C Sharp syntax


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. There are certain standard naming conventions to follow when selecting names for elements.
An identifier can:
An identifier cannot:
are predefined reserved words with special syntactic meaning. The language has two types of keyword — contextual and reserved. The reserved keywords such as or may only be used as keywords. The contextual keywords such as or 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 the @ character to distinguish it. This facilitates reuse of.NET code written in other languages.
Using a keyword as an identifier:

string @out; // @out is an ordinary identifier, distinct from the 'out' keyword,
// which retains its special meaning

Literals

Digit separators

The underscore symbol separates 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, next to the decimal in floating point values, next to the exponent character and next to the type specifier.

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

C# 3.0 introduced type inference, allowing the type specifier of a variable declaration to be replaced by the keyword, 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;

See also
Constants are immutable values.

When declaring a local variable or a field with the 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.

class Foo

The keyword does a similar thing to fields. Like fields marked as they cannot change once initialized. The difference is that you can choose to initialize them in a constructor, or to a value that is not known until run-time. This only works on fields. 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 you can use the braces to create new scopes like so:

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.

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 the C# application is the method. There can only be one, and it is a static method in a class. The method usually returns and is passed command-line arguments as an array of strings.

static void Main
// OR Main method can be defined without parameters.
static void Main

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

static int Main

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

directive

The directive loads a specific namespace from a referenced assembly. It is usually placed in the top 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;

Operators

Operator overloading

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

public static Foo operator+

These are the overloadable operators:
Operators
,,,,,,, Unary operators
,,,,,,,,, Binary operators
, !=,,, <=, >=Comparison operators, must be overloaded in pairs

See also
The cast operator is not overloadable but you 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 = 2;

operator

The 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;
FileStream fstream = stream as FileStream; // Will return an object.
String str = stream as String; // Will return null.

Null coalesce operator

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 = otherwiseValue;

Control structures

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

Conditional structures

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

statement

The 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 ... ;

Multi-line with else-block :

if
...
else
...

Recommended coding conventions for an if-statement.

if
else if
else

statement

The 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 is typically used to end a case. An unconditional in a case section can also be used to end a case. See also how 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

Iteration structures

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

loop


while

loop


do
while ;

loop

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

for

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

int i = 0;
while

loop

The statement is derived from the 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

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 statement

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

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

Note that the label need not be positioned after the statement; it may be before it in the source file.
The statement can be used in statements to jump from one case to another or to fall through from one case to the next.

switch

statement

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

int e = 10;
for

statement

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

int ch;
while ) != -1)

The 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
finally

The statements within the block are executed, and if any of them throws an exception, execution of the block is discontinued and the exception is handled by the block. There may be multiple blocks, in which case the first block with an exception variable whose type matches the type of the thrown exception is executed.
If no block matches the type of the thrown exception, the execution of the outer block containing the 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 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 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 block are always executed after the and blocks, whether or not an exception was thrown. Such blocks are useful for providing clean-up code.
Either a block, a block, or both, must follow the 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 you declare 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 keyword. They are very similar to classes but are more suitable for lightweight types. Some important syntactical differences between a and a 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 are named values representing integer values.

enum Season

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;

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 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 = 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 = season; // Casting enum-value to integer value.
season++; // Season.Spring becomes Season.Summer.
season--; // Season.Summer becomes Season.Spring.

Values can be combined using the bitwise-OR operator.

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

See also
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;
numbers = 2;
numbers = 5;
int x = numbers;
Initializers
Array initializers provide convenient syntax for initialization of arrays.

// Long syntax
int numbers = new int;
// 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;
numbers = 2;
int numbers2 = new int ;

See also
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 by default, but can be declared as to be visible outside of the class or to be visible by any descendants of the class.
class
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;
string parts = text.Split;

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

StringBuilder sb = new StringBuilder;
sb.Append;
sb.Append;
sb.AppendLine;

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;
Initializing the delegate with lambda expression. addition = => 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;
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 = => ;
public event EventHandler Click

See also
Nullable types were introduced in C# 2.0 firstly to enable value types to be .

int? n = 2;
n = null;
Console.WriteLine;

In reality this is the same as using the struct.

Nullable n = 2;
n = null;
Console.WriteLine;

Pointers

C# has and allows pointers to selected types in unsafe context: methods and codeblock marked. These are syntactically the same as pointers in C and C++. However, runtime-checking is disabled inside blocks.

static void Main

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;

See also
Type 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 and has been designed specifically with the goal of interoping with dynamically typed languages like IronPython and IronRuby.
Dynamic-support also eases interop with COM objects.

