.NET Documentation Comments

This article describes the syntax of the C# programming language. The features described are compatible with .NET Framework and Mono.

Basics

Identifier

An identifier is the name of an element in the code. It can contain letters, digits and underscores (_), and is case sensitive (FOO is different from foo). The language imposes the following restrictions on identifier names:
* They cannot start with a digit;
* They cannot start with a symbol, unless it is a keyword;
* They cannot contain more than 511 characters.

Identifier names may be prefixed by an at sign (@), but this is insignificant; @name is the same identifier as name.

Microsoft has published naming conventions for identifiers in C#, which recommend the use of PascalCase for the names of types and most type members, and camelCase for variables and for private or internal fields. However, these naming conventions are not enforced in the language.

Keywords

Keywords are predefined reserved words with special syntactic meaning. The language has two types of keyword — contextual and reserved. The reserved keywords such as false or byte may only be used as keywords. The contextual keywords such as where or from are only treated as keywords in certain situations. If an identifier is needed which would be the same as a reserved keyword, it may be prefixed by an at sign to distinguish it. For example, @out is interpreted as an identifier, whereas out as a keyword. This syntax facilitates reuse of .NET code written in other languages.

The following C# keywords are reserved words:

* abstract
* as
* base
* bool
* break
* byte
* case
* catch
* char
* checked
* class
* const
* continue
* decimal
* default
* delegate
* do
* double
* else
* enum
* event
* explicit
* extern
* false
* finally
* fixed
* float
* for
* foreach
* goto
* if
* implicit
* in
* int
* interface
* internal
* is
* lock
* long
* namespace
* new
* null
* object
* operator
* out
* override
* params
* private
* protected
* public
* readonly
* ref
* return
* sbyte
* sealed
* short
* sizeof
* stackalloc
* static
* string
* struct
* switch
* this
* throw
* true
* try
* typeof
* uint
* ulong
* unchecked
* unsafe
* ushort
* using
* virtual
* void
* volatile
* while

A contextual keyword is used to provide a specific meaning in the code, but it is not a reserved word in C#. Some contextual keywords, such as partial and where, have special meanings in multiple contexts. The following C# keywords are contextual:

* add
* allows
* alias
* and
* ascending
* args
* async
* await
* by
* descending
* dynamic
* equals
* from
* get
* global
* group
* init
* into
* join
* let
* managed
* nameof
* nint
* not
* notnull
* nuint
* on
* or
* orderby
* partial
* record
* remove
* required
* select
* set
* unmanaged
* value
* var
* when
* where
* with
* yield

Preprocessor directives

Although C# does not have a separate preprocessor, unlike C and C++ which use the C preprocessor, these directives are processed as if there were one.

* #nullable
* #if
* #elif
* #else
* #endif
* #define
* #undef
* #region
* #endregion
* #error
* #warning
* #line
* #pragma

Literals

Digit separators

Starting in C# 7.0, the underscore symbol can be used to separate digits in number values for readability purposes. The compiler ignores these underscores.

int bin = 0b1101_0010_1011_0100;
int hex = 0x2F_BB_4A_F1;
int dec = 1_000_500_954;
double real = 1_500.200_2e-1_000;

Generally, it may be put only between digit characters. It cannot be put at the beginning () or the end of the value ( or ), next to the decimal in floating point values (), next to the exponent character (), or next to the type specifier ().

Variables

Variables are identifiers associated with values. They are declared by writing the variable's type and name, and are optionally initialized in the same statement.

Declare

int myInt; // Declaring an uninitialized variable called 'myInt', of type 'int'

Assigning

int myInt; // Declaring an uninitialized variable
myInt = 35; // Assigning the variable a value

Initialize

int myInt = 35; // Declaring and initializing the variable

Multiple variables of the same type can be declared and initialized in one statement.

int a, b; // Declaring multiple variables of the same type

int a = 2, b = 3; // Declaring and initializing multiple variables of the same type

Local variable type inference

:''This is a feature of C# 3.0.''

C# 3.0 introduced type inference, allowing the type specifier of a variable declaration to be replaced by the keyword var, if its actual type can be statically determined from the initializer. This reduces repetition, especially for types with multiple generic type-parameters, and adheres more closely to the DRY principle.

var myChars = new char[] ; // or char[] myChars = new char[] ;

var myNums = new List(); // or List myNums = new List();

Constants

Constants are immutable values.

const

When declaring a local variable or a field with the const keyword as a prefix the value must be given when it is declared. After that it is locked and cannot change. They can either be declared in the context as a field or a local variable. Constants are implicitly static.

const double PI = 3.14;

This shows both uses of the keyword.

public class Foo

readonly

The readonly keyword does a similar thing to fields. Like fields marked as const they cannot change once initialized. The difference is that one can choose to initialize them in a constructor, or to a value that is not known until run-time. This only works on fields. readonly fields can either be members of an instance or static class members.

