Rounding Errors

float and double internally represent numbers in base 2. For this reason, only numbers expressible in base 2 are represented precisely. Practically, this means most literals with a fractional component (which are in base 10) will not be represented precisely; for example:

float tenth = 0.1f;                       // Not quite 0.1
float one   = 1f;
Console.WriteLine (one - tenth * 10f);    // -1.490116E-08

This is why float and double are bad for financial calculations. In contrast, decimal works in base 10 and so can precisely represent numbers expressible in base 10 (as well as its factors, base 2 and base 5). Because real literals are in base 10, decimal can precisely represent numbers such as 0.1. However, neither double nor decimal can precisely represent a fractional number whose base 10 representation is recurring:

decimal m = 1M / 6M;               // 0.1666666666666666666666666667M
double  d = 1.0 / 6.0;             // 0.16666666666666666

This leads to accumulated rounding errors:

decimal notQuiteWholeM = m+m+m+m+m+m;  // 1.0000000000000000000000000002M
double  notQuiteWholeD = d+d+d+d+d+d;  // 0.99999999999999989

which breaks equality and comparison operations:

Console.WriteLine (notQuiteWholeM == 1M);   // False
Console.WriteLine (notQuiteWholeD < 1.0);   // True

Boolean Type and Operators

C#’s bool type (aliasing the System.Boolean type) is a logical value that can be assigned the literal true or false.

Although a Boolean value requires only one bit of storage, the runtime will use one byte of memory because this is the minimum chunk that the runtime and processor can efficiently work with. To avoid space inefficiency in the case of arrays, .NET provides a BitArray class in the System.Collections namespace that is designed to use just one bit per Boolean value.

bool Conversions

No casting conversions can be made from the bool type to numeric types, or vice versa.

Equality and Comparison Operators

== and != test for equality and inequality of any type but always return a bool value.⁠3 Value types typically have a very simple notion of equality:

int x = 1;
int y = 2;
int z = 1;
Console.WriteLine (x == y);         // False
Console.WriteLine (x == z);         // True

For reference types, equality, by default, is based on reference, as opposed to the actual value of the underlying object (more on this in Chapter 6):

public class Dude
  public string Name;
  public Dude (string n) { Name = n; }
Dude d1 = new Dude ("John");
Dude d2 = new Dude ("John");
Console.WriteLine (d1 == d2);       // False
Dude d3 = d1;
Console.WriteLine (d1 == d3);       // True

The equality and comparison operators, ==, !=, <, >, >=, and <=, work for all numeric types, but you should use them with caution with real numbers (as we saw in “Real-Number Rounding Errors”). The comparison operators also work on enum type members by comparing their underlying integral-type values. We describe this in “Enums” in Chapter 3.

We explain the equality and comparison operators in greater detail in “Operator Overloading” in Chapter 4, and in “Equality Comparison” and “Order Comparison” in Chapter 6.

Conditional Operators

The && and || operators test for and and or conditions. They are frequently used in conjunction with the ! operator, which expresses not. In the following example, the UseUmbrella method returns true if it’s rainy or sunny (to protect us from the rain or the sun), as long as it’s not also windy (umbrellas are useless in the wind):

static bool UseUmbrella (bool rainy, bool sunny, bool windy)
  return !windy && (rainy || sunny);

The && and || operators short-circuit evaluation when possible. In the preceding example, if it is windy, the expression (rainy || sunny) is not even evaluated. Short-circuiting is essential in allowing expressions such as the following to run without throwing a NullReferenceException:

if (sb != null && sb.Length > 0) ...

The & and | operators also test for and and or conditions:

return !windy & (rainy | sunny);

The difference is that they do not short-circuit. For this reason, they are rarely used in place of conditional operators.


Unlike in C and C++, the & and | operators perform (non-short-circuiting) Boolean comparisons when applied to bool expressions. The & and | operators perform bitwise operations only when applied to numbers.

Conditional operator (ternary operator)

The conditional operator (more commonly called the ternary operator because it’s the only operator that takes three operands) has the form q ? a : b; thus, if condition q is true, a is evaluated, else b is evaluated:

static int Max (int a, int b)
  return (a > b) ? a : b;

The conditional operator is particularly useful in LINQ expressions (Chapter 8).

Strings and Characters

C#’s char type (aliasing the System.Char type) represents a Unicode character and occupies 2 bytes (UTF-16). A char literal is specified within single quotes:

char c = 'A';       // Simple character

Escape sequences express characters that cannot be expressed or interpreted literally. An escape sequence is a backslash followed by a character with a special meaning; for example:

char newLine = '\n';
char backSlash = '\\';

Table 2-2 shows the escape sequence characters.

Table 2-2. Escape sequence characters
Char Meaning Value
\' Single quote 0x0027
\" Double quote 0x0022
\\ Backslash 0x005C
\0 Null 0x0000
\a Alert 0x0007
\b Backspace 0x0008
\f Form feed 0x000C
\n Newline 0x000A
\r Carriage return 0x000D
\t Horizontal tab 0x0009
\v Vertical tab 0x000B

The \u (or \x) escape sequence lets you specify any Unicode character via its four-digit hexadecimal code:

char copyrightSymbol = '\u00A9';
char omegaSymbol     = '\u03A9';
char newLine         = '\u000A';

char Conversions

An implicit conversion from a char to a numeric type works for the numeric types that can accommodate an unsigned short. For other numeric types, an explicit conversion is required.

String Type

C#’s string type (aliasing the System.String type, covered in depth in Chapter 6) represents an immutable (unmodifiable) sequence of Unicode characters. A string literal is specified within double quotes:

string a = "Heat";

string is a reference type rather than a value type. Its equality operators, however, follow value-type semantics:

string a = "test";
string b = "test";
Console.Write (a == b);  // True

The escape sequences that are valid for char literals also work inside strings:

string a = "Here's a tab:\t";

The cost of this is that whenever you need a literal backslash, you must write it twice:

string a1 = "\\\\server\\fileshare\\helloworld.cs";

To avoid this problem, C# allows verbatim string literals. A verbatim string literal is prefixed with @ and does not support escape sequences. The following verbatim string is identical to the preceding one:

string a2 = @"\\server\fileshare\helloworld.cs";

A verbatim string literal can also span multiple lines:

string escaped  = "First Line\r\nSecond Line";
string verbatim = @"First Line
Second Line";

// True if your text editor uses CR-LF line separators:
Console.WriteLine (escaped == verbatim);

You can include the double-quote character in a verbatim literal by writing it twice:

string xml = @"<customer id=""123""></customer>";

String concatenation

The + operator concatenates two strings:

string s = "a" + "b";

One of the operands might be a nonstring value, in which case ToString is called on that value:

string s = "a" + 5;  // a5

Using the + operator repeatedly to build up a string is inefficient: a better solution is to use the System.Text.StringBuilder type (described in Chapter 6).

String interpolation

A string preceded with the $ character is called an interpolated string. Interpolated strings can include expressions enclosed in braces:

int x = 4;
Console.Write ($"A square has {x} sides");  // Prints: A square has 4 sides

Any valid C# expression of any type can appear within the braces, and C# will convert the expression to a string by calling its ToString method or equivalent. You can change the formatting by appending the expression with a colon and a format string (format strings are described in “string.Format and composite format strings” in Chapter 6):

string s = $"255 in hex is {byte.MaxValue:X2}";  // X2 = 2-digit hexadecimal
// Evaluates to "255 in hex is FF"

Interpolated strings must complete on a single line, unless you also specify the verbatim string operator:

int x = 2;
// Note that $ must appear before @ prior to C# 8:
string s = $@"this spans {
x} lines";

To include a brace literal in an interpolated string, repeat the desired brace character.

