C# 4.0 语言规范详解

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"CSharp 4.0 语言规范" C# 4.0 是一种面向对象的编程语言，由微软公司开发，用于构建基于.NET Framework的应用程序。C# 4.0 在其早期版本的基础上引入了多项新特性，旨在提高开发效率、增强代码的灵活性和互操作性。以下是对C# 4.0 语言规范的关键知识点的详细说明： 1. **多态方法调用（Dynamic Typing）**：C# 4.0 引入了`dynamic`关键字，允许在运行时进行类型检查和方法调用。这使得与动态语言如Python或JavaScript的互操作变得更加简单，不再需要繁琐的装箱和拆箱操作。 2. **命名参数和可选参数**：C# 4.0 允许在方法调用时指定参数名称，提高了代码可读性。同时，引入了可选参数，允许方法定义时提供默认值，调用者可以不提供这些参数的值。 3. **协变和逆变（Covariance and Contravariance）**：在C# 4.0中，接口和委托支持协变和逆变，这意味着可以更灵活地使用泛型类型参数。协变允许将更具体的类型赋值给更通用的类型，而逆变则允许使用更通用的类型作为参数。 4. **匿名类型**：C# 4.0 继续支持匿名类型，这在处理临时数据集合时非常有用。匿名类型允许开发者声明没有明确类型的变量，系统会自动生成类型名称。 5. **动态对象（Dynamic Objects）**：通过`System.Dynamic`命名空间，C# 4.0 支持动态对象，这些对象的成员可以在运行时动态解析。这对于与动态语言如IronPython或IronRuby交互非常方便。 6. **改进的委托和事件**：C# 4.0 对委托和事件进行了优化，可以更轻松地合并和拆分委托实例，以及使用Lambda表达式来简化事件处理程序的声明。 7. **扩展方法（Extension Methods）**：C# 4.0 继续支持扩展方法，这是一种可以在现有类上添加新方法而无需继承或使用装饰器的设计模式。这种方法有助于保持类的封闭性，同时增加可重用性。 8. **泛型接口和类的默认实现**：C# 4.0 允许在泛型接口中提供默认实现，使得实现该接口的类可以选择使用默认实现或覆盖它。 9. **XML 文档注释（XML Documentation Comments）**：C# 4.0 提供了一种机制，可以通过在源代码中添加特殊的注释来生成文档XML，这有助于生成API文档。 10. **并行编程支持**：虽然不是C# 4.0独有的特性，但.NET Framework 4.0包含的Task Parallel Library (TPL)提供了对并行编程的强大支持，C# 4.0可以充分利用这些库。以上是C# 4.0 语言规范中的主要知识点，它们极大地丰富了C#的功能，使其成为现代软件开发的有力工具。通过理解并应用这些特性，开发者可以编写出更加高效、可维护且易于协作的代码。

C# LANGUAGE

LANGUAGE

LANGUAGE SPECIFICATION

SPECIFICATION

xvi

Microsoft

Microsoft Corporation

Corporation

Corporation 1999-2010

1999-2010

1999-2010 .

. All

All

All Rights

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Rights Reserved.

Reserved.

B.2.4 Expressions ......................................................................................................................................... 479

B.2.5 Statements ........................................................................................................................................... 486

B.2.6 Namespaces ........................................................................................................................................ 489

B.2.7 Classes ................................................................................................................................................ 490

B.2.8 Structs ................................................................................................................................................. 497

B.2.9 Arrays .................................................................................................................................................. 498

B.2.10 Interfaces ........................................................................................................................................... 498

B.2.11 Enums ............................................................................................................................................... 499

B.2.12 Delegates ........................................................................................................................................... 500

B.2.13 Attributes .......................................................................................................................................... 500

B.3 Grammar extensions for unsafe code ......................................................................................................... 502

C. References

References

References ......................................................................................................................................................

......................................................................................................................................................

...................................................................................................................................................... 505

505

Chapter

Chapter 18

18 Unsafe

Unsafe

Unsafe code

code

Microsoft

Microsoft Corporation

Corporation

Corporation 1999-2010.