Code blocks

Curly braces are used to signify a code block and a new scope. Class members and the body of a method are examples of what can live inside these braces in various contexts.

Inside of method bodies, braces can be used to create new scopes:

void DoSomething()

Program structure

A C# application consists of classes and their members. Classes and other types exist in namespaces but can also be nested inside other classes.

Main method

Whether it is a console or a graphical interface application, the program must have an entry point of some sort. The entry point of a C# application is the Main method. There can only be one declaration of this method, and it is a static method in a class. It usually returns void and is passed command-line arguments as an array of strings.

static void Main(string[] args) // string[] args can be omitted if the program doesn't have any command-line arguments.

The main method is also allowed to return an integer value if specified.

static int Main(string[] args)

Async Main

''This is a feature of C# 7.1.''

Asynchronous Tasks can be awaited in the Main method by declaring the method's return type as Task.

static async Task Main(string[] args)

All the combinations of Task, or Task, and with, or without, the string[] args parameter are supported.

Top-level statements

''This is a feature of C# 9.0.''

Similar to in scripting languages, top-level statements removes the ceremony of having to declare the Program class with a Main method.

Instead, statements can be written directly in one specific file, and that file will be the entry point of the program. Code in other files will still have to be defined in classes.

This was introduced to make C# less verbose, and thus more accessible for beginners to get started.
using System;

Console.WriteLine("Hello World!");
Types are declared after the statements, and will be automatically available from the statements above them.

Namespaces

Namespaces are a part of a type name and they are used to group and/or distinguish named entities from other ones.

System.IO.DirectoryInfo // DirectoryInfo is in the System.IO-namespace

A namespace is defined like this:

namespace FooNamespace

A namespace can be put inside a namespace like this:

namespace FooNamespace

It can also be done like this:

namespace FooNamspace

namespace FooNamspace.BarNamespace

In C# 10 and later, namespaces can also be defined using ''file-scoped declarations'' by doing the following:

namespace FooNamespace; // the brackets are omitted here in favor of a semicolon.

using directive

The using directive loads a specific namespace from a referenced assembly. It is usually placed in the top (or header) of a code file but it can be placed elsewhere if wanted, e.g. inside classes.

using System;
using System.Collections;

The directive can also be used to define another name for an existing namespace or type. This is sometimes useful when names are too long and less readable.

using Net = System.Net;
using DirInfo = System.IO.DirectoryInfo;

directive

The directive loads the static members of a specified type into the current scope, making them accessible directly by the name of the member.
using static System.Console;

WriteLine("Hello, World!");

Operators

Operator overloading

Some of the existing operators can be overloaded by writing an overload method.

public static Foo operator+(Foo foo, Bar bar)

These are the overloadable operators:

*''Assignment operators'' ( etc.) are combinations of a binary operator and the assignment operator () and will be evaluated using the ordinary operators, which can be overloaded.
*''Cast operators'' () cannot be overloaded, but it is possible to define conversion operators.
*''Array indexing'' () operator is not overloadable, but it is possible to define new indexers.

Conversion operators

The cast operator is not overloadable, but one can write a conversion operator method which lives in the target class. Conversion methods can define two varieties of operators, implicit and explicit conversion operators. The implicit operator will cast without specifying with the cast operator () and the explicit operator requires it to be used.

Implicit conversion operator


class Foo

// Implicit conversion
Foo foo = 2;

Explicit conversion operator

class Foo

// Explicit conversion
Foo foo = (Foo)2;

as operator

The as operator will attempt to do a silent cast to a given type. It will return the object as the new type if possible, and otherwise will return null.

Stream stream = File.Open(@"C:\Temp\data.dat");
FileStream fstream = stream as FileStream; // Will return an object.

String str = stream as String; // Will return null.

Null coalesce operator

:''This is a feature of C# 2.0.''
The following:

return ifNotNullValue ?? otherwiseValue;

is shorthand for:

return ifNotNullValue != null ? ifNotNullValue : otherwiseValue;

Meaning that if the content of variable is not null, that content will be returned, otherwise the content of variable is returned.

C# 8.0 introduces null-coalescing assignment, such that

variable ??= otherwiseValue;

is equivalent to

if (variable is null) variable = otherwiseValue;

Control structures

C# inherits most of the control structures of C/C++ and also adds new ones like the foreach statement.