String comparisons

string does not support < and > operators for comparisons. You must use the string’s CompareTo method, described in Chapter 6.


An array represents a fixed number of variables (called elements) of a particular type. The elements in an array are always stored in a contiguous block of memory, providing highly efficient access.

An array is denoted with square brackets after the element type:

char[] vowels = new char[5];    // Declare an array of 5 characters

Square brackets also index the array, accessing a particular element by position:

vowels[0] = 'a';
vowels[1] = 'e';
vowels[2] = 'i';
vowels[3] = 'o';
vowels[4] = 'u';
Console.WriteLine (vowels[1]);      // e

This prints “e” because array indexes start at 0. We can use a for loop statement to iterate through each element in the array. The for loop in this example cycles the integer i from 0 to 4:

for (int i = 0; i < vowels.Length; i++)
  Console.Write (vowels[i]);            // aeiou

The Length property of an array returns the number of elements in the array. After an array has been created, you cannot change its length. The System.Collection namespace and subnamespaces provide higher-level data structures, such as dynamically sized arrays and dictionaries.

An array initialization expression lets you declare and populate an array in a single step:

char[] vowels = new char[] {'a','e','i','o','u'};

or simply:

char[] vowels = {'a','e','i','o','u'};

All arrays inherit from the System.Array class, providing common services for all arrays. These members include methods to get and set elements regardless of the array type. We describe them in “The Array Class” in Chapter 7.

Default Element Initialization

Creating an array always preinitializes the elements with default values. The default value for a type is the result of a bitwise zeroing of memory. For example, consider creating an array of integers. Because int is a value type, this allocates 1,000 integers in one contiguous block of memory. The default value for each element will be 0:

int[] a = new int[1000];
Console.Write (a[123]);            // 0

Value types versus reference types

Whether an array element type is a value type or a reference type has important performance implications. When the element type is a value type, each element value is allocated as part of the array, as shown here:

public struct Point { public int X, Y; }

Point[] a = new Point[1000];
int x = a[500].X;                  // 0

Had Point been a class, creating the array would have merely allocated 1,000 null references:

public class Point { public int X, Y; }

Point[] a = new Point[1000];
int x = a[500].X;                  // Runtime error, NullReferenceException

To avoid this error, we must explicitly instantiate 1,000 Points after instantiating the array:

Point[] a = new Point[1000];
for (int i = 0; i < a.Length; i++) // Iterate i from 0 to 999
   a[i] = new Point();             // Set array element i with new point

An array itself is always a reference-type object, regardless of the element type. For instance, the following is legal:

int[] a = null;

Indices and Ranges (C# 8)

C# 8 introduces indices and ranges to simplify working with elements or portions of an array.


Indices and ranges also work with the CLR types Span<T> and ReadOnlySpan<T> (see “Span<T> and Memory<T>” in Chapter 5).

You can also make your own types work with indices and ranges, by defining an indexer of type Index or Range (see “Indexers” in Chapter 3).


Indices let you refer to elements relative to the end of an array, with the ^ operator. ^1 refers to the last element, ^2 refers to the second-to-last element, and so on:

char[] vowels = new char[] {'a','e','i','o','u'};
char lastElement  = vowels [^1];   // 'u'
char secondToLast = vowels [^2];   // 'o'

(^0 equals the length of the array, so vowels[^0] generates an error.)

C# implements indices with the help of the Index type, so you can also do the following:

Index first = 0;
Index last = ^1;
char firstElement = vowels [first];   // 'a'
char lastElement = vowels [last];     // 'u'


Ranges let you “slice” an array by using the .. operator:

char[] firstTwo =  vowels [..2];    // 'a', 'e'
char[] lastThree = vowels [2..];    // 'i', 'o', 'u'
char[] middleOne = vowels [2..3];   // 'i'

The second number in the range is exclusive, so ..2 returns the elements before vowels[2].

You can also use the ^ symbol in ranges. The following returns the last two characters:

char[] lastTwo = vowels [^2..];     // 'o', 'u'

C# implements ranges with the help of the Range type, so you can also do the following:

Range firstTwoRange = 0..2;
char[] firstTwo = vowels [firstTwoRange];   // 'a', 'e'

Multidimensional Arrays

Multidimensional arrays come in two varieties: rectangular and jagged. Rectangular arrays represent an n-dimensional block of memory, and jagged arrays are arrays of arrays.

Rectangular arrays

Rectangular arrays are declared using commas to separate each dimension. The following declares a rectangular two-dimensional array for which the dimensions are 3 by 3:

int[,] matrix = new int[3,3];

The GetLength method of an array returns the length for a given dimension (starting at 0):

for (int i = 0; i < matrix.GetLength(0); i++)
  for (int j = 0; j < matrix.GetLength(1); j++)
    matrix[i,j] = i * 3 + j;

You can initialize a rectangular array with explicit values. The following code creates an array identical to the previous example:

int[,] matrix = new int[,]

Jagged arrays

Jagged arrays are declared using successive square brackets to represent each dimension. Here is an example of declaring a jagged two-dimensional array for which the outermost dimension is 3:

int[][] matrix = new int[3][];

Interestingly, this is new int[3][] and not new int[][3]. Eric Lippert has written an excellent article on why this is so.

The inner dimensions aren’t specified in the declaration because, unlike a rectangular array, each inner array can be an arbitrary length. Each inner array is implicitly initialized to null rather than an empty array. You must manually create each inner array:

for (int i = 0; i < matrix.Length; i++)
  matrix[i] = new int[3];                    // Create inner array
  for (int j = 0; j < matrix[i].Length; j++)
    matrix[i][j] = i * 3 + j;

You can initialize a jagged array with explicit values. The following code creates an array identical to the previous example with an additional element at the end:

int[][] matrix = new int[][]
  new int[] {0,1,2},
  new int[] {3,4,5},
  new int[] {6,7,8,9}

Simplified Array Initialization Expressions

There are two ways to shorten array initialization expressions. The first is to omit the new operator and type qualifications:

char[] vowels = {'a','e','i','o','u'};

int[,] rectangularMatrix =

int[][] jaggedMatrix =
  new int[] {0,1,2},
  new int[] {3,4,5},
  new int[] {6,7,8,9}

The second approach is to use the var keyword, which instructs the compiler to implicitly type a local variable:

var i = 3;           // i is implicitly of type int
var s = "sausage";   // s is implicitly of type string

// Therefore:

var rectMatrix = new int[,]    // rectMatrix is implicitly of type int[,]

var jaggedMat = new int[][]    // jaggedMat is implicitly of type int[][]
  new int[] {0,1,2},
  new int[] {3,4,5},
  new int[] {6,7,8,9}

Implicit typing can be taken one stage further with arrays: you can omit the type qualifier after the new keyword and have the compiler infer the array type:

var vowels = new[] {'a','e','i','o','u'};   // Compiler infers char[]

For this to work, the elements must all be implicitly convertible to a single type (and at least one of the elements must be of that type, and there must be exactly one best type), as in the following example:

var x = new[] {1,10000000000};   // all convertible to long

Bounds Checking

All array indexing is bounds-checked by the runtime. An IndexOutOfRangeException is thrown if you use an invalid index:

int[] arr = new int[3];
arr[3] = 1;               // IndexOutOfRangeException thrown

Array bounds checking is necessary for type safety and simplifies debugging.