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

Anonymous types

Anonymous types are nameless classes that are generated by the compiler. They are only consumable and yet very useful in a scenario like where you have a LINQ query which returns an object on and you just want to return some specific values. Then you can 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. Boxing in C# is implicit.
Unboxing is the operation of converting a value of a reference type 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 = 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.

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 and can be overridden.
Classes inherit either directly or indirectly through another base class.
Members
Some of the members of the class:
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.
See also
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 require some more work than classes. For example, you need to explicitly create a default constructor which takes no arguments to initialize the struct and its members. The compiler will create a default one for classes. All fields and properties of a struct must have been initialized before an instance is created. Structs do not have finalizers and cannot inherit from another class like classes do. However, they inherit from, that inherits from. Structs are more suitable for smaller constructs of data.
This is a short summary of the differences:

Declaration

A class is declared like this:

class Foo
Partial class
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.

// File1.cs
partial class Foo
// File2.cs
partial class Foo

Initialization

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

Foo foo = new Foo;

For structs it is optional to explicitly call a constructor because the default one is called automatically. You just need to declare it and it gets initialized with standard values.
Object initializers
Provides a more convenient way of initializing public fields and properties of an object. Constructor calls are optional when there is a default constructor.

Person person = new Person ;
// Equal to
Person person = new Person;
person.Name = "John Doe";
person.Age = 39;
Collection initializers
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.

List list = new List ;
// Equal to
List list = new List;
list.Add;
list.Add;
list.Add;
list.Add;

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;

Accessing a member through a pointer
In unsafe code, members of a value 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
The 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.
Unnested typesMembers
yesyes
noyes
noyes
yes yes
noyes
noyes

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. They may take parameters like any other method.

class Foo

Constructors can be,, or.
See also
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 your own.
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.
See also
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;
Console.WriteLine;

See also
One can explicitly make arguments be passed by reference when calling a method with parameters preceded by keywords or. These managed pointers come in handy when passing variables that you want to be modified inside the method by reference. The main difference between the two is that an parameter must have been assigned within the method by the time the method returns, while ref need not assign a value.

void PassRef
int Z;
PassRef;
void PassOut
int Q;
PassOut;
Optional parameters
C# 4.0 introduces optional parameters with default values as seen in C++. For example:

void Increment
int x = 0;
Increment; // dx takes the default value of 1
Increment; // 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
OpenFile; // use default values for both "mode" and "access"
OpenFile; // use default value for "access"
OpenFile; // use default value for "mode"
OpenFile;
// 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;
console.writeline;

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

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

static extern double Pow;

Fields

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

class Foo

Fields can be initialized directly when declared.

class Foo

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

class Person
// Using a property
Person person = new Person;
person.Name = "Robert";

Modifiers for properties:
Modifiers for property accessors:
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
A feature of C# 3.0 is auto-implemented properties. You 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.

class IntList
// Using an indexer
IntList 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.

class A
class B : A

See also
Methods marked provide an implementation, but they can be overridden by the inheritors by using the keyword.
The implementation is chosen by the actual type of the object and not the type of the variable.

class Operation
class NewOperation : Operation
When overloading a non-virtual method with another signature, the keyword 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:

NewOperation 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 classes are classes that only serve as templates and you can not 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 the class that inherits the member.

abstract class Mammal
class Human : Mammal
The modifier can be combined with the others as an optional modifier for classes to make them uninheritable.

internal sealed class _FOO

Interfaces

Interfaces are data structures that contain member definitions and not actual implementation. They are useful when you want to define a contract between members in different types that have different implementations. You 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 you derive a class from another class using the notation.
Implicit implementation
When implicitly implementing an interface the members of the interface have to be.

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
You 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.
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:
Generics 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 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
List list = new List;
list.Add;
list.Add;

Generic interfaces


interface IEnumerable

Generic delegates


delegate R Func;

Generic methods


public static T CombineArrays
string a = new string ;
string b = new string ;
string c = CombineArrays;
double da = new double ;
double db = new double ;
double dc = CombineArrays;
// 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 a generic, you can then substitute a concrete type for the type-parameter you gave in its declaration. Type parameters may be constrained by use of the keyword and a constraint specification, any of the six comma separated constraints may be used:
ConstraintExplanation
type parameter must be a value type
type parameter must be a reference type
type parameter must have a constructor with no parameters
type parameter must inherit from
type parameter must be, or must implement this interface
naked type parameter constraint