Conditional structures

These structures control the flow of the program through given conditions.

if statement

The if statement is entered when the given condition is true. Single-line case statements do not require block braces although it is mostly preferred by convention.

Simple one-line statement:

if (i

3) ... ;

Multi-line with else-block (without any braces):

if (i

2)
...
else
...

Recommended coding conventions for an if-statement.

if (i

3)

else if (i

2)

else

switch statement

The switch construct serves as a filter for different values. Each value leads to a "case". It is not allowed to fall through case sections and therefore the keyword break is typically used to end a case. An unconditional return in a case section can also be used to end a case. See also how goto statement can be used to fall through from one case to the next. Many cases may lead to the same code though. The default case handles all the other cases not handled by the construct.

switch (ch)

Iteration structures

Iteration statements are statements that are repeatedly executed when a given condition is evaluated as true.

while loop

while (i

true)

do ... while loop

do

while (i

true);

for loop

The for loop consists of three parts: ''declaration'', ''condition'' and ''counter expression''. Any of them can be left out as they are optional.

for (int i = 0; i < 10; i++)

Is equivalent to this code represented with a while statement, except here the variable is not local to the loop.

int i = 0;
while (i < 10)

foreach loop

The foreach statement is derived from the for statement and makes use of a certain pattern described in C#'s language specification in order to obtain and use an enumerator of elements to iterate over.

Each item in the given collection will be returned and reachable in the context of the code block. When the block has been executed the next item will be returned until there are no items remaining.

foreach (int i in intList)

Jump statements

Jump statements are inherited from C/C++ and ultimately assembly languages through it. They simply represent the jump-instructions of an assembly language that controls the flow of a program.

Labels and goto statement

Labels are given points in code that can be jumped to by using the goto statement.

start:
.......
goto start;

Note that the label need not be positioned after the goto statement; it may be before it in the source file.

The goto statement can be used in switch statements to jump from one case to another or to fall through from one case to the next.

switch (n)

break statement

The break statement breaks out of the closest loop or switch statement. Execution continues in the statement after the terminated statement, if any.

int e = 10;
for (int i = 0; i < e; i++)

continue statement

The continue statement discontinues the current iteration of the current control statement and begins the next iteration.

int ch;
while ((ch = Console.Read()) != -1)

The while loop in the code above reads characters by calling , skipping the statements in the body of the loop if the characters are spaces.

Exception handling

Runtime exception handling method in C# is inherited from Java and C++.

The base class library has a class called from which all other exception classes are derived. An -object contains all the information about a specific exception and also the inner exceptions that were caused.
Programmers may define their own exceptions by deriving from the class.

An exception can be thrown this way:

throw new NotImplementedException();

statements

Exceptions are managed within blocks.

try

catch (Exception ex)

finally

The statements within the try block are executed, and if any of them throws an exception, execution of the block is discontinued and the exception is handled by the catch block. There may be multiple catch blocks, in which case the first block with an exception variable whose type matches the type of the thrown exception is executed.

If no catch block matches the type of the thrown exception, the execution of the outer block (or method) containing the try ... catch statement is discontinued, and the exception is passed up and outside the containing block or method. The exception is propagated upwards through the call stack until a matching catch block is found within one of the currently active methods. If the exception propagates all the way up to the top-most method without a matching catch block being found, the entire program is terminated and a textual description of the exception is written to the standard output stream.

The statements within the finally block are always executed after the try and catch blocks, whether or not an exception was thrown. Such blocks are useful for providing clean-up code.

Either a catch block, a finally block, or both, must follow the try block.

Types

C# is a statically typed language like C and C++. That means that every variable and constant gets a fixed type when it is being declared. There are two kinds of types: ''value types'' and ''reference types''.

Value types

Instances of value types reside on the stack, i.e. they are bound to their variables. If one declares a variable for a value type the memory gets allocated directly. If the variable gets out of scope the object is destroyed with it.

Structures

Structures are more commonly known as ''structs''. Structs are user-defined value types that are declared using the struct keyword. They are very similar to classes but are more suitable for lightweight types. Some important syntactical differences between a class and a struct are presented later in this article.

struct Foo

The primitive data types are all structs.

=Pre-defined types

=
These are the primitive datatypes.

Note: () is not a struct and is not a primitive type.

Enumerations

Enumerated types (declared with enum) are named values representing integer values.

enum Season

Enum variables are initialized by default to zero. They can be assigned or initialized to the named values defined by the enumeration type.