Generally, the performance hit from bounds checking is minor, and the Just-In-Time (JIT) compiler can perform optimizations, such as determining in advance whether all indexes will be safe before entering a loop, thus avoiding a check on each iteration. In addition, C# provides “unsafe” code that can explicitly bypass bounds checking (see “Unsafe Code and Pointers” in Chapter 4).

Variables and Parameters

A variable represents a storage location that has a modifiable value. A variable can be a local variable, parameter (value, ref, out, or in), field (instance or static), or array element.

The Stack and the Heap

The stack and the heap are the places where variables reside. Each has very different lifetime semantics.


The stack is a block of memory for storing local variables and parameters. The stack logically grows and shrinks as a method or function is entered and exited. Consider the following method (to avoid distraction, input argument checking is ignored):

static int Factorial (int x)
  if (x == 0) return 1;
  return x * Factorial (x-1);

This method is recursive, meaning that it calls itself. Each time the method is entered, a new int is allocated on the stack, and each time the method exits, the int is deallocated.


The heap is the memory in which objects (i.e., reference-type instances) reside. Whenever a new object is created, it is allocated on the heap, and a reference to that object is returned. During a program’s execution, the heap begins filling up as new objects are created. The runtime has a garbage collector that periodically deallocates objects from the heap, so your program does not run out of memory. An object is eligible for deallocation as soon as it’s not referenced by anything that’s itself alive.

In the following example, we begin by creating a StringBuilder object referenced by the variable ref1 and then write out its content. That StringBuilder object is then immediately eligible for garbage collection because nothing subsequently uses it.

Then, we create another StringBuilder referenced by variable ref2 and copy that reference to ref3. Even though ref2 is not used after that point, ref3 keeps the same StringBuilder object alive—ensuring that it doesn’t become eligible for collection until we’ve finished using ref3.

using System;
using System.Text;

class Test
  static void Main()
    StringBuilder ref1 = new StringBuilder ("object1");
    Console.WriteLine (ref1);
    // The StringBuilder referenced by ref1 is now eligible for GC.

    StringBuilder ref2 = new StringBuilder ("object2");
    StringBuilder ref3 = ref2;
    // The StringBuilder referenced by ref2 is NOT yet eligible for GC.

    Console.WriteLine (ref3);                   // object2

Value-type instances (and object references) live wherever the variable was declared. If the instance was declared as a field within a class type, or as an array element, that instance lives on the heap.


You can’t explicitly delete objects in C#, as you can in C++. An unreferenced object is eventually collected by the garbage collector.

The heap also stores static fields. Unlike objects allocated on the heap (which can be garbage-collected), these live until the application domain is torn down.

Definite Assignment

C# enforces a definite assignment policy. In practice, this means that outside of an unsafe context, it’s impossible to access uninitialized memory. Definite assignment has three implications:

  • Local variables must be assigned a value before they can be read.

  • Function arguments must be supplied when a method is called (unless marked as optional; see “Optional parameters”).

  • All other variables (such as fields and array elements) are automatically initialized by the runtime.

For example, the following code results in a compile-time error:

static void Main()
  int x;
  Console.WriteLine (x);        // Compile-time error

Fields and array elements are automatically initialized with the default values for their type. The following code outputs 0 because array elements are implicitly assigned to their default values:

static void Main()
  int[] ints = new int[2];
  Console.WriteLine (ints[0]);    // 0

The following code outputs 0, because fields are implicitly assigned a default value:

class Test
  static int x;
  static void Main() { Console.WriteLine (x); }   // 0

Default Values

All type instances have a default value. The default value for the predefined types is the result of a bitwise zeroing of memory:

Type Default value
All reference types null
All numeric and enum types 0
char type '\0'
bool type false

You can obtain the default value for any type via the default keyword:

Console.WriteLine (default (decimal));   // 0

From C# 7.1, you can optionally omit the type when it can be inferred:

decimal d = default;

The default value in a custom value type (i.e., struct) is the same as the default value for each field defined by the custom type.


A method may have a sequence of parameters. Parameters define the set of arguments that must be provided for that method. In the following example, the method Foo has a single parameter named p, of type int:

static void Foo (int p)
  p = p + 1;                 // Increment p by 1
  Console.WriteLine (p);     // Write p to screen

static void Main()
  Foo (8);                  // Call Foo with an argument of 8

You can control how parameters are passed with the ref, in, and out modifiers:

Parameter modifier Passed by Variable must be definitely assigned
(None) Value Going in
ref Reference Going in
in Reference (read-only) Going in
out Reference Going out

Passing arguments by value

By default, arguments in C# are passed by value, which is by far the most common case. This means that a copy of the value is created when passed to the method:

class Test
  static void Foo (int p)
    p = p + 1;                // Increment p by 1
    Console.WriteLine (p);    // Write p to screen

  static void Main()
    int x = 8;
    Foo (x);                  // Make a copy of x
    Console.WriteLine (x);    // x will still be 8

Assigning p a new value does not change the contents of x, because p and x reside in different memory locations.

Passing a reference-type argument by value copies the reference, but not the object. In the following example, Foo sees the same StringBuilder object that Main instantiated, but has an independent reference to it. In other words, sb and fooSB are separate variables that reference the same StringBuilder object:

class Test
  static void Foo (StringBuilder fooSB)
    fooSB.Append ("test");
    fooSB = null;

  static void Main()
    StringBuilder sb = new StringBuilder();
    Foo (sb);
    Console.WriteLine (sb.ToString());    // test

Because fooSB is a copy of a reference, setting it to null doesn’t make sb null. (If, however, fooSB was declared and called with the ref modifier, sb would become null.)

The ref modifier

To pass by reference, C# provides the ref parameter modifier. In the following example, p and x refer to the same memory locations:

class Test
  static void Foo (ref int p)
    p = p + 1;               // Increment p by 1
    Console.WriteLine (p);   // Write p to screen

  static void Main()
    int x = 8;
    Foo (ref  x);            // Ask Foo to deal directly with x
    Console.WriteLine (x);   // x is now 9

Now assigning p a new value changes the contents of x. Notice how the ref modifier is required both when writing and when calling the method.4 This makes it very clear what’s going on.

The ref modifier is essential in implementing a swap method (in “Generics” in Chapter 3, we show how to write a swap method that works with any type):

class Test
  static void Swap (ref string a, ref string b)
    string temp = a;
    a = b;
    b = temp;

  static void Main()
    string x = "Penn";
    string y = "Teller";
    Swap (ref x, ref y);
    Console.WriteLine (x);   // Teller
    Console.WriteLine (y);   // Penn

A parameter can be passed by reference or by value, regardless of whether the parameter type is a reference type or a value type.

The out modifier

An out argument is like a ref argument except for the following:

  • It need not be assigned before going into the function.

  • It must be assigned before it comes out of the function.

The out modifier is most commonly used to get multiple return values back from a method; for example:

class Test
  static void Split (string name, out string firstNames,
                     out string lastName)
     int i = name.LastIndexOf (' ');
     firstNames = name.Substring (0, i);
     lastName   = name.Substring (i + 1);

  static void Main()
    string a, b;
    Split ("Stevie Ray Vaughan", out a, out b);
    Console.WriteLine (a);                      // Stevie Ray
    Console.WriteLine (b);                      // Vaughan

Like a ref parameter, an out parameter is passed by reference.