Covariance and contravariance

Generic interfaces and delegates can have their type parameters marked as covariant or contravariant, using keywords and, 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 strings = new List;
PrintAll; // IEnumerable is implicitly converted to IEnumerable

For contravariance, the existing interface has been redefined as follows:

public interface IComparer

Therefore, any class that implements for some class is also considered to be compatible with for all classes and interfaces that are extended from. It makes it possible to write code such as:

IComparer objectComparer = GetComparer;
IComparer stringComparer = objectComparer;

Enumerators

An enumerator is an iterator.
Enumerators are typically obtained by calling the method of an object implementing the interface. Container classes typically implement this interface. However, the foreach statement in C# can operate on any object providing such a method, even if it doesn't implement. This interface was expanded into generic version in.NET 2.0.
The following shows a simple use of iterators in C# 2.0:

// explicit version
IEnumerator iter = list.GetEnumerator;
while )
Console.WriteLine;
// implicit version
foreach
Console.WriteLine;

Generator functionality

The.NET 2.0 Framework allowed C# to introduce an iterator that provides generator functionality, using a construct similar to in Python. With a, the function automatically keeps its state during the iteration.

// Method that takes an iterable input
// and returns all even numbers.
public static IEnumerable GetEven
//using the method to output only even numbers from the array
static void Main

LINQ

LINQ, short for Language Integrated Queries, is a.NET Framework feature which simplifies the handling of data. Mainly it adds support that allows you to query arrays, collections, and databases. It also introduces binders, which makes it easier to access to databases and their data.

Query syntax

The LINQ query syntax was introduced in C# 3.0 and lets you write SQL-like queries in C#.

var list = new List;
var result = from i in list
where i > 1
select i;

The statements are compiled into method calls, whereby almost only the names of the methods are specified. Which methods are ultimately used is determined by normal overload resolution. Thus, the end result of the translation is affected by what symbols are in scope.
What differs from SQL is that the from-statement comes first and not last as in SQL. This is because it seems more natural writing like this in C# and supports "Intellisense".

Anonymous methods

Anonymous methods, or in their present form more commonly referred to as "lambda expressions", is a feature which allows you to write inline closure-like functions in your code.
There are various ways to create anonymous methods. Prior to C# 3.0 there was limited support by using delegates.
See also
  • Anonymous function
  • Closure

    Anonymous delegates

Anonymous delegates are functions pointers that hold anonymous methods. The purpose is to make it simpler to use delegates by simplifying the process of assigning the function. Instead of declaring a separate method in code the programmer can use the syntax to write the code inline and the compiler will then generate an anonymous function for it.

Func f = delegate ;

Lambda expressions

Lambda expressions provide a simple syntax for inline functions that are similar to closures. Functions with parameters infer the type of the parameters if other is not explicitly specified.

// =>
// With parameters
n => n 2
=> a + b
=>
// With explicitly typed parameters
=> a + b
// No parameters
=> return 0
// Assigning lambda to delegate
Func f = => a + b;

Multi-statement lambdas have bodies enclosed by braces and inside of them code can be written like in standard methods.

=>

Lambda expressions can be passed as arguments directly in method calls similar to anonymous delegates but with a more aesthetic syntax.

var list = stringList.Where;

Lambda expressions are essentially compiler-generated methods that are passed via delegates. These methods are reserved for the compiler only and can not be used in any other context.

Extension methods

Extension methods are a form of syntactic sugar providing the illusion of adding new methods to the existing class outside its definition. In practice, an extension method is a static method that is callable as if it were an instance method; the receiver of the call is bound to the first parameter of the method, decorated with keyword :

public static class StringExtensions
string s = "foo";
s.Left; // same as StringExtensions.Left;

See also
  • Decorator pattern

    Local functions

Local functions can be defined in the body of another method, constructor or property’s getter and setter. Such functions have access to all variables in the enclosing scope, including parent method local variables. They are in scope for the entire method, regardless of whether they’re invoked before or after their declaration. Access modifiers cannot be used with local functions. Also they do not support function overloading. It means there cannot be two local functions in the same method with the same name even if the signatures don’t overlap. After a compilation, a local function is transformed into a private static method, but when defined it cannot be marked static.
In code example below, the Sum method is a local function inside Main method. So it can be used only inside its parent method Main:

static void Main

Miscellaneous

Closure blocks

C# implements closure blocks by means of the . The statement accepts an expression which results in an object implementing, and the compiler generates code that guarantees the object's disposal when the scope of the -statement is exited. The statement is syntactic sugar. It makes the code more readable than the equivalent block.

