C#高级程序设计（二）——代理

本质上讲，代理提供了方法的封装，它把方法以某种方式封装在一个代理对象里，通过代理对象来执行调用方法、移除或者添加代理对象等操作。从另一种意义上讲，你可以把代理类型看作单一方法的接口，代理对象看作实现了该接口的对象。
代理对象基础：代理类型
下面的代码定义了一个代理类型
[csharp]
internal delegate void Feedback(Int32 value);

C#编译器实际上将这个代理类型转化成为一个继承自MulticastDelegate的类。
[csharp]
internal class Feedback : System.MulticastDelegate
{
// Constructor
public Feedback(Object object, IntPtr method);
// Method with same prototype as specified by the source code
public virtual void Invoke(Int32 value);
// Methods allowing the callback to be called asynchronously
public virtual IAsyncResult BeginInvoke(Int32 value,
AsyncCallback callback, Object object);
public virtual void EndInvoke(IAsyncResult result);
}
MulticastDelegate继承自Delegate类，下面是代理类的签名：
[csharp]
public abstract class Delegate : ICloneable, ISerializable
    {
        protected Delegate(object target, string method);
        protected Delegate(Type target, string method);

        public static bool operator !=(Delegate d1, Delegate d2);
        public static bool operator ==(Delegate d1, Delegate d2);

        public MethodInfo Method { get; }
        public object Target { get; }

        public virtual object Clone();
        public static Delegate Combine(params Delegate[] delegates);
        public static Delegate Combine(Delegate a, Delegate b);
        protected virtual Delegate CombineImpl(Delegate d);
        public static Delegate CreateDelegate(Type type, MethodInfo method);
        public static Delegate CreateDelegate(Type type, MethodInfo method, bool throwOnBindFailure);
        public static Delegate CreateDelegate(Type type, object firstArgument, MethodInfo method);
        public static Delegate CreateDelegate(Type type, object target, string method);
        public static Delegate CreateDelegate(Type type, Type target, string method);
        public static Delegate CreateDelegate(Type type, object firstArgument, MethodInfo method, bool throwOnBindFailure);
        public static Delegate CreateDelegate(Type type, object target, string method, bool ignoreCase);
        public static Delegate CreateDelegate(Type type, Type target, string method, bool ignoreCase);
        public static Delegate CreateDelegate(Type type, object target, string method, bool ignoreCase, bool throwOnBindFailure);
        public static Delegate CreateDelegate(Type type, Type target, string method, bool ignoreCase, bool throwOnBindFailure);
        public object DynamicInvoke(params object[] args);
        protected virtual object DynamicInvokeImpl(object[] args);
        public override bool Equals(object obj);
        public override int GetHashCode();
        public virtual Delegate[] GetInvocationList();
        protected virtual MethodInfo GetMethodImpl();
        public virtual void GetObjectData(SerializationInfo info, StreamingContext context);
        public static Delegate Remove(Delegate source, Delegate value);
        public static Delegate RemoveAll(Delegate source, Delegate value);
        protected virtual Delegate RemoveImpl(Delegate d);
    }

这样的转化实际上定义了代理对象所有可能的操作，下面将一一列出
一、构造并初始化代理对象
最常见的方式是使用下面的方式来构造并初始化代理对象
[csharp]
Feedback fbInstance = new Feedback(new Program().FeedbackToFile);

然而，随着C#版本的升级，越来越多和代理有关的特性随之引入，同时也改变了代理对象的构造并初始化方式，例如在C#3.0中就可以用匿名方法或者lambda表达式来构造并初始化代理对象。
二、调用代理对象封装的方法
这实际上是调用代理定义时编译器所生成的Invoke或者BeginInvoke方法，一个用于同步调用，另一个用于异步调用。
[csharp]
fbInstance.Invoke(1);

但是，C#做了进一步优化，你可以直接通过代理对象来调用其所封装的方法，例如：
[csharp]
fbInstance(1);

三、连接和移除代理对象
前面看到的代理对象都是封装一个方法，实际上，代理对象可以将一系列方法封装成一个链表，这一特性取决于Delegate类的Combine和Remove方法。
[csharp]
fbChain = (Feedback) Delegate.Combine(fbChain, fb1);

实际上，C#对这一用法也提供了优化，可以使用+、+=、-、-=运算符来添加或移除代理对象：
[csharp]
fbChain += fb1;

代理的扩展：事件
C#事件的引入类似属性的引入，属性是对字段的封装，而事件对象是对代理对象的封装。
当定义如下事件时：
[csharp]
public event EventHandler<NewMailEventArgs> NewMail;

C#编译器实际上会将其转换成如下的代码：
[csharp]
// 1. A PRIVATE delegate field that is initialized to null
private EventHandler<NewMailEventArgs> NewMail = null;
// 2. A PUBLIC add_Xxx method (where Xxx is the Event name)
// Allows methods to register interest in the event.
public void add_NewMail(EventHandler<NewMailEventArgs> value) {
// The loop and the call to CompareExchange is all just a fancy way
// of adding a delegate to the event in a thread-safe way
EventHandler<NewMailEventArgs>prevHandler;
EventHandler<NewMailEventArgs> newMail = this.NewMail;
do {
prevHandler = newMail;
EventHandler<NewMailEventArgs>newHandler =
(EventHandler<NewMailEventArgs>) Delegate.Combine(prevHandler, value);
newMail = Interlocked.CompareExchange<EventHandler<NewMailEventArgs>>(
ref this.NewMail, newHandler, prevHandler);
} while (newMail != prevHandler);
}
// 3. A PUBLIC remove_Xxx method (where Xxx is the Event name)
// Allows methods to unregister interest in the event.
public void remove_NewMail(EventHandler<NewMailEventArgs> value) {
// The loop and the call to CompareExchange is all just a fancy way
// of removing a delegate from the event in a thread-safe way
EventHandler<NewMailEventArgs> prevHandler;
EventHandler<NewMailEventArgs> newMail = this.NewMail;
do {
prevHandler = newMail;
EventHandler<New

补充：软件开发 , C# ,