C Sharp语法

C Sharp语法（英语：Syntax (programming languages)）是编写该语言程序的一套规则。也适用于.NET Framework and Mono。

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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:

start with an underscore: _
contain an underscore: _
contain a digit: 0123456789
contain both upper case and lower case Unicode letters. Case is sensitive (FOO is different from foo)
begin with an @ sign (but this is insignificant; @name is the same identifier as name).

An identifier cannot:

start with a digit
start with a symbol, unless it is a keyword (check Keywords)
contain more than 511 characters
contain @ sign after its first character

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 Template:C sharp or Template:C sharp may only be used as keywords. The contextual keywords such as Template:C sharp or Template:C sharp are only treated as keywords in certain situations.^[1] 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.^[2]

C# keywords, reserved words
Template:C sharp	Template:C sharp	Template:C sharp	Template:C sharp
Template:C sharp	Template:C sharp ²	Template:C sharp	Template:C sharp
Template:C sharp	Template:C sharp	Template:C sharp	Template:C sharp
Template:C sharp	Template:C sharp	Template:C sharp	Template:C sharp
Template:C sharp	Template:C sharp	Template:C sharp	Template:C sharp ²
Template:C sharp	Template:C sharp	Template:C sharp	Template:C sharp
Template:C sharp	Template:C sharp	Template:C sharp	Template:C sharp
Template:C sharp	Template:C sharp	Template:C sharp	Template:C sharp ²
Template:C sharp	Template:C sharp ²	Template:C sharp	Template:C sharp
Template:C sharp	Template:C sharp	Template:C sharp	Template:C sharp
Template:C sharp ²	Template:C sharp	Template:C sharp	Template:C sharp
Template:C sharp	Template:C sharp	Template:C sharp	Template:C sharp
Template:C sharp	Template:C sharp	Template:C sharp	Template:C sharp ²
Template:C sharp	Template:C sharp	Template:C sharp	Template:C sharp
Template:C sharp	Template:C sharp	Template:C sharp	Template:C sharp
Template:C sharp	Template:C sharp	Template:C sharp	Template:C sharp
`sizeof`	Template:C sharp	Template:C sharp	Template:C sharp
Template:C sharp ²	Template:C sharp	Template:C sharp	Template:C sharp
Template:C sharp	Template:C sharp	Template:C sharp	Template:C sharp
Template:C sharp	Template:C sharp	Template:C sharp	Template:C sharp
Template:C sharp ²	Template:C sharp	Template:C sharp	Template:C sharp
Template:C sharp	Template:C sharp ¹^[3]²	Template:C sharp ¹
^{1, 2} These are contextual keywords; thus (unlike actual keywords), it is possible to define variables and types using these names, but they act like keywords when appearing in specific positions in code. Contextual keywords were introduced in C# 2.0, and all keywords to be introduced in the future of the language will be contextual.

Using a keyword as an identifier:

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

Literals

Integers
decimal	Template:C sharp
hexadecimal	Template:C sharp
binary	Template:C sharp
Floating-point values
float	Template:C sharp
double	Template:C sharp
decimal	Template:C sharp
Characters
char	Template:C sharp
Strings
string	Template:C sharp Template:C sharp, Template:C sharp [verbatim strings (preceded by @) may include line-break and carriage return characters] Template:C sharp Interpolated string. As a verbatim string: Template:C sharp
Character escapes in strings
Unicode character	Template:C sharp followed by the hexadecimal unicode code point
Null character¹	Template:C sharp
Tab	Template:C sharp
Backspace	Template:C sharp
Carriage return	Template:C sharp
Form feed	Template:C sharp
Backslash	Template:C sharp
Single quote	Template:C sharp
Double quote	Template:C sharp
Line feed	Template:C sharp
¹Strings in C# are not null terminated

Digit separators

This is a feature of C# 7.0.

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 (_121) or the end of the value (121_ or 121.05_), next to the decimal in floating point values (10_.0), next to the exponent character (1.1e_1) and next to the type specifier (10_f).

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 Template:C sharp, 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[] {'A', 'Ö'}; // or char[] myChars = new char[] {'A', 'Ö'};

var myNums = new List<int>();  // or List<int> myNums = new List<int>();

Constants

Constants are immutable values.

Template:C sharp

When declaring a local variable or a field with the Template:C sharp 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
{
    const double X = 3;

    Foo()
    {
        const int Y = 2;
    }
}

Template:C sharp

The Template:C sharp keyword does a similar thing to fields. Like fields marked as Template:C sharp 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. Template:C sharp fields can either be members of an instance or static class members.

Code blocks

Curly braces Template:C sharp 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()
{
    int a;

    {
        int b;
        a = 1;
    }

    a = 2;
    b = 3; // Will fail because the variable is declared in an inner scope.
}

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.