Season season;
season = Season.Spring;

Enum type variables are integer values. Addition and subtraction between variables of the same type is allowed without any specific cast but multiplication and division is somewhat more risky and requires an explicit cast. Casts are also required for converting enum variables to and from integer types. However, the cast will not throw an exception if the value is not specified by the type definition.

season = (Season)2; // cast 2 to an enum-value of type Season.
season = season + 1; // Adds 1 to the value.
season = season + season2; // Adding the values of two enum variables.
int value = (int)season; // Casting enum-value to integer value.

season++; // Season.Spring (1) becomes Season.Summer (2).
season--; // Season.Summer (2) becomes Season.Spring (1).

Values can be combined using the bitwise-OR operator .

Color myColors = Color.Green , Color.Yellow , Color.Blue;

Reference types

Variables created for reference types are typed managed references. When the constructor is called, an object is created on the heap and a reference is assigned to the variable. When a variable of an object goes out of scope the reference is broken and when there are no references left the object gets marked as garbage. The garbage collector will then soon collect and destroy it.

A reference variable is null when it does not reference any object.

Arrays

An array type is a reference type that refers to a space containing one or more elements of a certain type. All array types derive from a common base class, . Each element is referenced by its index just like in C++ and Java.

An array in C# is what would be called a dynamic array in C++.

int[] numbers = new int[2];
numbers[0] = 2;
numbers[1] = 5;
int x = numbers[0];

=Initializers

=
Array initializers provide convenient syntax for initialization of arrays.

// Long syntax
int[] numbers = new int[5];
// Short syntax
int[] numbers2 = ;
// Inferred syntax
var numbers3 = new[] ;

=Multi-dimensional arrays

=
Arrays can have more than one dimension, for example 2 dimensions to represent a grid.

int numbers = new int, 3
numbers ,2= 2;

int numbers2 = new int, 3;

See also
* Jagged array

Classes

Classes are self-describing user-defined reference types. Essentially all types in the .NET Framework are classes, including structs and enums, that are compiler generated classes. Class members are private by default, but can be declared as public to be visible outside of the class or protected to be visible by any descendants of the class.

=Strings

=
The class, or simply , represents an immutable sequence of unicode characters ().

Actions performed on a string will always return a new string.

string text = "Hello World!";
string substr = text.Substring(0, 5);
string[] parts = text.Split(new char[]);

The class can be used when a mutable "string" is wanted.

var sb = new StringBuilder();
sb.Append('H');
sb.Append("el");
sb.AppendLine("lo!");

Interface

Interfaces are data structures that contain member definitions with no actual implementation. A variable of an interface type is a reference to an instance of a class which implements this interface. See #Interfaces.

Delegates

C# provides type-safe object-oriented function pointers in the form of ''delegates''.

class Program

Initializing the delegate with an anonymous method.

addition = delegate(int a, int b) ;

Initializing the delegate with lambda expression.

addition = (a, b) => a + b;

Events

''Events'' are pointers that can point to multiple methods. More exactly they bind method pointers to one identifier. This can therefore be seen as an extension to delegates. They are typically used as triggers in UI development. The form used in C# and the rest of the Common Language Infrastructure is based on that in the classic Visual Basic.

delegate void MouseEventHandler(object sender, MouseEventArgs e);

public class Button : System.Windows.Controls.Control

An event requires an accompanied ''event handler'' that is made from a special delegate that in a platform specific library like in Windows Presentation Foundation and Windows Forms usually takes two parameters: ''sender'' and the ''event arguments''. The type of the event argument-object derive from the EventArgs class that is a part of the CLI base library.

Once declared in its class the only way of invoking the event is from inside of the owner. A listener method may be implemented outside to be triggered when the event is fired.

public class MainWindow : System.Windows.Controls.Window

Custom event implementation is also possible:

private EventHandler _clickHandles = (s, e) => ;

public event EventHandler Click

See also
* Event-driven programming

Nullable types

:''This is a feature of C# 2.0.''

Nullable types were introduced in C# 2.0 firstly to enable value types to be null (useful when working with a database).

int? n = 2;
n = null;

Console.WriteLine(n.HasValue);

In reality this is the same as using the struct.

Nullable n = 2;
n = null;

Console.WriteLine(n.HasValue);

Pointers

C# has and allows pointers to selected types (some primitives, enums, strings, pointers, and even arrays and structs if they contain only types that can be pointed) in unsafe context: methods and codeblock marked unsafe. These are syntactically the same as pointers in C and C++. However, runtime-checking is disabled inside unsafe blocks.

static void Main(string[] args)

Structs are required only to be pure structs with no members of a managed reference type, e.g. a string or any other class.

public struct MyStruct

public struct MyContainerStruct

In use:

MyContainerStruct x;
MyContainerStruct* ptr = &x;

byte value = ptr->Byte;

Dynamic

:''This is a feature of C# 4.0 and .NET Framework 4.0.''
Type dynamic is a feature that enables dynamic runtime lookup to C# in a static manner. Dynamic denotes a variable with an object with a type that is resolved at runtime, as opposed to compile-time, as normally is done.