Out variables and discards

From C# 7, you can declare variables on the fly when calling methods with out parameters. We can shorten the Main method in our preceding example as follows:

  static void Main()
    Split ("Stevie Ray Vaughan", out string a, out string b);
    Console.WriteLine (a);                      // Stevie Ray
    Console.WriteLine (b);                      // Vaughan

When calling methods with multiple out parameters, sometimes you’re not interested in receiving values from all the parameters. In such cases, you can discard the ones in which you’re not interested by using an underscore:

Split ("Stevie Ray Vaughan", out string a, out _);   // Discard the 2nd param
Console.WriteLine (a);

In this case, the compiler treats the underscore as a special symbol, called a discard. You can include multiple discards in a single call. Assuming SomeBigMethod has been defined with seven out parameters, we can ignore all but the fourth, as follows:

SomeBigMethod (out _, out _, out _, out int x, out _, out _, out _);

For backward compatibility, this language feature will not take effect if a real underscore variable is in scope:

string _;
Split ("Stevie Ray Vaughan", out string a, out _);
Console.WriteLine (_);     // Vaughan

Implications of passing by reference

When you pass an argument by reference, you alias the storage location of an existing variable rather than create a new storage location. In the following example, the variables x and y represent the same instance:

class Test
  static int x;

  static void Main() { Foo (out x); }

  static void Foo (out int y)
    Console.WriteLine (x);                // x is 0
    y = 1;                                // Mutate y
    Console.WriteLine (x);                // x is 1

The in modifier

An in parameter is similar to a ref parameter except that the argument’s value cannot modified by the method (doing so generates a compile-time error). This modifier is most useful when passing a large value type to the method because it allows the compiler to avoid the overhead of copying the argument prior to passing it in while still protecting the original value from modification.

Overloading solely on the presence of in is permitted:

void Foo (   SomeBigStruct a) { ... }
void Foo (in SomeBigStruct a) { ... }

To call the second overload, the caller must use the in modifier:

SomeBigStruct x = ...;
Foo (x);      // Calls the first overload
Foo (in x);   // Calls the second overload

When there’s no ambiguity:

void Bar (in SomeBigStruct a) { ... }

use of the in modifier is optional for the caller:

Bar (x);     // OK (calls the 'in' overload)
Bar (in x);  // OK (calls the 'in' overload)

To make this example meaningful, SomeBigStruct would be defined as a struct (see “Structs” in Chapter 3).

The params modifier

You can specify the params parameter modifier on the last parameter of a method so that the method accepts any number of arguments of a particular type. The parameter type must be declared as an array, as shown in the following example:

class Test
  static int Sum (params int[] ints)
    int sum = 0;
    for (int i = 0; i < ints.Length; i++)
      sum += ints[i];                       // Increase sum by ints[i]
    return sum;

  static void Main()
    int total = Sum (1, 2, 3, 4);
    Console.WriteLine (total);              // 10

You can also supply a params argument as an ordinary array. The first line in Main is semantically equivalent to this:

int total = Sum (new int[] { 1, 2, 3, 4 } );

Optional parameters

Methods, constructors, and indexers (Chapter 3) can declare optional parameters. A parameter is optional if it specifies a default value in its declaration:

void Foo (int x = 23) { Console.WriteLine (x); }

You can omit optional parameters when calling the method:

Foo();     // 23

The default argument of 23 is actually passed to the optional parameter x—the compiler bakes the value 23 into the compiled code at the calling side. The preceding call to Foo is semantically identical to:

Foo (23);

because the compiler simply substitutes the default value of an optional parameter wherever it is used.


Adding an optional parameter to a public method that’s called from another assembly requires recompilation of both assemblies—just as though the parameter were mandatory.

The default value of an optional parameter must be specified by a constant expression or a parameterless constructor of a value type. Optional parameters cannot be marked with ref or out.

Mandatory parameters must occur before optional parameters in both the method declaration and the method call (the exception is with params arguments, which still always come last). In the following example, the explicit value of 1 is passed to x, and the default value of 0 is passed to y:

void Foo (int x = 0, int y = 0) { Console.WriteLine (x + ", " + y); }

void Test()
  Foo(1);    // 1, 0

To do the converse (pass a default value to x and an explicit value to y) you must combine optional parameters with named arguments.

Named arguments

Rather than identifying an argument by position, you can identify an argument by name:

void Foo (int x, int y) { Console.WriteLine (x + ", " + y); }

void Test()
  Foo (x:1, y:2);  // 1, 2

Named arguments can occur in any order. The following calls to Foo are semantically identical:

Foo (x:1, y:2);
Foo (y:2, x:1);

A subtle difference is that argument expressions are evaluated in the order in which they appear at the calling site. In general, this makes a difference only with interdependent side-effecting expressions such as the following, which writes 0, 1:

int a = 0;
Foo (y: ++a, x: --a);  // ++a is evaluated first

Of course, you would almost certainly avoid writing such code in practice!

You can mix named and positional arguments:

Foo (1, y:2);

However, there is a restriction: positional arguments must come before named arguments unless they are used in the correct position. So, we could call Foo like this:

Foo (x:1, 2);         // OK. Arguments in the declared positions

but not like this:

Foo (y:2, 1);         // Compile-time error. y isn't in the first position

Named arguments are particularly useful in conjunction with optional parameters. For instance, consider the following method:

void Bar (int a = 0, int b = 0, int c = 0, int d = 0) { ... }

We can call this supplying only a value for d, as follows:

Bar (d:3);

This is particularly useful when calling COM APIs, which we discuss in detail in Chapter 25.

Ref Locals

C# 7 added an esoteric feature, whereby you can define a local variable that references an element in an array or field in an object:

int[] numbers = { 0, 1, 2, 3, 4 };
ref int numRef = ref numbers [2];

In this example, numRef is a reference to numbers[2]. When we modify numRef, we modify the array element:

numRef *= 10;
Console.WriteLine (numRef);        // 20
Console.WriteLine (numbers [2]);   // 20

The target for a ref local must be an array element, field, or local variable; it cannot be a property (Chapter 3). Ref locals are intended for specialized micro-optimization scenarios and are typically used in conjunction with ref returns.

Ref Returns


The Span<T> and ReadOnlySpan<T> types that we describe in Chapter 24 use ref returns to implement a highly efficient indexer. Outside such scenarios, ref returns are not commonly used; you can consider them a micro-optimization feature.

You can return a ref local from a method. This is called a ref return:

static string x = "Old Value";

static ref string GetX() => ref x;    // This method returns a ref

static void Main()
  ref string xRef = ref GetX();       // Assign result to a ref local
  xRef = "New Value";
  Console.WriteLine (x);              // New Value

If you omit the ref modifier on the calling side, it reverts to returning an ordinary value:

string localX = GetX();  // Legal: localX is an ordinary non-ref variable.

You also can use ref returns when defining a property or indexer:

static ref string Prop => ref x;

Such a property is implicitly writable, despite there being no set accessor:

Prop = "New Value";

You can prevent such modification by using ref readonly:

static ref readonly string Prop => ref x;

The ref readonly modifier prevents modification while still enabling the performance gain of returning by reference. The gain would be very small in this case, because x is of type string (a reference type): no matter how long the string, the only inefficiency that we can hope to avoid is the copying of a single 32- or 64-bit reference. Real gains can occur with custom value types (see “Structs” in Chapter 3), but only if the struct is marked as readonly (otherwise, the compiler will perform a defensive copy).