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

static void Main(string[] args)
{
}
// OR Main method can be defined without parameters.
static void Main()
{
}

A Template:C sharp method is also allowed to return an integer value if specified.

static int Main(string[] args)
{
    return 0;
}

Async Main

This is a feature of C# 7.1.

Asynchronous Tasks can be awaited in the Main method by declaring it to return type Task.

static async Task Main(string[] args)
{
    await DoWorkAsync(42);
}

All the combinations of Task, or Task<int>, 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
{
    // Members
}

Template:C sharp directive

The Template:C sharp 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;

Template:C sharp directive

The Template:C sharp 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 category	Operators
Arithmetic	Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp
Logical (boolean and bitwise)	Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp
String concatenation	Template:C sharp
Increment, decrement	Template:C sharp, Template:C sharp
Shift	Template:C sharp, Template:C sharp
Relational (conditional)	Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp
Assignment	Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp
Member access	Template:C sharp, Template:C sharp, Template:C sharp
Indexing	Template:C sharp
Cast	Template:C sharp
Conditional (ternary)	Template:C sharp
Delegate concatenation and removal	Template:C sharp, Template:C sharp
Object creation	Template:C sharp
Type information	Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp
Overflow exception control	Template:C sharp, Template:C sharp
Indirection and Address	Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp
Coalesce	Template:C sharp
Lambda expression	Template:C sharp

Operator overloading

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

public static Foo operator+(Foo foo, Bar bar)
{
    return new Foo(foo.Value + bar.Value);
}

These are the overloadable operators:

Operators
Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp	Unary operators
Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp, Template:C sharp	Binary operators
`==`, `!=`, Template:C sharp, Template:C sharp, `<=`, `>=`	Comparison operators, must be overloaded in pairs

Assignment operators (Template:C sharp etc.) are combinations of a binary operator and the assignment operator (Template:C sharp) and will be evaluated using the ordinary operators, which can be overloaded.
Cast operators (Template:C sharp) cannot be overloaded, but you can define conversion operators.
Array indexing (Template:C sharp) operator is not overloadable, but you can define new indexers.

Conversion operators

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 (Template:C sharp) and the explicit operator requires it to be used.

Implicit conversion operator

class Foo
{
    public int Value;
    public static implicit operator Foo(int value)
    {
        return new Foo(value);
    }
}
// Implicit conversion
Foo foo = 2;

Explicit conversion operator

class Foo
{
    public int Value;
    public static explicit operator Foo(int value)
    {
        return new Foo(value);
    }
}
// Explicit conversion
Foo foo = (Foo)2;

Template:C sharp operator

The Template:C sharp 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 Template:C sharp is not null, that content will be returned, otherwise the content of variable Template:C sharp 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 Template:C sharp statement.

Conditional structures

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

Template:C sharp statement

The Template:C sharp 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
{
    ...
}

Template:C sharp statement

The Template:C sharp 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 Template:C sharp is typically used to end a case. An unconditional Template:C sharp in a case section can also be used to end a case. See also how Template:C sharp 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)
{
    case 'A':
        statement;
        ...
        break;
    case 'B':
        statement;
        break;
    case 'C': // A switch section can have multiple case labels.
    case 'D':
        ...
        break;
    default:
        ...
        break;
}

Iteration structures

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

Template:C sharp loop

while (i == true)
{
    ...
}

Template:C sharp loop

do
{

}
while (i == true);

Template:C sharp loop

The Template:C sharp 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 Template:C sharp statement, except here the Template:C sharp variable is not local to the loop.

int i = 0;
while (i < 10)
{
    //...
    i++;
}

Template:C sharp loop

The Template:C sharp statement is derived from the Template:C sharp 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 Template:C sharp statement

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

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

Note that the label need not be positioned after the Template:C sharp statement; it may be before it in the source file.

The Template:C sharp statement can be used in Template:C sharp statements to jump from one case to another or to fall through from one case to the next.

switch(n)
{
    case 1:
        Console.WriteLine("Case 1");
        break;
    case 2:
        Console.WriteLine("Case 2");
        goto case 1;
    case 3:
        Console.WriteLine("Case 3");
    case 4: // Compilation will fail here as cases cannot fall through in C#.
        Console.WriteLine("Case 4");
        goto default; // This is the correct way to fall through to the next case.
    case 5:  // Multiple labels for the same code are OK
    case 6:
    default:
        Console.WriteLine("Default");
        break;  // Even default must not reach the end point
}

Template:C sharp statement

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

int e = 10;
for (int i = 0; i < e; i++)
{
    while (true)
    {
        break;
    }
    // Will break to this point.
}

Template:C sharp statement

The Template:C sharp statement discontinues the current iteration of the current control statement and begins the next iteration.

int ch;
while ((ch = Console.Read()) != -1)
{
   	if (ch == ' ')
      		continue;    // Skips the rest of the while-loop

   	// Rest of the while-loop
   	...
}

The Template:C sharp loop in the code above reads characters by calling Template:C sharp, 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 Template:C sharp from which all other exception classes are derived. An Template:C sharp-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 Template:C sharp class.