This feature takes advantage of the Dynamic Language Runtime (DLR) and has been designed specifically with the goal of interoperation with dynamically typed languages like IronPython and IronRuby (Implementations of Python and Ruby for .NET).

Dynamic-support also eases interoperation with COM objects.

dynamic x = new Foo();
x.DoSomething(); // Will compile and resolved at runtime. An exception will be thrown if invalid.

Anonymous types

:''This is a feature of C# 3.0.''
Anonymous types are nameless classes that are generated by the compiler. They are only consumable and yet very useful in a scenario like where one has a LINQ query which returns an object on select and one just wants to return some specific values. Then, define an anonymous type containing auto-generated read-only fields for the values.

When instantiating another anonymous type declaration with the same signature the type is automatically inferred by the compiler.

var carl = new ; // Name of the type is only known by the compiler.
var mary = new ; // Same type as the expression above

Boxing and unboxing

''Boxing'' is the operation of converting a value of a value type into a value of a corresponding reference type.Archer, Part 2, Chapter 4:The Type System Boxing in C# is implicit.

''Unboxing'' is the operation of converting a value of a reference type (previously boxed) into a value of a value type. Unboxing in C# requires an explicit type cast.

Example:

int foo = 42; // Value type.
object bar = foo; // foo is boxed to bar.
int foo2 = (int)bar; // Unboxed back to value type.

Object-oriented programming (OOP)

C# has direct support for object-oriented programming.

Objects

An object is created with the type as a template and is called an ''instance'' of that particular type.

In C#, objects are either references or values. No further syntactical distinction is made between those in code.

Object class

All types, even value types in their boxed form, implicitly inherit from the class, the ultimate base class of all objects. This class contains the most common methods shared by all objects. Some of these are virtual and can be overridden.

Classes inherit either directly or indirectly through another base class.

Members

Some of the members of the Object class:

* - Supports comparisons between objects.
* - Performs cleanup operations before an object is automatically reclaimed. (Default destructor)
* - Gets the number corresponding to the value of the object to support the use of a hash table.
* - Gets the Type of the current instance.
* - Creates a human-readable text string that describes an instance of the class. Usually it returns the name of the type.

Classes

Classes are fundamentals of an object-oriented language such as C#. They serve as a template for objects. They contain members that store and manipulate data in a real-life like way.

Differences between classes and structs

Although classes and structures are similar in both the way they are declared and how they are used, there are some significant differences. Classes are reference types and structs are value types. A structure is allocated on the stack when it is declared and the variable is bound to its address. It directly contains the value. Classes are different because the memory is allocated as objects on the heap. Variables are rather managed pointers on the stack which point to the objects. They are references.

Structures differ from classes in several other ways. For example, while both offer an implicit default constructor which takes no arguments, one cannot redefine it for structs. Explicitly defining a differently-parametrized constructor will suppress the implicit default constructor in classes, but not in structs. All fields of a struct must be initialized in those kinds of constructors. Structs do not have finalizers and cannot inherit from another class like classes do. Implicitly, they are sealed and inherit from (which inherits from ). Structs are more suitable for smaller amounts of data.

This is a short summary of the differences:

Declaration

A class is declared like this:

class Foo

=Partial class

=
:''This is a feature of C# 2.0.''

A partial class is a class declaration whose code is divided into separate files. The different parts of a partial class must be marked with keyword partial.

// File1.cs
partial class Foo

// File2.cs
partial class Foo

The usual reason for using partial classes is to split some class into a programmer-maintained and a tool-maintained part, i.e. some code is automatically generated by a user-interface designing tool or something alike.

Initialization

Before one can use the members of the class, initialize the variable with a reference to an object. To create it, call the appropriate constructor using the new keyword. It has the same name as the class.

var foo = new Foo();

For ''structs'' it is optional to explicitly call a constructor because the default one is called automatically. It is just needed to declare it and it gets initialized with standard values.