Attempting to define an explicit set accessor on a ref return property or indexer is illegal.

var—Implicitly Typed Local Variables

It is often the case that you declare and initialize a variable in one step. If the compiler is able to infer the type from the initialization expression, you can use the keyword var (introduced in C# 3.0) in place of the type declaration; for example:

var x = "hello";
var y = new System.Text.StringBuilder();
var z = (float)Math.PI;

This is precisely equivalent to the following:

string x = "hello";
System.Text.StringBuilder y = new System.Text.StringBuilder();
float z = (float)Math.PI;

Because of this direct equivalence, implicitly typed variables are statically typed. For example, the following generates a compile-time error:

var x = 5;
x = "hello";    // Compile-time error; x is of type int

var can decrease code readability in the case when you can’t deduce the type purely by looking at the variable declaration. For example:

Random r = new Random();
var x = r.Next();

What type is x?

In “Anonymous Types” in Chapter 4, we will describe a scenario in which the use of var is mandatory.

Expressions and Operators

An expression essentially denotes a value. The simplest kinds of expressions are constants and variables. Expressions can be transformed and combined using operators. An operator takes one or more input operands to output a new expression.

Here is an example of a constant expression:


We can use the * operator to combine two operands (the literal expressions 12 and 30), as follows:

12 * 30

We can build complex expressions because an operand can itself be an expression, such as the operand (12 * 30) in the following example:

1 + (12 * 30)

Operators in C# can be classed as unary, binary, or ternary, depending on the number of operands they work on (one, two, or three). The binary operators always use infix notation, in which the operator is placed between the two operands.

Primary Expressions

Primary expressions include expressions composed of operators that are intrinsic to the basic plumbing of the language. Here is an example:

Math.Log (1)

This expression is composed of two primary expressions. The first expression performs a member lookup (with the . operator), and the second expression performs a method call (with the () operator).

Void Expressions

A void expression is an expression that has no value, such as this:

Console.WriteLine (1)

Because it has no value, you cannot use a void expression as an operand to build more complex expressions:

1 + Console.WriteLine (1)      // Compile-time error

Assignment Expressions

An assignment expression uses the = operator to assign the result of another expression to a variable; for example:

x = x * 5

An assignment expression is not a void expression—it has a value of whatever was assigned, and so can be incorporated into another expression. In the following example, the expression assigns 2 to x and 10 to y:

y = 5 * (x = 2)

You can use this style of expression to initialize multiple values:

a = b = c = d = 0

The compound assignment operators are syntactic shortcuts that combine assignment with another operator:

x *= 2    // equivalent to x = x * 2
x <<= 1   // equivalent to x = x << 1

(A subtle exception to this rule is with events, which we describe in Chapter 4: the += and -= operators here are treated specially and map to the event’s add and remove accessors.)

Operator Precedence and Associativity

When an expression contains multiple operators, precedence and associativity determine the order of their evaluation. Operators with higher precedence execute before operators of lower precedence. If the operators have the same precedence, the operator’s associativity determines the order of evaluation.


The following expression:

1 + 2 * 3

is evaluated as follows because * has a higher precedence than +:

1 + (2 * 3)

Left-associative operators

Binary operators (except for assignment, lambda, and null-coalescing operators) are left-associative; in other words, they are evaluated from left to right. For example, the following expression:

8 / 4 / 2

is evaluated as follows:

( 8 / 4 ) / 2    // 1

You can insert parentheses to change the actual order of evaluation:

8 / ( 4 / 2 )    // 4

Right-associative operators

The assignment operators, as well as the lambda, null coalescing, and conditional operators, are right-associative; in other words, they are evaluated from right to left. Right associativity allows multiple assignments such as the following to compile:

x = y = 3;

This first assigns 3 to y and then assigns the result of that expression (3) to x.

Operator Table

Table 2-3 lists C#’s operators in order of precedence. Operators in the same category have the same precedence. We explain user-overloadable operators in “Operator Overloading” in Chapter 4.

Table 2-3. C# operators (categories in order of precedence)
Category Operator symbol Operator name Example User-overloadable
Primary . Member access x.y No
  ?. and ?[] Null-conditional x?.y or x?[0] No
  -> (unsafe) Pointer to struct x->y No
  () Function call x() No
  [] Array/index a[x] Via indexer
  ++ Post-increment x++ Yes
  −− Post-decrement x−− Yes
  new Create instance new Foo() No
  stackalloc Unsafe stack allocation stackalloc(10) No
  typeof Get type from identifier typeof(int) No
  nameof Get name of identifier nameof(x) No
  checked Integral overflow check on checked(x) No
  unchecked Integral overflow check off unchecked(x) No
  default Default value default(char) No
Unary await Await await myTask No
  sizeof Get size of struct sizeof(int) No
  + Positive value of +x Yes
  Negative value of −x Yes
  ! Not !x Yes
  ~ Bitwise complement ~x Yes
  ++ Pre-increment ++x Yes
  −− Pre-decrement −−x Yes
  () Cast (int)x No
  * (unsafe) Value at address *x No
  & (unsafe) Address of value &x No
Range .. Start and end of a range of indices x..y No
Multiplicative * Multiply x * y Yes
  / Divide x / y Yes
  % Remainder x % y Yes
Additive + Add x + y Yes
  Subtract x − y Yes
Shift << Shift left x << 1 Yes
  >> Shift right x >> 1 Yes
Relational < Less than x < y Yes
  > Greater than x > y Yes
  <= Less than or equal to x <= y Yes
  >= Greater than or equal to x >= y Yes
  is Type is or is subclass of x is y No
  as Type conversion x as y No
Equality == Equals x == y Yes
  != Not equals x != y Yes
Logical And & And x & y Yes
Logical Xor ^ Exclusive Or x ^ y Yes
Logical Or | Or x | y Yes
Conditional And && Conditional And x && y Via &
Conditional Or || Conditional Or x || y Via |
Null coalescing ?? Null coalescing x ?? y No
Conditional ?: Conditional isTrue ? thenThisValue : elseThisValue No
Assignment & Lambda = Assign x = y No
  *= Multiply self by x *= 2 Via *
  /= Divide self by x /= 2 Via /
  += Add to self x += 2 Via +
  −= Subtract from self x −= 2 Via
  <<= Shift self left by x <<= 2 Via <<
  >>= Shift self right by x >>= 2 Via >>
  &= And self by x &= 2 Via &
  ^= Exclusive-Or self by x ^= 2 Via ^
  |= Or self by x |= 2 Via |
  ??= Null-coalescing assignment x ??= 0 No
  => Lambda x => x + 1 No

Null Operators

C# provides three operators to make it easier to work with nulls: the null-coalescing operator, the null-coalescing assignment operator, and the null-conditional operator.

Null-Coalescing Operator

The ?? operator is the null-coalescing operator. It says, “If the operand to the left is non-null, give it to me; otherwise, give me another value.” For example:

string s1 = null;
string s2 = s1 ?? "nothing";   // s2 evaluates to "nothing"

If the lefthand expression is non-null, the righthand expression is never evaluated. The null-coalescing operator also works with nullable value types (see “Nullable Value Types” in Chapter 4).