An exception can be thrown this way:

    throw new NotImplementedException();

Template:C sharp statements

Exceptions are managed within Template:C sharp blocks.

try
{
    // Statements which may throw exceptions
    ...
}
catch (Exception ex)
{
    // Exception caught and handled here
    ...
}
finally
{
    // Statements always executed after the try/catch blocks
    ...
}

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

If no Template:C sharp block matches the type of the thrown exception, the execution of the outer block (or method) containing the Template:C sharp 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 Template:C sharp block is found within one of the currently active methods. If the exception propagates all the way up to the top-most Template:C sharp method without a matching Template:C sharp 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 Template:C sharp block are always executed after the Template:C sharp and Template:C sharp blocks, whether or not an exception was thrown. Such blocks are useful for providing clean-up code.

Either a Template:C sharp block, a Template:C sharp block, or both, must follow the Template:C sharp 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 Template:C sharp keyword. They are very similar to classes but are more suitable for lightweight types. Some important syntactical differences between a Template:C sharp and a Template:C sharp are presented later in this article.

struct Foo
{
    ...
}

The primitive data types are all structs.

Pre-defined types

These are the primitive datatypes.

Primitive types
Type name	BCL equivalent	Value	Range	Size	Default value
Template:C sharp	Template:C sharp	integer	−128 through +127	8-bit (1-byte)	Template:C sharp
Template:C sharp	Template:C sharp	integer	−32,768 through +32,767	16-bit (2-byte)	Template:C sharp
Template:C sharp	Template:C sharp	integer	−2,147,483,648 through +2,147,483,647	32-bit (4-byte)	Template:C sharp
Template:C sharp	Template:C sharp	integer	−9,223,372,036,854,775,808 through +9,223,372,036,854,775,807	64-bit (8-byte)	Template:C sharp
Template:C sharp	Template:C sharp	unsigned integer	0 through 255	8-bit (1-byte)	Template:C sharp
Template:C sharp	Template:C sharp	unsigned integer	0 through 65,535	16-bit (2-byte)	Template:C sharp
Template:C sharp	Template:C sharp	unsigned integer	0 through 4,294,967,295	32-bit (4-byte)	Template:C sharp
Template:C sharp	Template:C sharp	unsigned integer	0 through 18,446,744,073,709,551,615	64-bit (8-byte)	Template:C sharp
Template:C sharp	Template:C sharp	signed decimal number	−79,228,162,514,264,337,593,543,950,335 through +79,228,162,514,264,337,593,543,950,335	128-bit (16-byte)	Template:C sharp
Template:C sharp	Template:C sharp	floating point number	±1.401298E−45 through ±3.402823E+38	32-bit (4-byte)	Template:C sharp
Template:C sharp	Template:C sharp	floating point number	±4.94065645841246E−324 through ±1.79769313486232E+308	64-bit (8-byte)	Template:C sharp
Template:C sharp	Template:C sharp	Boolean	Template:C sharp or Template:C sharp	8-bit (1-byte)	Template:C sharp
Template:C sharp	Template:C sharp	single Unicode character	Template:C sharp through Template:C sharp	16-bit (2-byte)	Template:C sharp

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

Enumerations

Enumerated types (Template:C sharp) are named values representing integer values.

enum Season
{
    Winter = 0,
    Spring = 1,
    Summer = 2,
    Autumn = 3,
    Fall = Autumn    // Autumn is called Fall in American English.
}

Template:C sharp 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;

Template:C sharp 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 Template:C sharp variables to and from integer types. However, the cast will not throw an exception if the value is not specified by the Template:C sharp 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 Template:C sharp.

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 Template:C sharp 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, Template:C sharp. 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]{ 20, 1, 42, 15, 34 };
// Short syntax
int[] numbers2 = { 20, 1, 42, 15, 34 };
// Inferred syntax
var numbers3 = new[] { 20, 1, 42, 15, 34 };

Multi-dimensional arrays

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

int[,] numbers = new int[3, 3];
numbers[1,2] = 2;

int[,] numbers2 = new int[3, 3] { {2, 3, 2}, {1, 2, 6}, {2, 4, 5} };

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 Template:C sharp by default, but can be declared as Template:C sharp to be visible outside of the class or Template:C sharp to be visible by any descendants of the class.

Template:C sharp class

The Template:C sharp class, or simply Template:C sharp, represents an immutable sequence of unicode characters (Template:C sharp).

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 Template:C sharp class can be used when a mutable "string" is wanted.