=Object initializers

=
:''This is a feature of C# 3.0.''
Provides a more convenient way of initializing public fields and properties of an object. Constructor calls are optional when there is a default constructor.

var person = new Person
;

// Equal to
var person = new Person();
person.Name = "John Doe";
person.Age = 39;

=Collection initializers

=
:''This is a feature of C# 3.0.''
Collection initializers give an array-like syntax for initializing collections. The compiler will simply generate calls to the Add-method. This works for classes that implement the interface .

var list = new List ;

// Equal to
var list = new List();
list.Add(2);
list.Add(5);
list.Add(6);
list.Add(6);

Accessing members

Members of an instance and static members of a class are accessed using the operator.

Accessing an instance member

Instance members can be accessed through the name of a variable.

string foo = "Hello";
string fooUpper = foo.ToUpper();

Accessing a static class member

Static members are accessed by using the name of the class or other type.

int r = string.Compare(foo, fooUpper);

Accessing a member through a pointer

In ''unsafe code'', members of a value (struct type) referenced by a pointer are accessed with the operator just like in C and C++.

POINT p;
p.X = 2;
p.Y = 6;
POINT* ptr = &p;
ptr->Y = 4;

Modifiers

Modifiers are keywords used to modify declarations of types and type members. Most notably there is a sub-group containing the access modifiers.

=Class modifiers

=
*abstract - Specifies that a class only serves as a base class. It must be implemented in an inheriting class. A precondition for allowing the class to have abstract methods.
*sealed - Specifies that a class cannot be inherited.

=Class member modifiers

=
*abstract - Declares a method to be available in all derived non-abstract classes.
*const - Specifies that a variable is a constant value that has to be initialized when it gets declared.
*event - Declares an event.
*extern - Specifies that a method signature without a body uses a DLL-import.
*override - Specifies that a method or property declaration is an override of a virtual member or an implementation of a member of an abstract class.
*readonly - Declares a field that can only be assigned values as part of the declaration or in a constructor in the same class.
*unsafe - Specifies an unsafe context, which allows the use of pointers.
*virtual - Specifies that a method or property declaration can be overridden by a derived class.
*volatile - Specifies a field which may be modified by an external process and prevents an optimizing compiler from making guesses about the persistence of the current value of the field.

=static modifier

=
The static modifier states that a member belongs to the class and not to a specific object. Classes marked static are only allowed to contain static members. Static members are sometimes referred to as ''class members'' since they apply to the class as a whole and not to its instances.

public class Foo

// Calling the class method.
Foo.Something();

=Access modifiers

=
The ''access modifiers'', or ''inheritance modifiers'', set the accessibility of classes, methods, and other members. Something marked can be reached from anywhere. members can only be accessed from inside of the class they are declared in and will be hidden when inherited. Members with the modifier will be , but accessible when inherited. classes and members will only be accessible from the inside of the declaring assembly.

Classes and structs are implicitly and members are implicitly if they do not have an access modifier.

public class Foo

This table defines where the access modifiers can be used.

Constructors

A constructor is a special method that is called automatically when an object is created. Its purpose is to initialize the members of the object. Constructors have the same name as the class and do not return anything explicitly. Implicitly, they will return the newly created object when called via the new operator. They may take parameters like any other method. The parameter-less constructor is special because it can be specified as a necessary constraint for a generic type parameter.

class Foo

Constructors can be public, private, protected or internal.

Destructor

The destructor is called when the object is being collected by the garbage collector to perform some manual clean-up. There is a default destructor method called that can be overridden by declaring one.

The syntax is similar to the one of constructors. The difference is that the name is preceded by a ~ and it cannot contain any parameters. There cannot be more than one destructor.

class Foo

Finalizers are always private.

Methods

Like in C and C++ there are functions that group reusable code. The main difference is that functions, just like in Java, have to reside inside of a class. A function is therefore called a ''method''. A method has a return value, a name and usually some parameters initialized when it is called with some arguments. It can either belong to an instance of a class or be a static member.

class Foo

A method is called using notation on a specific variable, or as in the case of static methods, the name of a type.

Foo foo = new Foo();
int r = foo.Bar(7, 2);

Console.WriteLine(r);

=ref and out parameters

=
One can explicitly make arguments be passed by reference when calling a method with parameters preceded by keywords ref or out. These managed pointers come in handy when passing variables that one wants to be modified inside the method by reference. The main difference between the two is that an out parameter must have been assigned within the method by the time the method returns. ref may or may not assign a new value, but the parameter variable has to be initialized before calling the function.

void PassRef(ref int x)

int Z = 7;
PassRef(ref Z);

void PassOut(out int x)

int Q;
PassOut(out Q);

=Optional parameters

=
:''This is a feature of C# 4.0.''
C# 4.0 introduces optional parameters with default values as seen in C++. For example:

void Increment(ref int x, int dx = 1)

int x = 0;
Increment(ref x); // dx takes the default value of 1
Increment(ref x, 2); // dx takes the value 2