Null-Coalescing Assignment Operator (C# 8)

The ??= operator is the null-coalescing assignment operator. It says, “If the operand to the left is null, assign the right operand to the left operand.” For example:

string s1 = null;
s1 ??= "something";
Console.WriteLine (s1);  // something

s1 ??= "everything";
Console.WriteLine (s1);  // something

The operator is useful to replace the pattern

if (myVariable == null) myVariable = someDefault;


myVariable ??= someDefault;

Null-Conditional Operator

The ?. operator is the null-conditional or “Elvis” operator (after the Elvis emoticon). It allows you to call methods and access members just like the standard dot operator except that if the operand on the left is null, the expression evaluates to null instead of throwing a NullReferenceException:

System.Text.StringBuilder sb = null;
string s = sb?.ToString();  // No error; s instead evaluates to null

The last line is equivalent to the following:

string s = (sb == null ? null : sb.ToString());

Upon encountering a null, the Elvis operator short-circuits the remainder of the expression. In the following example, s evaluates to null, even with a standard dot operator between ToString() and ToUpper():

System.Text.StringBuilder sb = null;
string s = sb?.ToString().ToUpper();   // s evaluates to null without error

Repeated use of Elvis is necessary only if the operand immediately to its left might be null. The following expression is robust to both x being null and x.y being null:


It is equivalent to the following (except that x.y is evaluated only once):

x == null ? null
          : (x.y == null ? null : x.y.z)

The final expression must be capable of accepting a null. The following is illegal:

System.Text.StringBuilder sb = null;
int length = sb?.ToString().Length;   // Illegal : int cannot be null

We can fix this with the use of nullable value types (see “Nullable Value Types” in Chapter 4). If you’re already familiar with nullable value types, here’s a preview:

int? length = sb?.ToString().Length;   // OK: int? can be null

You can also use the null-conditional operator to call a void method:


If someObject is null, this becomes a “no-operation” rather than throwing a Null​ReferenceException.

You can use the null-conditional operator with the commonly used type members that we describe in Chapter 3, including methods, fields, properties, and indexers. It also combines well with the null-coalescing operator:

System.Text.StringBuilder sb = null;
string s = sb?.ToString() ?? "nothing";   // s evaluates to "nothing"


Functions comprise statements that execute sequentially in the textual order in which they appear. A statement block is a series of statements appearing between braces (the {} tokens).

Declaration Statements

A declaration statement declares a new variable, optionally initializing the variable with an expression. A declaration statement ends in a semicolon. You may declare multiple variables of the same type in a comma-separated list:

string someWord = "rosebud";
int someNumber = 42;
bool rich = true, famous = false;

A constant declaration is like a variable declaration except that it cannot be changed after it has been declared, and the initialization must occur with the declaration (see “Constants” in Chapter 3):

const double c = 2.99792458E08;
c += 10;                        // Compile-time error

Local variables

The scope of a local variable or local constant extends throughout the current block. You cannot declare another local variable with the same name in the current block or in any nested blocks:

static void Main()
  int x;
    int y;
    int x;            // Error - x already defined
    int y;            // OK - y not in scope
  Console.Write (y);  // Error - y is out of scope

A variable’s scope extends in both directions throughout its code block. This means that if we moved the initial declaration of x in this example to the bottom of the method, we’d get the same error. This is in contrast to C++ and is somewhat peculiar, given that it’s not legal to refer to a variable or constant before it’s declared.

Expression Statements

Expression statements are expressions that are also valid statements. An expression statement must either change state or call something that might change state. Changing state essentially means changing a variable. Following are the possible expression statements:

  • Assignment expressions (including increment and decrement expressions)

  • Method call expressions (both void and nonvoid)

  • Object instantiation expressions

Here are some examples:

// Declare variables with declaration statements:
string s;
int x, y;
System.Text.StringBuilder sb;

// Expression statements
x = 1 + 2;                 // Assignment expression
x++;                       // Increment expression
y = Math.Max (x, 5);       // Assignment expression
Console.WriteLine (y);     // Method call expression
sb = new StringBuilder();  // Assignment expression
new StringBuilder();       // Object instantiation expression

When you call a constructor or a method that returns a value, you’re not obliged to use the result. However, unless the constructor or method changes state, the statement is completely useless:

new StringBuilder();     // Legal, but useless
new string ('c', 3);     // Legal, but useless
x.Equals (y);            // Legal, but useless

Selection Statements

C# has the following mechanisms to conditionally control the flow of program execution:

  • Selection statements (if, switch)

  • Conditional operator (?:)

  • Loop statements (while, do-while, for, foreach)

This section covers the simplest two constructs: the if statement and the switch statement.

The if statement

An if statement executes a statement if a bool expression is true:

if (5 < 2 * 3)
  Console.WriteLine ("true");       // true

The statement can be a code block:

if (5 < 2 * 3)
  Console.WriteLine ("true");
  Console.WriteLine ("Let's move on!");

The else clause

An if statement can optionally feature an else clause:

if (2 + 2 == 5)
  Console.WriteLine ("Does not compute");
  Console.WriteLine ("False");        // False

Within an else clause, you can nest another if statement:

if (2 + 2 == 5)
  Console.WriteLine ("Does not compute");
  if (2 + 2 == 4)
    Console.WriteLine ("Computes");    // Computes

Changing the flow of execution with braces

An else clause always applies to the immediately preceding if statement in the statement block:

if (true)
  if (false)
    Console.WriteLine ("executes");

This is semantically identical to the following:

if (true)
  if (false)
    Console.WriteLine ("executes");

We can change the execution flow by moving the braces:

if (true)
  if (false)
  Console.WriteLine ("does not execute");

With braces, you explicitly state your intention. This can improve the readability of nested if statements—even when not required by the compiler. A notable exception is with the following pattern:

static void TellMeWhatICanDo (int age)
  if (age >= 35)
    Console.WriteLine ("You can be president!");
  else if (age >= 21)
    Console.WriteLine ("You can drink!");
  else if (age >= 18)
    Console.WriteLine ("You can vote!");
    Console.WriteLine ("You can wait!");

Here, we’ve arranged the if and else statements to mimic the elseif construct of other languages (and C#’s #elif preprocessor directive). Visual Studio’s auto-formatting recognizes this pattern and preserves the indentation. Semantically, though, each if statement following an else statement is functionally nested within the else clause.

The switch statement

switch statements let you branch program execution based on a selection of possible values that a variable might have. switch statements can result in cleaner code than multiple if statements because switch statements require an expression to be evaluated only once:

static void ShowCard (int cardNumber)
  switch (cardNumber)
    case 13:
      Console.WriteLine ("King");
    case 12:
      Console.WriteLine ("Queen");
    case 11:
      Console.WriteLine ("Jack");
    case -1:                         // Joker is -1
      goto case 12;                  // In this game joker counts as queen
    default:                         // Executes for any other cardNumber
      Console.WriteLine (cardNumber);

This example demonstrates the most common scenario, which is switching on constants. When you specify a constant, you’re restricted to the built-in integral types, bool, char, enum types, and the string type.

At the end of each case clause, you must specify explicitly where execution is to go next, with some kind of jump statement (unless your code ends in an infinite loop). Here are the options:

  • break (jumps to the end of the switch statement)

  • goto case x (jumps to another case clause)

  • goto default (jumps to the default clause)

  • Any other jump statement—namely, return, throw, continue, or goto label

When more than one value should execute the same code, you can list the common cases sequentially:

switch (cardNumber)
  case 13:
  case 12:
  case 11:
    Console.WriteLine ("Face card");
    Console.WriteLine ("Plain card");

This feature of a switch statement can be pivotal in terms of producing cleaner code than multiple if-else statements.