    StringBuilder 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
{
    // Delegate type:
    delegate int Operation(int a, int b);

    static int Add(int i1, int i2)
    {
        return i1 + i2;
    }

    static int Sub(int i1, int i2)
    {
        return i1 - i2;
    }

    static void Main()
    {
        // Instantiate the delegate and assign the method to it.
        Operation op = Add;

        // Call the method that the delegate points to.
        int result1 = op(2, 3);  // 5

        op = Sub;
        int result2 = op(10, 2); // 8
    }
}

Initializing the delegate with an anonymous method.

  addition = delegate(int a, int b){ return a + 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
{
    event MouseEventHandler OnClick;

    /* Imaginary trigger function */
    void click()
    {
        this.OnClick(this, new MouseEventArgs(data));
    }
}

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
{
    private Button button1;

    public MainWindow()
    {
        button1 = new Button();
        button1.Text = "Click me!";

        /* Subscribe to the event */
        button1.ClickEvent += button1_OnClick;

        /* Alternate syntax that is considered old:
           button1.MouseClick += new MouseEventHandler(button1_OnClick); */
    }

    protected void button1_OnClick(object sender, MouseEventArgs e)
    {
        MessageBox.Show("Clicked!");
    }
}

Custom event implementation is also possible:

	private EventHandler clickHandles = (s, e) => { };

	public event EventHandler Click
	{
		add
		{
			// Some code to run when handler is added...
			...

			clickHandles += value;
		}
		remove
		{
			// Some code to run when handler is removed...
			...

			clickHandles -= value;
		}
	}

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 Template:C sharp (useful when working with a database).

int? n = 2;
n = null;

Console.WriteLine(n.HasValue);

In reality this is the same as using the Template:C sharp struct.

Nullable<int> 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^[4]) in unsafe context: methods and codeblock marked Template:C sharp. These are syntactically the same as pointers in C and C++. However, runtime-checking is disabled inside Template:C sharp blocks.

static void Main(string[] args)
{
    unsafe
    {
        int a = 2;
        int* b = &a;

        Console.WriteLine("Address of a: {0}. Value: {1}", (int)&a, a);
        Console.WriteLine("Address of b: {0}. Value: {1}. Value of *b: {2}", (int)&b, (int)b, *b);

        // Will output something like:
        // Address of a: 71953600. Value: 2
        // Address of b: 71953596. Value: 71953600. Value of *b: 2
    }
}

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 char Character;
    public int Integer;
}

public struct MyContainerStruct
{
    public byte Byte;
    public MyStruct MyStruct;
}

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 Template:C sharp 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 interopingThis is not English^{[需要解释]} with dynamically typed languages like IronPython and IronRuby (Implementations of Python and Ruby for .NET).

Dynamic-support also eases interopThis is not English^{[需要解释]} 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 you have a LINQ query which returns an object on Template:C sharp 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 = "Carl", Age = 35 }; // Name of the type is only known by the compiler.
var mary = new { Name = "Mary", Age = 22 }; // 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.^[5] 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.^[5] 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.

Template:C sharp class

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

Classes inherit Template:C sharp either directly or indirectly through another base class.

Members
Some of the members of the Template:C sharp class:

Template:C sharp - Supports comparisons between objects.
Template:C sharp - Performs cleanup operations before an object is automatically reclaimed. (Default destructor)
Template:C sharp - Gets the number corresponding to the value of the object to support the use of a hash table.
Template:C sharp - Gets the Type of the current instance.
Template:C sharp - 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 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 Template:C sharp, that inherits from Template:C sharp. Structs are more suitable for smaller constructs of data.

This is a short summary of the differences:

	Default constructor	Finalizer	Member initialization	Inheritance
Classes	not required (auto generated¹)	yes	not required	yes (if base class is not Template:C sharp)
Structs	required (auto generated²)	no	required	not supported
¹Generated only if no constructor was provided ²Always auto generated, and cannot be written by the programmer

Declaration

A class is declared like this:

class Foo
{
    // Member declarations
}

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 Template:C sharp.

// 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 Template:C sharp 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

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.

Person person = new Person {
    Name = "John Doe",
    Age = 39
};

// Equal to
Person 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 Template:C sharp.

List<int> list = new List<int> {2, 5, 6, 6};

// Equal to
List<int> list = new List<int>();
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 Template:C sharp 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 Template:C sharp 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

Template:C sharp - Specifies that a class only serves as a base class. It must be implemented in an inheriting class.
Template:C sharp - Specifies that a class cannot be inherited.

Class member modifiers

Template:C sharp - Specifies that a variable is a constant value that has to be initialized when it gets declared.
Template:C sharp - Declares an event.
Template:C sharp - Specifies that a method signature without a body uses a DLL-import.
Template:C sharp - Specifies that a method or property declaration is an override of a virtual member or an implementation of a member of an abstract class.
Template:C sharp - Declares a field that can only be assigned values as part of the declaration or in a constructor in the same class.
Template:C sharp - Specifies an unsafe context, which allows the use of pointers.
Template:C sharp - Specifies that a method or property declaration can be overridden by a derived class.
Template:C sharp - Specifies a field which may be modified by an external process and prevents an optimizing compiler from modifying the use of the field.