In addition, to complement optional parameters, it is possible to explicitly specify parameter names in method calls, allowing to selectively pass any given subset of optional parameters for a method. The only restriction is that named parameters must be placed after the unnamed parameters. Parameter names can be specified for both optional and required parameters, and can be used to improve readability or arbitrarily reorder arguments in a call. For example:

Stream OpenFile(string name, FileMode mode = FileMode.Open,
FileAccess access = FileAccess.Read)

OpenFile("file.txt"); // use default values for both "mode" and "access"
OpenFile("file.txt", mode: FileMode.Create); // use default value for "access"
OpenFile("file.txt", access: FileAccess.Read); // use default value for "mode"
OpenFile(name: "file.txt", access: FileAccess.Read, mode: FileMode.Create);
// name all parameters for extra readability,
// and use order different from method declaration

Optional parameters make interoperating with COM easier. Previously, C# had to pass in every parameter in the method of the COM component, even those that are optional. For example:

object fileName = "Test.docx";
object missing = System.Reflection.Missing.Value;

doc.SaveAs(ref fileName,
ref missing, ref missing, ref missing,
ref missing, ref missing, ref missing,
ref missing, ref missing, ref missing,
ref missing, ref missing, ref missing,
ref missing, ref missing, ref missing);
console.writeline("File saved successfully");

With support for optional parameters, the code can be shortened as

doc.SaveAs(ref fileName);

=extern

=
A feature of C# is the ability to call native code. A method signature is simply declared without a body and is marked as extern. The attribute also needs to be added to reference the desired DLL file.

llImport("win32.dll")static extern double Pow(double a, double b);

Fields

Fields, or instance variables, can be declared inside the class body to store data.

class Foo

Fields can be initialized directly when declared (unless declared in struct).

class Foo

Modifiers for fields:
*const - Makes the field a constant.
*private - Makes the field private (default).
*protected - Makes the field protected.
*public - Makes the field public.
*readonly - Allows the field to be initialized only once in a constructor.
*static - Makes the field a static member, i.e. a class variable.

Properties

Properties bring field-like syntax and combine them with the power of methods. A property can have two accessors: get and set.

public class Person

// Using a property
var person = new Person();
person.Name = "Robert";

Modifiers for properties:
*private - Makes the property private (default).
*protected - Makes the property protected.
*public - Makes the property public.
*static - Makes the property a static member.

Modifiers for property accessors:
*private - Makes the accessor private.
*protected - Makes the accessor protected.
*public - Makes the accessor public.

The default modifiers for the accessors are inherited from the property. Note that the accessor's modifiers can only be equal or more restrictive than the property's modifier.

=Automatic properties

=
:''This is a feature of C# 3.0.''
A feature of C# 3.0 is auto-implemented properties. Define accessors without bodies and the compiler will generate a backing field and the necessary code for the accessors.

public double Width

Indexers

Indexers add array-like indexing capabilities to objects. They are implemented in a way similar to properties.

internal class IntList

// Using an indexer
var list = new IntList();
list = 2;

Inheritance

Classes in C# may only inherit from one class. A class may derive from any class that is not marked as sealed.

class A

class B : A

=virtual

=
Methods marked virtual provide an implementation, but they can be overridden by the inheritors by using the override keyword.

The implementation is chosen by the actual type of the object and not the type of the variable.

class Operation

class NewOperation : Operation

=new

=
When overloading a non-virtual method with another signature, the keyword new may be used. The used method will be chosen by the type of the variable instead of the actual type of the object.

class Operation

class NewOperation : Operation

This demonstrates the case:

var operation = new NewOperation();

// Will call "double Do()" in NewOperation
double d = operation.Do();

Operation operation_ = operation;

// Will call "int Do()" in Operation
int i = operation_.Do();

=abstract

=
Abstract classes are classes that only serve as templates and one cannot initialize an object of that type. Otherwise, it is just like an ordinary class.

There may be abstract members too. Abstract members are members of abstract classes that do not have any implementation. They must be overridden by any non-abstract class that inherits the member.

abstract class Mammal

class Human : Mammal

=sealed

=
The sealed modifier can be combined with the others as an optional modifier for classes to make them uninheritable,
or for methods to disallow overriding them in derived classes.

internal sealed class Foo

public class Bar

public class Baz : Bar

Interfaces

Interfaces are data structures that contain member definitions and not actual implementation. They are useful when one wants to define a contract between members in different types that have different implementations. One can declare definitions for methods, properties, and indexers. Interface members are implicitly public. An interface can either be implicitly or explicitly implemented.

interface IBinaryOperation

Implementing an interface

An interface is implemented by a class or extended by another interface in the same way a class is derived from another class using the notation.