1999-2010.

1999-2010. All

All

All Rights

Rights

Rights Reserved.

Reserved.

1. Introduction

Introduction

C# (pronounced “ See Sharp ” ) is a simple, modern, object-oriented, and type-safe programming language. C#

has its roots in the C family of languages and will be immediately familiar to C, C++, and Java programmers.

C# is standardized by ECMA International as the

ECMA-334

standard and by ISO/IEC as the

ISO/IEC

ISO/IEC 23270

23270

standard. Microsoft ’ s C# compiler for the .NET Framework is a conforming implementation of both of these

standards.

C# is an object-oriented language, but C# further includes support for

component-oriented

programming.

Contemporary software design increasingly relies on software components in the form of self-contained and

self-describing packages of functionality. Key to such components is that they present a programming model

with properties, methods, and events; they have attributes that provide declarative information about the

component; and they incorporate their own documentation. C# provides language constructs to directly support

these concepts, making C# a very natural language in which to create and use software components.

Several C# features aid in the construction of robust and durable applications:

Garbage

Garbage collection

collection

automatically

reclaims memory occupied by unused objects;

exception

exception handling

handling

provides a structured and extensible

approach to error detection and recovery; and the

type-safe

design of the language makes it impossible to read

from uninitialized variables, to index arrays beyond their bounds, or to perform unchecked type casts.

C# has a

unified

unified type

type

type system

system

. All C# types, including primitive types such as int and double , inherit from a

single root object type. Thus, all types share a set of common operations, and values of any type can be stored,

transported, and operated upon in a consistent manner. Furthermore, C# supports both user-defined reference

types and value types, allowing dynamic allocation of objects as well as in-line storage of lightweight structures.

To ensure that C# programs and libraries can evolve over time in a compatible manner, much emphasis has been

placed on

versioning

in C# ’ s design. Many programming languages pay little attention to this issue, and, as a

result, programs written in those languages break more often than necessary when newer versions of dependent

libraries are introduced. Aspects of C# ’ s design that were directly influenced by versioning considerations

include the separate virtual and override modifiers, the rules for method overload resolution, and support

for explicit interface member declarations.

The rest of this chapter describes the essential features of the C# language. Although later chapters describe

rules and exceptions in a detail-oriented and sometimes mathematical manner, this chapter strives for clarity and

brevity at the expense of completeness. The intent is to provide the reader with an introduction to the language

that will facilitate the writing of early programs and the reading of later chapters.

1.1

1.1 Hello

Hello

Hello world

world

The “ Hello, World ” program is traditionally used to introduce a programming language. Here it is in C#:

using System;

class Hello

{

static void Main() {

Console.WriteLine("Hello, World");

}

C# source files typically have the file extension

.cs

. Assuming that the “ Hello, World ” program is stored in the

file hello.cs , the program can be compiled with the Microsoft C# compiler using the command line

C# LANGUAGE

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Reserved.

csc hello.cs

which produces an executable assembly named hello.exe . The output produced by this application when it is

run is

Hello, World

The “ Hello, World ” program starts with a using directive that references the System namespace. Namespaces

provide a hierarchical means of organizing C# programs and libraries. Namespaces contain types and other

namespaces — for example, the System namespace contains a number of types, such as the Console class

referenced in the program, and a number of other namespaces, such as IO and Collections . A using

directive that references a given namespace enables unqualified use of the types that are members of that

namespace. Because of the using directive, the program can use Console.WriteLine as shorthand for

System.Console.WriteLine

The Hello class declared by the “ Hello, World ” program has a single member, the method named Main . The

Main

method is declared with the

static

modifier. While instance methods can reference a particular

enclosing object instance using the keyword this , static methods operate without reference to a particular

object. By convention, a static method named Main serves as the entry point of a program.

The output of the program is produced by the

WriteLine

method of the

Console

class in the

System

namespace. This class is provided by the .NET Framework class libraries, which, by default, are automatically

referenced by the Microsoft C# compiler. Note that C# itself does not have a separate runtime library. Instead,

the .NET Framework

the runtime library of C#.