Template:C sharp modifier

The Template:C sharp 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
{
    public static void Something()
    {
        ...
    }
}
// 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 Template:C sharp can be reached from anywhere. Template:C sharp members can only be accessed from inside of the class they are declared in and will be hidden when inherited. Members with the Template:C sharp modifier will be Template:C sharp, but accessible when inherited. Template:C sharp classes and members will only be accessible from the inside of the declaring assembly.

Classes and structs are implicitly Template:C sharp and members are implicitly Template:C sharp if they do not have an access modifier.

public class Foo
{
    public int Do()
    {
        return 0;
    }

    public class Bar
    {

    }
}

This table defines where the access modifiers can be used.

	Unnested types	Members (incl. nested types)
Template:C sharp	yes	yes
Template:C sharp	no	yes
Template:C sharp	no	yes
Template:C sharp	yes (default)	yes
Template:C sharp	no	yes
Template:C sharp	no	yes (default)

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
{
    Foo()
    {
        ...
    }
}

Constructors can be Template:C sharp, Template:C sharp, Template:C sharp or Template:C sharp.

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 Template:C sharp 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
{
    ...

    ~Foo()
    {
        ...
    }
}

Finalizers are always Template:C sharp.

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
{
    int Bar(int a, int b)
    {
        return a%b;
    }
}

A method is called using Template:C sharp 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);

Template:C sharp and Template:C sharp parameters

One can explicitly make arguments be passed by reference when calling a method with parameters preceded by keywords Template:C sharp or Template:C sharp. 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 Template:C sharp parameter must have been assigned within the method by the time the method returns, while ref need not assign a value.

void PassRef(ref int x)
{
    if (x == 2)
        x = 10;
}
int Z;
PassRef(ref Z);

void PassOut(out int x)
{
    x = 2;
}
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)
{
  x += dx;
}

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);

Template:C sharp

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

[DllImport("win32.dll")]
static extern double Pow(double a, double b);

Fields

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

class Foo
{
    double foo;
}

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

class Foo
{
    double foo = 2.3;
}

Modifiers for fields:

Template:C sharp - Makes the field a constant.
Template:C sharp - Makes the field private (default).
Template:C sharp - Makes the field protected.
Template:C sharp - Makes the field public.
Template:C sharp - Allows the field to be initialized only once in a constructor.
Template:C sharp - Makes the field a static member.

Properties

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

class Person
{
    string name;

    string Name
    {
        get { return name; }
        set { name = value; }
    }
}

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

Modifiers for properties:

Template:C sharp - Makes the property private (default).
Template:C sharp - Makes the property protected.
Template:C sharp - Makes the property public.
Template:C sharp - Makes the property a static member.

Modifiers for property accessors:

Template:C sharp - Makes the accessor private.
Template:C sharp - Makes the accessor protected.
Template:C sharp - 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. You define accessors without bodies and the compiler will generate a backing field and the necessary code for the accessors.

public double Width
{
    get;
    private set;
}

Indexers

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

class IntList
{
   int[] items;

   int this[int index]
   {
        get { return this.items[index]; }
        set { this.items[index] = value; }
    }
}

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

Inheritance

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

class A
{

}

class B : A
{

}

Template:C sharp

Methods marked Template:C sharp provide an implementation, but they can be overridden by the inheritors by using the Template:C sharp keyword.

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

class Operation
{
    public virtual int Do()
    {
        return 0;
    }
}

class NewOperation : Operation
{
    public override int Do()
    {
        return 1;
    }
}

Template:C sharp

When overloading a non-virtual method with another signature, the keyword Template:C sharp 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
{
    public int Do()
    {
        return 0;
    }
}

class NewOperation : Operation
{
    public new double Do()
    {
        return 4.0;
    }
}

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();

Template:C sharp

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
{
    public abstract void Walk();
}

class Human : Mammal
{
    public override void Walk()
    {

    }

    ...
}

Template:C sharp

The Template:C sharp 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
{
    double A { get; set; }
    double B { get; set; }

    double GetResult();
}

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 Template:C sharp notation.