Implicit implementation

When implicitly implementing an interface the members of the interface have to be public.

public class Adder : IBinaryOperation

public class Multiplier : IBinaryOperation

In use:

IBinaryOperation op = null;
double result;

// Adder implements the interface IBinaryOperation.

op = new Adder();
op.A = 2;
op.B = 3;

result = op.GetResult(); // 5

// Multiplier also implements the interface.

op = new Multiplier();
op.A = 5;
op.B = 4;

result = op.GetResult(); // 20

Explicit implementation

One can also explicitly implement members. The members of the interface that are explicitly implemented by a class are accessible only when the object is handled as the interface type.

public class Adder : IBinaryOperation

In use:

Adder add = new Adder();

// These members are not accessible:
// add.A = 2;
// add.B = 3;
// double result = add.GetResult();

// Cast to the interface type to access them:
IBinaryOperation add2 = add;
add2.A = 2;
add2.B = 3;

double result = add2.GetResult();

Note: The properties in the class that extends are auto-implemented by the compiler and a backing field is automatically added (see #Automatic properties).

Extending multiple interfaces

Interfaces and classes are allowed to extend multiple interfaces.

class MyClass : IInterfaceA, IInterfaceB

Here is an interface that extends two interfaces.

interface IInterfaceC : IInterfaceA, IInterfaceB

Interfaces vs. abstract classes

Interfaces and abstract classes are similar. The following describes some important differences:
* An abstract class may have member variables as well as non-abstract methods or properties. An interface cannot.
* A class or abstract class can only inherit from one class or abstract class.
* A class or abstract class may implement one or more interfaces.
* An interface can only extend other interfaces.
* An abstract class may have non-public methods and properties (also abstract ones). An interface can only have public members.
* An abstract class may have constants, static methods and static members. An interface cannot.
* An abstract class may have constructors. An interface cannot.

Generics

:''This is a feature of C# 2.0 and .NET Framework 2.0.''

Generics (or parameterized types, parametric polymorphism) use type parameters, which make it possible to design classes and methods that do not specify the type used until the class or method is instantiated. The main advantage is that one can use generic type parameters to create classes and methods that can be used without incurring the cost of runtime casts or boxing operations, as shown here:

// Declare the generic class.

public class GenericList

class TestGenericList

When compared with C++ templates, C# generics can provide enhanced safety, but also have somewhat limited capabilities. For example, it is not possible to call arithmetic operators on a C# generic type. Unlike C++ templates, .NET parameterized types are instantiated at runtime rather than by the compiler; hence they can be cross-language whereas C++ templates cannot. They support some features not supported directly by C++ templates such as type constraints on generic parameters by use of interfaces. On the other hand, C# does not support non-type generic parameters.

Unlike generics in Java, .NET generics use reification to make parameterized types first-class objects in the Common Language Infrastructure (CLI) Virtual Machine, which allows for optimizations and preservation of the type information.

Using generics

Generic classes

Classes and structs can be generic.

public class List

var list = new List();
list.Add(6);
list.Add(2);

Generic interfaces

interface IEnumerable

Generic delegates

delegate R Func(T1 a1, T2 a2);

Generic methods

public static T[] CombineArrays(T[] a, T[] b)

string[] a = new string[] ;
string[] b = new string[] ;
string[] c = CombineArrays(a, b);

double[] da = new double[] ;
double[] db = new double[] ;
double[] dc = CombineArrays(da, db);

// c is a string array containing
// dc is a double array containing

Type-parameters

Type-parameters are names used in place of concrete types when defining a new generic. They may be associated with classes or methods by placing the type parameter in angle brackets . When instantiating (or calling) a generic, one can then substitute a concrete type for the type-parameter one gave in its declaration. Type parameters may be constrained by use of the where keyword and a constraint specification, any of the six comma separated constraints may be used:

Covariance and contravariance

:''This is a feature of C# 4.0 and .NET Framework 4.0.''

Generic interfaces and delegates can have their type parameters marked as covariant or contravariant, using keywords out and in, respectively. These declarations are then respected for type conversions, both implicit and explicit, and both compile-time and run-time. For example, the existing interface has been redefined as follows:

interface IEnumerable

Therefore, any class that implements for some class is also considered to be compatible with for all classes and interfaces that extends, directly, or indirectly. In practice, it makes it possible to write code such as:

void PrintAll(IEnumerable