1.2

1.2 Program

Program

Program structure

structure

The key organizational concepts in C# are

programs

namespaces

types

members

, and

assemblies

. C#

programs consist of one or more source files. Programs declare types, which contain members and can be

organized into namespaces. Classes and interfaces are examples of types. Fields, methods, properties, and events

are examples of members. When C# programs are compiled, they are physically packaged into assemblies.

Assemblies typically have the file extension .exe or .dll , depending on whether they implement

applications

libraries

The example

using System;

namespace Acme.Collections

{

public class Stack

{

Entry top;

public void Push(object data) {

top = new Entry(top, data);

}

public object Pop() {

if (top == null) throw new InvalidOperationException();

object result = top.data;

top = top.next;

return result;

}

class Entry

{

public Entry next;

public object data;

Chapter

Chapter 18

18 Unsafe

Unsafe

Unsafe code

code

Microsoft

Microsoft Corporation

Corporation

Corporation 1999-2010.

1999-2010.

1999-2010. All

All

All Rights

Rights

Rights Reserved.

Reserved.

public Entry(Entry next, object data) {

this.next = next;

this.data = data;

}

declares a class named Stack in a namespace called Acme.Collections . The fully qualified name of this

class is Acme.Collections.Stack . The class contains several members: a field named top , two methods

named Push and Pop , and a nested class named Entry . The Entry class further contains three members: a

field named

, a field named

data

, and a constructor. Assuming that the source code of the example is

stored in the file acme.cs , the command line

csc /t:library acme.cs

compiles the example as a library (code without a Main entry point) and produces an assembly named

acme.dll

Assemblies contain executable code in the form of

Intermediate

Intermediate Language

Language

(IL) instructions, and symbolic

information in the form of

metadata

. Before it is executed, the IL code in an assembly is automatically

converted to processor-specific code by the Just-In-Time (JIT) compiler of .NET Common Language Runtime.

Because an assembly is a self-describing unit of functionality containing both code and metadata, there is no

need for #include directives and header files in C#. The public types and members contained in a particular

assembly are made available in a C# program simply by referencing that assembly when compiling the program.

For example, this program uses the

Acme.Collections.Stack

class from the

acme.dll

assembly:

using System;

using Acme.Collections;

class Test

{

static void Main() {

Stack s = new Stack();

s.Push(1);

s.Push(10);

s.Push(100);

Console.WriteLine(s.Pop());

}

If the program is stored in the file

test.cs

, when

test.cs

is compiled, the

acme.dll

assembly can be

referenced using the compiler ’ s /r option:

csc /r:acme.dll test.cs

This creates an executable assembly named test.exe , which, when run, produces the output:

100

C# permits the source text of a program to be stored in several source files. When a multi-file C# program is

compiled, all of the source files are processed together, and the source files can freely reference each other —

conceptually, it is as if all the source files were concatenated into one large file before being processed. Forward

declarations are never needed in C# because, with very few exceptions, declaration order is insignificant. C#

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LANGUAGE

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1999-2010 .

. All

All

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Rights Reserved.

Reserved.

does not limit a source file to declaring only one public type nor does it require the name of the source file to

match a type declared in the source file.

1.3

1.3 Types

Types

Types and

and

and variables

variables

There are two kinds of types in C#:

value

value types

types

and

reference

reference types

types

. Variables of value types directly contain

their data whereas variables of reference types store references to their data, the latter being known as objects.

With reference types, it is possible for two variables to reference the same object and thus possible for

operations on one variable to affect the object referenced by the other variable. With value types, the variables

each have their own copy of the data, and it is not possible for operations on one to affect the other (except in

the case of

ref

and

out

parameter variables).

C# ’ s value types are further divided into

simple

simple types

types

enum

enum types

types

struct

struct types

types

, and

nullable

nullable types

types

, and C# ’ s

reference types are further divided into

class

class types

types

interface

interface types

types

array

array types

types

, and

delegate

delegate types

types

The following table provides an overview of C# ’ s type system.

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