Implicit implementation

When implicitly implementing an interface the members of the interface have to be Template:C sharp.

public class Adder : IBinaryOperation
{
    public double A { get; set; }
    public double B { get; set; }

    public double GetResult()
    {
        return A + B;
    }
}

public class Multiplier : IBinaryOperation
{
    public double A { get; set; }
    public double B { get; set; }

    public double GetResult()
    {
        return A*B;
    }
}

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
{
    double IBinaryOperation.A { get; set; }
    double IBinaryOperation.B { get; set; }

    double IBinaryOperation.GetResult()
    {
        return ((IBinaryOperation)this).A + ((IBinaryOperation)this).B;
    }
}

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 Template:C sharp 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:^[6]

// Declare the generic class.

public class GenericList<T>
{
    void Add(T input) { }
}

class TestGenericList
{
    private class ExampleClass { }
    static void Main()
    {
        // Declare a list of type int.
        GenericList<int> list1 = new GenericList<int>();

        // Declare a list of type string.
        GenericList<string> list2 = new GenericList<string>();

        // Declare a list of type ExampleClass.
        GenericList<ExampleClass> list3 = new GenericList<ExampleClass>();
    }
}

When compared with C++ templates, C# generics can provide enhanced safety, but also have somewhat limited capabilities.^[7] For example, it is not possible to call arithmetic operators on a C# generic type.^[8] 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.^[9]

Using generics

Generic classes

Classes and structs can be generic.

public class List<T>
{
    ...
    public void Add(T item)
    {
         ...
    }
}

List<int> list = new List<int>();
list.Add(6);
list.Add(2);

Generic interfaces

interface IEnumerable<T>
{
    ...
}

Generic delegates

delegate R Func<T1, T2, R>(T1 a1, T2 a2);

Generic methods

public static T[] CombineArrays<T>(T[] a, T[] b)
{
    T[] newArray = new T[a.Length + b.Length];
    a.CopyTo(newArray, 0);
    b.CopyTo(newArray, a.Length);
    return newArray;
}

string[] a = new string[] { "a", "b", "c" };
string[] b = new string[] { "1", "2", "3" };
string[] c = CombineArrays(a, b);

double[] da = new double[] { 1.2, 2.17, 3.141592 };
double[] db = new double[] { 4.44, 5.6, 6.02 };
double[] dc = CombineArrays(da, db);

// c is a string array containing { "a", "b", "c", "1", "2", "3"}
// dc is a double array containing { 1.2, 2.17, 3.141592, 4.44, 5.6, 6.02}

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 Template:C sharp. When instantiating (or calling) 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 Template:C sharp keyword and a constraint specification, any of the six comma separated constraints may be used:^[10]

Constraint	Explanation
Template:C sharp	type parameter must be a value type
Template:C sharp	type parameter must be a reference type
Template:C sharp	type parameter must have a constructor with no parameters (must appear last)
Template:C sharp	type parameter must inherit from Template:C sharp
Template:C sharp	type parameter must be, or must implement this interface
Template:C sharp	naked type parameter constraint

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 Template:C sharp and Template:C sharp, 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 Template:C sharp has been redefined as follows:

interface IEnumerable<out T>
{
  IEnumerator<T> GetEnumerator();
}

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

void PrintAll(IEnumerable<object> objects)
{
  foreach (object o in objects)
  {
    System.Console.WriteLine(o);
  }
}

IEnumerable<string> strings = new List<string>();
PrintAll(strings); // IEnumerable<string> is implicitly converted to IEnumerable<object>

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

public interface IComparer<in T>
{
    int Compare(T x, T y);
}

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

IComparer<object> objectComparer = GetComparer();
IComparer<string> stringComparer = objectComparer;

Enumerators

An enumerator is an iterator. Enumerators are typically obtained by calling the Template:C sharp method of an object implementing the Template:C sharp 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 Template:C sharp. 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<MyType> iter = list.GetEnumerator();
while (iter.MoveNext())
    Console.WriteLine(iter.Current);

// implicit version
foreach (MyType value in list)
    Console.WriteLine(value);

Generator functionality

This is a feature of C# 2.0.

The .NET 2.0 Framework allowed C# to introduce an iterator that provides generator functionality, using a Template:C sharp construct similar to Template:C sharp in Python.^[11] With a Template:C sharp, the function automatically keeps its state during the iteration.

// Method that takes an iterable input (possibly an array)
// and returns all even numbers.
public static IEnumerable<int> GetEven(IEnumerable<int> numbers)
{
    foreach (int i in numbers)
    {
        if (i%2 == 0)
            yield return i;
    }
}

//using the method to output only even numbers from the array
static void Main()
{
      int[] numbers = { 1, 2, 3, 4, 5, 6};
      foreach (int i in GetEven(numbers))
        Console.WriteLine(i);  //outputs 2, 4 and 6
}

LINQ

This is a feature of C# 3.0 and .NET Framework 3.0.

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<int>{ 2, 7, 1, 3, 9 };

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# ^{[來源請求]} and supports "Intellisense" (Code completion in the editor).

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.

Anonymous delegates

This is a feature of C# 2.0.

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<int, int> f = delegate(int x) { return x*2; };

Lambda expressions

This is a feature of C# 3.0.

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.