public void Foo

Thread synchronization

C# provides the , which is yet another example of beneficial syntactic sugar. It works by marking a block of code as a critical section by mutual exclusion of access to a provided object. Like the statement, it works by the compiler generating a block in its place.

private static StreamWriter _writer;
public void ConcurrentMethod

Attributes

Attributes are entities of data that are stored as metadata in the compiled assembly. An attribute can be added to types and members like properties and methods. Attributes better maintenance of preprocessor directives.

public class $AnonymousType$120

The.NET Framework comes with predefined attributes that can be used. Some of them serve an important role at runtime while some are just for syntactic decoration in code like. It does only mark that it is a compiler-generated element. Programmer-defined attributes can also be created.
An attribute is essentially a class which inherits from the class. By convention, attribute classes end with "Attribute" in their name. This will not be required when using it.

public class EdibleAttribute : Attribute

Showing the attribute in use using the optional constructor parameters.

public class Peach : Fruit

Preprocessor

C# features "preprocessor directives" based on the C preprocessor that allow programmers to define symbols, but not macros. Conditionals such as,, and are also provided.
Directives such as give hints to editors for code folding. The block must be terminated with a directive.

public class Foo

Code comments

C# utilizes a double slash to indicate the rest of the line is a comment.

public class Foo

Multi-line comments can be indicated by a starting slash/asterisk and ending asterisk/forward slash.

public class Foo

Comments do not nest. These are two single comments:

// Can put /* */ */ */ /* /*


/* Can put /* /* /* but it ends with */

Single-line comments beginning with three slashes are used for XML documentation. This, however, is a convention used by Visual Studio and is not part of the language definition:

///
/// This class is very classy.
///

XML documentation system

C#'s documentation system is similar to Java's Javadoc, but based on XML. Two methods of documentation are currently supported by the C# compiler.
Single-line documentation comments, such as those commonly found in Visual Studio generated code, are indicated on a line beginning with.

public class Foo

Multi-line documentation comments, while defined in the version 1.0 language specification, were not supported until the.NET 1.1 release. These comments are designated by a starting forward slash/asterisk/asterisk and ending asterisk/forward slash.

public class Foo

There are some stringent criteria regarding white space and XML documentation when using the forward slash/asterisk/asterisk technique.
This code block:

/**
*
* A summary of the method.*/

produces a different XML comment than this code block:

/**
*
A summary of the method.*/

Syntax for documentation comments and their XML markup is defined in a non-normative annex of the ECMA C# standard. The same standard also defines rules for processing of such comments, and their transformation to a plain XML document with precise rules for mapping of Common Language Infrastructure identifiers to their related documentation elements. This allows any C# integrated development environment or other development tool to find documentation for any symbol in the code in a certain well-defined way.

Async-await syntax

As of.NET Framework 4 there is a task library that makes it easier to write parallel and multi-threaded applications through tasks.
C# 5.0 has native language support for asynchrony.
Consider this code that takes advantage of the task library directly:

public static class SomeAsyncCode
var t = SomeAsyncCode.GetContentAsync.ContinueWith;
t.Start;

Here is the same logic written in the async-await syntax:

public static class SomeAsyncCode
var xmlDocument = await SomeAsyncCode.GetContentAsync;
// The Task will be started on call with await.

Dialects

Spec#

Spec# is a dialect of C# that is developed in parallel with the standard implementation from Microsoft. It extends C# with specification language features and is a possible future feature to the C# language. It also adds syntax for the code contracts API that was introduced in.NET Framework 4.0. Spec# is being developed by Microsoft Research.
This sample shows two of the basic structures that are used when adding contracts to your code.

static void Main
requires args.Length > 0


  • is used to make a reference type non-nullable, e.g. you cannot set the value to. This in contrast of nullable types which allow value types to be set as.
  • indicates a condition that must be followed in the code. In this case the length of args is not allowed to be zero or less.

    Non-nullable types

Spec# extends C# with non-nullable types that simply checks so the variables of nullable types that has been set as non-nullable are not. If is then an exception will be thrown.

string! input

In use:

public Test

Preconditions

Preconditions are checked before a method is executed.

public Test
requires i > 0;

Postconditions

Postconditions are conditions that are ensured to be correct when a method has been executed.

public void Increment
ensures i > 0;

Checked exceptions

Spec# adds checked exceptions like those in Java.

public void DoSomething
throws SomeException; // SomeException : ICheckedException


Checked exceptions are problematic, because when a lower-level function adds a new exception type, the whole chain of methods using this method at some nested lower level must also change its contract. This violates the open/closed principle.
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