Switching on types


Switching on a type is a special case of switching on a pattern. A number of other (moderately useful) patterns were introduced in C# 7 and C# 8; see “Patterns” in Chapter 4 for a full discussion.

From C# 7, you can also switch on types:

static void Main()
  TellMeTheType (12);
  TellMeTheType ("hello");
  TellMeTheType (true);

static void TellMeTheType (object x)   // object allows any type.
  switch (x)
    case int i:
      Console.WriteLine ("It's an int!");
      Console.WriteLine ($"The square of {i} is {i * i}");
    case string s:
      Console.WriteLine ("It's a string");
      Console.WriteLine ($"The length of {s} is {s.Length}");
      Console.WriteLine ("I don't know what x is");

(The object type allows for a variable of any type; we discuss this fully in “Inheritance” and “The object Type” in Chapter 3.)

Each case clause specifies a type upon which to match, and a variable upon which to assign the typed value if the match succeeds (the “pattern” variable). Unlike with constants, there’s no restriction on what types you can use.

You can predicate a case with the when keyword:

switch (x)
  case bool b when b == true:     // Fires only when b is true
    Console.WriteLine ("True!");
  case bool b:
    Console.WriteLine ("False!");

The order of the case clauses can matter when switching on type (unlike when switching on constants). This example would give a different result if we reversed the two cases (in fact, it would not even compile, because the compiler would determine that the second case is unreachable). An exception to this rule is the default clause, which is always executed last, regardless of where it appears.

If you want to switch on a type, but are uninterested in its value, you can use a discard (_):

    case DateTime _:
      Console.WriteLine ("It's a DateTime");

You can stack multiple case clauses. The Console.WriteLine in the following code will execute for any floating-point type greater than 1,000:

switch (x)
  case float f when f > 1000:
  case double d when d > 1000:
  case decimal m when m > 1000:
    Console.WriteLine ("We can refer to x here but not f or d or m");

In this example, the compiler lets us consume the pattern variables f, d, and m, only in the when clauses. When we call Console.WriteLine, its unknown which one of those three variables will be assigned, so the compiler puts all of them out of scope.

You can mix and match constants and patterns in the same switch statement. And you can also switch on the null value:

case null:
  Console.WriteLine ("Nothing here");

switch expressions (C# 8)

From C# 8, you can use switch in the context of an expression. Assuming that cardNumber is of type int, the following illustrates its use:

string cardName = cardNumber switch
  13 => "King",
  12 => "Queen",
  11 => "Jack",
  _ => "Pip card"   // equivalent to 'default'

Notice that the switch keyword appears after the variable name, and that the case clauses are expressions (terminated by commas) rather than statements. switch expressions are more compact than their switch statement counterparts, and you can use them in LINQ queries (Chapter 8).

If you omit the default expression (_) and the switch fails to match, an exception is thrown.

You can also switch on multiple values (the tuple pattern):

int cardNumber = 12;
string suit = "spades";

string cardName = (cardNumber, suit) switch
  (13, "spades") => "King of spades",
  (13, "clubs") => "King of clubs",

Many more options are possible through the use of patterns (see “Patterns” in Chapter 4).

Iteration Statements

C# enables a sequence of statements to execute repeatedly with the while, do-while, for, and foreach statements.

while and do-while loops

while loops repeatedly execute a body of code while a bool expression is true. The expression is tested before the body of the loop is executed:

int i = 0;
while (i < 3)
  Console.WriteLine (i);


do-while loops differ in functionality from while loops only in that they test the expression after the statement block has executed (ensuring that the block is always executed at least once). Here’s the preceding example rewritten with a do-while loop:

int i = 0;
  Console.WriteLine (i);
while (i < 3);

for loops

for loops are like while loops with special clauses for initialization and iteration of a loop variable. A for loop contains three clauses as follows:

for (initialization-clause; condition-clause; iteration-clause)

Here’s what each clause does:

Initialization clause
Executed before the loop begins; used to initialize one or more iteration variables
Condition clause
The bool expression that, while true, will execute the body
Iteration clause
Executed after each iteration of the statement block; typically used to update the iteration variable

For example, the following prints the numbers 0 through 2:

for (int i = 0; i < 3; i++)
  Console.WriteLine (i);

The following prints the first 10 Fibonacci numbers (in which each number is the sum of the previous two):

for (int i = 0, prevFib = 1, curFib = 1; i < 10; i++)
  Console.WriteLine (prevFib);
  int newFib = prevFib + curFib;
  prevFib = curFib; curFib = newFib;

Any of the three parts of the for statement can be omitted. You can implement an infinite loop such as the following (though while(true) can be used, instead):

for (;;)
  Console.WriteLine ("interrupt me");

foreach loops

The foreach statement iterates over each element in an enumerable object. Most of the types in C# and .NET Core that represent a set or list of elements are enumerable. For example, both an array and a string are enumerable. Here is an example of enumerating over the characters in a string, from the first character through to the last:

foreach (char c in "beer")   // c is the iteration variable
  Console.WriteLine (c);


We define enumerable objects in “Enumeration and Iterators” in Chapter 4.

Jump Statements

The C# jump statements are break, continue, goto, return, and throw.


Jump statements obey the reliability rules of try statements (see “try Statements and Exceptions” in Chapter 4). This means that:

  • A jump out of a try block always executes the try’s finally block before reaching the target of the jump.

  • A jump cannot be made from the inside to the outside of a finally block (except via throw).

The break statement

The break statement ends the execution of the body of an iteration or switch statement:

int x = 0;
while (true)
  if (x++ > 5)
    break;      // break from the loop
// execution continues here after break

The continue statement

The continue statement forgoes the remaining statements in a loop and makes an early start on the next iteration. The following loop skips even numbers:

for (int i = 0; i < 10; i++)
  if ((i % 2) == 0)       // If i is even,
    continue;             // continue with next iteration

  Console.Write (i + " ");

OUTPUT: 1 3 5 7 9

The goto statement

The goto statement transfers execution to another label within a statement block. The form is as follows:

goto statement-label;

Or, when used within a switch statement:

goto case case-constant;    // (Only works with constants, not patterns)

A label is a placeholder in a code block that precedes a statement, denoted with a colon suffix. The following iterates the numbers 1 through 5, mimicking a for loop:

int i = 1;
if (i <= 5)
  Console.Write (i + " ");
  goto startLoop;

OUTPUT: 1 2 3 4 5

The goto case case-constant transfers execution to another case in a switch block (see “The switch statement”).

The return statement

The return statement exits the method and must return an expression of the method’s return type if the method is nonvoid:

static decimal AsPercentage (decimal d)
  decimal p = d * 100m;
  return p;             // Return to the calling method with value

A return statement can appear anywhere in a method (except in a finally block), and can be used more than once.

The throw statement

The throw statement throws an exception to indicate an error has occurred (see “try Statements and Exceptions” in Chapter 4):

if (w == null)
  throw new ArgumentNullException (...);

Miscellaneous Statements

The using statement provides an elegant syntax for calling Dispose on objects that implement IDisposable, within a finally block (see “try Statements and Exceptions” in Chapter 4 and “IDisposable, Dispose, and Close” in Chapter 12).