// [arguments] => [method-body]

// With parameters
n => n == 2
(a, b) => a + b
(a, b) => { a++; return a + b; }

// With explicitly typed parameters
(int a, int b) => a + b

// No parameters
() => return 0

// Assigning lambda to delegate
Func<int, int, int> f = (a, b) => a + b;

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

(a, b) => { a++; return a + b; }

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(n => n.Length > 2);

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

This is a feature of C# 3.0.

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 Template:C sharp:

public static class StringExtensions
{
    public static string Left(this string s, int n)
    {
        return s.Substring(0, n);
    }
}

string s = "foo";
s.Left(3); // same as StringExtensions.Left(s, 3);

See also

Decorator pattern

Local functions

This is a feature of C# 7.0.

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 (public, private, protected) 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.^[12] After a compilation, a local function is transformed into a private static method, but when defined it cannot be marked static.^[13]

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(string[] args)
{
    int Sum(int x, int y)
    {
        return x + y;
    }

    Console.WriteLine(Sum(10, 20));
    Console.ReadKey();
}

Miscellaneous

Closure blocks

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

public void Foo()
{
    using (var bar = File.Open("Foo.txt"))
    {
        // do some work
        throw new Exception();
        // bar will still get properly disposed.
    }
}

Thread synchronization

C# provides the Template:C sharp statement, 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 Template:C sharp statement, it works by the compiler generating a Template:C sharp block in its place.

private static StreamWriter _writer;

public void ConcurrentMethod()
{
    lock (_writer)
    {
        _writer.WriteLine("Line 1.");
        _writer.WriteLine("Followed by line 2.");
    }
}

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 can be used for better maintenance of preprocessor directives.

[CompilerGenerated]
public class $AnonymousType$120
{
    [CompilerGenerated]
    public string Name { get; set; }
}

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 Template:C sharp. 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 Template:C sharp class. By convention, attribute classes end with "Attribute" in their name. This will not be required when using it.

public class EdibleAttribute : Attribute
{
    public EdibleAttribute() : base()
    {

    }

    public EdibleAttribute(bool isNotPoisonous)
    {
        this.IsPoisonous = !isNotPoisonous;
    }

    public bool IsPoisonous { get; set; }
}

Showing the attribute in use using the optional constructor parameters.

[Edible(true)]
public class Peach : Fruit
{
   // Members if any
}

Preprocessor

C# features "preprocessor directives"^[14] (though it does not have an actual preprocessor) based on the C preprocessor that allow programmers to define symbols, but not macros. Conditionals such as Template:C sharp, Template:C sharp, and Template:C sharp are also provided.

Directives such as Template:C sharp give hints to editors for code folding. The Template:C sharp block must be terminated with a Template:C sharp directive.

public class Foo
{
    #region Constructors
    public Foo() {}
    public Foo(int firstParam) {}
    #endregion

    #region Procedures
    public void IntBar(int firstParam) {}
    public void StrBar(string firstParam) {}
    public void BoolBar(bool firstParam) {}
    #endregion
}

Code comments

C# utilizes a double slash (Template:C sharp) to indicate the rest of the line is a comment.

public class Foo
{
    // a comment
    public static void Bar(int firstParam) {}  // Also a comment
}

Multi-line comments can be indicated by a starting slash/asterisk (Template:C sharp) and ending asterisk/forward slash (Template:C sharp).

public class Foo
{
    /* A Multi-Line
       comment  */
    public static void Bar(int firstParam) {}
}

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:

    /// <summary>
    /// This class is very classy.
    /// </summary>

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 Template:C sharp.

public class Foo
{
    // / <summary>A summary of the method.</summary>
    // / <param name="firstParam">A description of the parameter.</param>
    // / <remarks>Remarks about the method.</remarks>
    public static void Bar(int firstParam) {}
}

Multi-line documentation comments, while defined in the version 1.0 language specification, were not supported until the .NET 1.1 release.^[15] These comments are designated by a starting forward slash/asterisk/asterisk (Template:C sharp) and ending asterisk/forward slash (Template:C sharp).^[16]

public class Foo
{
    /** <summary>A summary of the method.</summary>
     *  <param name="firstParam">A description of the parameter.</param>
     *  <remarks>Remarks about the method.</remarks> */
    public static void Bar(int firstParam) {}
}

There are some stringent criteria regarding white space and XML documentation when using the forward slash/asterisk/asterisk (Template:C sharp) technique.

This code block:

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

produces a different XML comment than this code block:^[16]

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

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 (CLI) identifiers to their related documentation elements. This allows any C# integrated development environment (IDE) or other development tool to find documentation for any symbol in the code in a certain well-defined way.

Async-await syntax

This is a feature of C# 5.0 and .NET Framework 4.0.

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
{
    public static Task<XDocument> GetContentAsync()
    {
        HttpClient httpClient = new HttpClient();
        return httpClient.GetStringAsync("www.contoso.com").ContinueWith((task) => {
            string responseBodyAsText = task.Result;
            return XDocument.Parse(responseBodyAsText);
        });
    }
}

var t = SomeAsyncCode.GetContentAsync().ContinueWith((task) => {
    var xmlDocument = task.Result;
});

t.Start();

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

public static class SomeAsyncCode
{
    public static async Task<XDocument> GetContentAsync()
    {
        HttpClient httpClient = new HttpClient();
        string responseBodyAsText = await httpClient.GetStringAsync("www.contoso.com");
        return XDocument.Parse(responseBodyAsText);
    }
}

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(string![] args)
        requires args.Length > 0
    {
        foreach(string arg in args)
        {

        }
    }

Template:C sharp is used to make a reference type non-nullable, e.g. you cannot set the value to Template:C sharp. This in contrast of nullable types which allow value types to be set as Template:C sharp.
Template:C sharp 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 Template:C sharp. If is Template:C sharp then an exception will be thrown.

   string! input

In use:

   public Test(string! input)
   {
      ...
   }

Preconditions

Preconditions are checked before a method is executed.

   public Test(int i)
      requires i > 0;
   {
      this.i = i;
   }

Postconditions

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

   public void Increment()
      ensures i > 0;
   {
      i++;
   }

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.^[17]

参见

参考文献

^ Schildt, Herbert, C# 3.0: The Complete Reference
^ Deitel, Harvey M.; Deitel, Paul J., C# for programmers
^ Constraints on Type Parameters (C# Programming Guide)
^ Pointer types (C# Programming Guide)
^ ^5.0 ^5.1 Archer, Part 2, Chapter 4:The Type System
^ Generics (C# Programming Guide). Microsoft. [August 7, 2011].
^ An Introduction to C# Generics. Microsoft.
^ Differences Between C++ Templates and C# Generics. Microsoft.
^ An Introduction to C# Generics. Microsoft. January 2005 [June 18, 2009].
^ Constraints on Type Parameters (C# Programming Guide) in Microsoft MSDN
^ yield. C# Language Reference. Microsoft. [2009-04-26].
^ .NET Framework - What's New in C# 7.0. msdn.microsoft.com. [2017-04-08] （英语）.
^ Thoughts on C# 7 Local Functions. Anton Sizikov. 2016-04-15 [2017-04-08].
^ C# Preprocessor Directives. C# Language Reference. Microsoft. [June 18, 2009].
^ Horton, Anson. C# XML documentation comments FAQ. 2006-09-11 [2007-12-11].
^ ^16.0 ^16.1 Delimiters for Documentation Tags. C# Programmer's Reference. Microsoft. January 1, 1970 [June 18, 2009].
^ Martin, Robert C., 7 Error Handling, Use Unchecked Exceptions, Clean Code: A Handbook of Agile Software Craftsmanship, Prentice Hall International, 11 August 2008, ISBN 978-0132350884

Archer, Tom. Inside C#. Microsoft Press. 2001. ISBN 0-7356-1288-9.
Bart de Smet on Spec#

外部链接

[1] Schildt, Herbert, C# 3.0: The Complete Reference

[2] Deitel, Harvey M.; Deitel, Paul J., C# for programmers

[3] Constraints on Type Parameters (C# Programming Guide)

[4] Pointer types (C# Programming Guide)

[insidecsharpp2ch4-5] 5.0 ^5.1 Archer, Part 2, Chapter 4:The Type System

[6] Generics (C# Programming Guide). Microsoft. [August 7, 2011].

[7] An Introduction to C# Generics. Microsoft.

[8] Differences Between C++ Templates and C# Generics. Microsoft.

[9] An Introduction to C# Generics. Microsoft. January 2005 [June 18, 2009].

[10] Constraints on Type Parameters (C# Programming Guide) in Microsoft MSDN

[11] yield. C# Language Reference. Microsoft. [2009-04-26].

[12] .NET Framework - What's New in C# 7.0. msdn.microsoft.com. [2017-04-08] （英语）.

[13] Thoughts on C# 7 Local Functions. Anton Sizikov. 2016-04-15 [2017-04-08].

[14] C# Preprocessor Directives. C# Language Reference. Microsoft. [June 18, 2009].

[15] Horton, Anson. C# XML documentation comments FAQ. 2006-09-11 [2007-12-11].

[Delimiters_for_Documentation_Tags-16] 16.0 ^16.1 Delimiters for Documentation Tags. C# Programmer's Reference. Microsoft. January 1, 1970 [June 18, 2009].

[17] Martin, Robert C., 7 Error Handling, Use Unchecked Exceptions, Clean Code: A Handbook of Agile Software Craftsmanship, Prentice Hall International, 11 August 2008, ISBN 978-0132350884

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