C# overloads the using keyword to have independent meanings in different contexts. Specifically, the using directive is different from the using statement.

The lock statement is a shortcut for calling the Enter and Exit methods of the Monitor class (see Chapters 14 and 23).


A namespace is a domain for type names. Types are typically organized into hierarchical namespaces, making them easier to find and avoiding conflicts. For example, the RSA type that handles public-key encryption is defined within the following namespace:


A namespace forms an integral part of a type’s name. The following code calls RSA’s Create method:

System.Security.Cryptography.RSA rsa =

Namespaces are independent of assemblies, which are units of deployment such as an .exe or .dll (described in Chapter 18).

Namespaces also have no impact on member visibility—public, internal, private, and so on.

The namepace keyword defines a namespace for types within that block; for example:

namespace Outer.Middle.Inner
  class Class1 {}
  class Class2 {}

The dots in the namespace indicate a hierarchy of nested namespaces. The code that follows is semantically identical to the preceding example:

namespace Outer
  namespace Middle
    namespace Inner
      class Class1 {}
      class Class2 {}

You can refer to a type with its fully qualified name, which includes all namespaces from the outermost to the innermost. For example, we could refer to Class1 in the preceding example as Outer.Middle.Inner.Class1.

Types not defined in any namespace are said to reside in the global namespace. The global namespace also includes top-level namespaces, such as Outer in our example.

The using Directive

The using directive imports a namespace, allowing you to refer to types without their fully qualified names. The following imports the previous example’s Outer.Middle.Inner namespace:

using Outer.Middle.Inner;

class Test
  static void Main()
    Class1 c;    // Don't need fully qualified name

It’s legal (and often desirable) to define the same type name in different namespaces. However, you’d typically do so only if it was unlikely for a consumer to want to import both namespaces at once. A good example is the TextBox class, which is defined both in System.Windows.Controls (WPF) and System.Windows.Forms.Controls (Windows Forms).

using static

The using static directive imports a type rather than a namespace. All static members of the imported type can then be used without qualification. In the following example, we call the Console class’s static WriteLine method without needing to refer to the type:

using static System.Console;

class Test
  static void Main() { WriteLine ("Hello"); }

The using static directive imports all accessible static members of the type, including fields, properties, and nested types (Chapter 3). You can also apply this directive to enum types, in which case their members are imported. So, if we import the following enum type:

using static System.Windows.Visibility;

we can specify Hidden instead of Visibility.Hidden:

var textBox = new TextBox { Visibility = Hidden };   // XAML-style

Should an ambiguity arise between multiple static imports, the C# compiler is not smart enough to infer the correct type from the context and will generate an error.

Rules Within a Namespace

Name scoping

You can use names declared in outer namespaces unqualified within inner namespaces. In this example, Class1 does not need qualification within Inner:

namespace Outer
  class Class1 {}

  namespace Inner
    class Class2 : Class1  {}

If you want to refer to a type in a different branch of your namespace hierarchy, you can use a partially qualified name. In the following example, we base SalesReport on Common.ReportBase:

namespace MyTradingCompany
  namespace Common
    class ReportBase {}
  namespace ManagementReporting
    class SalesReport : Common.ReportBase  {}

Name hiding

If the same type name appears in both an inner and an outer namespace, the inner name wins. To refer to the type in the outer namespace, you must qualify its name:

namespace Outer
  class Foo { }

  namespace Inner
    class Foo { }

    class Test
      Foo f1;         // = Outer.Inner.Foo
      Outer.Foo f2;   // = Outer.Foo

All type names are converted to fully qualified names at compile time. Intermediate Language (IL) code contains no unqualified or partially qualified names.

Repeated namespaces

You can repeat a namespace declaration, as long as the type names within the namespaces don’t conflict:

namespace Outer.Middle.Inner
  class Class1 {}

namespace Outer.Middle.Inner
  class Class2 {}

We can even break the example into two source files such that we could compile each class into a different assembly.

Source file 1:

namespace Outer.Middle.Inner
  class Class1 {}

Source file 2:

namespace Outer.Middle.Inner
  class Class2 {}

Nested using directives

You can nest a using directive within a namespace. This allows you to scope the using directive within a namespace declaration. In the following example, Class1 is visible in one scope, but not in another:

namespace N1
  class Class1 {}

namespace N2
  using N1;

  class Class2 : Class1 {}

namespace N2
  class Class3 : Class1 {}   // Compile-time error

Aliasing Types and Namespaces

Importing a namespace can result in type-name collision. Rather than importing the entire namespace, you can import just the specific types that you need, giving each type an alias:

using PropertyInfo2 = System.Reflection.PropertyInfo;
class Program { PropertyInfo2 p; }

An entire namespace can be aliased, as follows:

using R = System.Reflection;
class Program { R.PropertyInfo p; }

Advanced Namespace Features


Extern aliases allow your program to reference two types with the same fully qualified name (i.e., the namespace and type name are identical). This is an unusual scenario and can occur only when the two types come from different assemblies. Consider the following example.

Library 1, compiled to Widgets1.dll:

namespace Widgets
  public class Widget {}

Library 2, compiled to Widgets2.dll:

namespace Widgets
  public class Widget {}

Application, which references Widgets1.dll and Widgets2.dll:

using Widgets;

class Test
  static void Main()
    Widget w = new Widget();

The application cannot compile, because Widget is ambiguous. Extern aliases can resolve the ambiguity. The first step is to modify the application’s .csproj file, assigning a unique alias to each reference:

  <Reference Include="Widgets1">
  <Reference Include="Widgets2">

The second step is to use the extern alias directive:

extern alias W1;
extern alias W2;

class Test
  static void Main()
    W1.Widgets.Widget w1 = new W1.Widgets.Widget();
    W2.Widgets.Widget w2 = new W2.Widgets.Widget();

Namespace alias qualifiers

As we mentioned earlier, names in inner namespaces hide names in outer namespaces. However, sometimes even the use of a fully qualified type name does not resolve the conflict. Consider the following example:

namespace N
  class A
    static void Main() => new A.B();     // Instantiate class B
    public class B {}                    // Nested type

namespace A
  class B {}

The Main method could be instantiating either the nested class B, or the class B within the namespace A. The compiler always gives higher precedence to identifiers in the current namespace—in this case, the nested B class.

To resolve such conflicts, a namespace name can be qualified, relative to one of the following:

  • The global namespace—the root of all namespaces (identified with the contextual keyword global)

  • The set of extern aliases

The :: token performs namespace alias qualification. In this example, we qualify using the global namespace (this is most commonly seen in autogenerated code to avoid name conflicts):

namespace N
  class A
    static void Main()
      System.Console.WriteLine (new A.B());
      System.Console.WriteLine (new global::A.B());

    public class B {}

namespace A
  class B {}

Here is an example of qualifying with an alias (adapted from the example in “Extern”):

extern alias W1;
extern alias W2;

class Test
  static void Main()
    W1::Widgets.Widget w1 = new W1::Widgets.Widget();
    W2::Widgets.Widget w2 = new W2::Widgets.Widget();

1 A minor caveat is that very large long values lose some precision when converted to double.

2 Technically, decimal is a floating-point type, too, although it’s not referred to as such in the C# language specification.

3 It’s possible to overload these operators (Chapter 4) such that they return a non-bool type, but this is almost never done in practice.

4 An exception to this rule is when calling Component Object Model (COM) methods. We discuss this in Chapter 24.