c# 关键字delegate、event（委托与事件）[MSDN原文摘录][1]

A delegate is a type that safely encapsulates a method, similar to a function pointer in C and C++. Unlike C function pointers, delegates are object-oriented, type safe, and secure. The type of a delegate is defined by the name of the delegate. The following example declares a delegate named Del that can encapsulate a method that takes a string as an argument and returns void:
（委托是一种安全地封装方法的类型，它与 C 和 C++ 中的函数指针类似。 与 C 中的函数指针不同，委托是面向对象的、类型安全的和保险的。 委托的类型由委托的名称定义。 下面的示例声明了一个名为 Del 的委托，该委托可以封装一个采用字符串作为参数并返回 void 的方法。）

public delegate void Del(string message);

A delegate object is normally constructed by providing the name of the method the delegate will wrap, or with an anonymous Method. Once a delegate is 
instantiated, a method call made to the delegate will be passed by the delegate to that method. The parameters passed to the delegate by the caller are passed to the method, and the return value, if any, from the method is returned to the caller by the delegate. This is known as invoking the delegate. An instantiated delegate can be invoked as if it were the wrapped method itself. For example:
（构造委托对象时，通常提供委托将包装的方法的名称或使用匿名方法。 实例化委托后，委托将把对它进行的方法调用传递给方法。 调用方传递给委托的参数被传递给方法，来自方法的返回值（如果有）由委托返回给调用方。 这被称为调用委托。 可以将一个实例化的委托视为被包装的方法本身来调用该委托。 例如：）

// Create a method for a delegate.
public static void DelegateMethod(string message)
{
    System.Console.WriteLine(message);
}

// Instantiate the delegate.
Del handler = DelegateMethod;

// Call the delegate.
handler("Hello World");

Delegate types are derived from the Delegate class in the .NET Framework. Delegate types are sealed—they cannot be derived from— and it is not possible to derive custom classes from Delegate. Because the instantiated delegate is an object, it can be passed as a parameter, or assigned to a property. This allows a method to accept a delegate as a parameter, and call the delegate at some later time. This is known as an asynchronous callback, and is a common method of notifying a caller when a long process has completed. When a delegate is used in this fashion, the code using the delegate does not need any knowledge of the implementation of the method being used. The functionality is similar to the encapsulation interfaces provide. For more information, see When to Use Delegates Instead of Interfaces.
Another common use of callbacks is defining a custom comparison method and passing that delegate to a sort method. It allows the caller‘s code to become part of the sort algorithm. The following example method uses the Del type as a parameter:
（委托类型派生自 .NET Framework 中的 Delegate 类。 委托类型是密封的，不能从 Delegate 中派生委托类型，也不可能从中派生自定义类。 由于实例化委托是一个对象，所以可以将其作为参数进行传递，也可以将其赋值给属性。 这样，方法便可以将一个委托作为参数来接受，并且以后可以调用该委托。 这称为异步回调，是在较长的进程完成后用来通知调用方的常用方法。 以这种方式使用委托时，使用委托的代码无需了解有关所用方法的实现方面的任何信息。 此功能类似于接口所提供的封装。 有关更多信息，请参见何时使用委托而不使用接口。
回调的另一个常见用法是定义自定义的比较方法并将该委托传递给排序方法。 它允许调用方的代码成为排序算法的一部分。 下面的示例方法使用 Del 类型作为参数：）

public void MethodWithCallback(int param1, int param2, Del callback)
{
    callback("The number is: " + (param1 + param2).ToString());
}

MethodWithCallback(1, 2, handler);//（PS:注意回调这种用法）

When a delegate is constructed to wrap an instance method, the delegate references both the instance and the method. A delegate has no knowledge of the instance type aside from the method it wraps, so a delegate can refer to any type of object as long as there is a method on that object that matches the delegate signature. When a delegate is constructed to wrap a static method, it only references the method. Consider the following declarations:
（将委托构造为包装实例方法时，该委托将同时引用实例和方法。 除了它所包装的方法外，委托不了解实例类型，所以只要任意类型的对象中具有与委托签名相匹配的方法，委托就可以引用该对象。 将委托构造为包装静态方法时，它只引用方法。 考虑下列声明：）

public class MethodClass
{
    public void Method1(string message) { }
    public void Method2(string message) { }
}

Along with the static DelegateMethod shown previously, we now have three methods that can be wrapped by a Del instance.
A delegate can call more than one method when invoked. This is referred to as multicasting. To add an extra method to the delegate‘s list of methods—the invocation list—simply requires adding two delegates using the addition or addition assignment operators (‘+‘ or ‘+=‘). For example:
（加上前面显示的静态 DelegateMethod，现在我们有三个方法可由 Del 实例进行包装。
调用委托时，它可以调用多个方法。 这称为多路广播。 若要向委托的方法列表（调用列表）中添加额外的方法，只需使用加法运算符或加法赋值运算符（“+”或“+=”）添加两个委托。 例如：）

MethodClass obj = new MethodClass();
Del d1 = obj.Method1;
Del d2 = obj.Method2;
Del d3 = DelegateMethod;

//Both types of assignment are valid.
Del allMethodsDelegate = d1 + d2;
allMethodsDelegate += d3;

At this point allMethodsDelegate contains three methods in its invocation list—Method1, Method2, and DelegateMethod. The original three delegates, d1, d2, 
and d3, remain unchanged. When allMethodsDelegate is invoked, all three methods are called in order. If the delegate uses reference parameters, the reference is passed sequentially to each of the three methods in turn, and any changes by one method are visible to the next method. When any of the methods throws an exception that is not caught within the method, that exception is passed to the caller of the delegate and no subsequent methods in the invocation list are called. If the delegate has a return value and/or out parameters, it returns the return value and parameters of the last method invoked. To remove a method from the invocation list, use the decrement or decrement assignment operator (‘-‘ or ‘-=‘). For example:
（此时，allMethodsDelegate 在其调用列表中包含三个方法 -- Method1、Method2 和 DelegateMethod。 原来的三个委托 d1、d2 和 d3 保持不变。 调用 allMethodsDelegate 时，将按顺序调用所有这三个方法。 如果委托使用引用参数，则引用将依次传递给三个方法中的每个方法，由一个方法引起的更改对下一个方法是可见的。 如果任一方法引发了异常，而在该方法内未捕获该异常，则该异常将传递给委托的调用方，并且不再对调用列表中后面的方法进行调用。 如果委托具有返回值和/或输出参数，它将返回最后调用的方法的返回值和参数。 若要从调用列表中移除方法，请使用减法运算符或减法赋值运算符（“-”或“-=”）。 例如：

//remove Method1
allMethodsDelegate -= d1;

// copy AllMethodsDelegate while removing d2
Del oneMethodDelegate = allMethodsDelegate - d2;

Because delegate types are derived from System.Delegate, the methods and properties defined by that class can be called on the delegate. For example, to find the number of methods in a delegate‘s invocation list, you may write:
（由于委托类型派生自 System.Delegate，所以可在委托上调用该类定义的方法和属性。 例如，为了找出委托的调用列表中的方法数，您可以编写下面的代码：）

int invocationCount = d1.GetInvocationList().GetLength(0);

delegate void Delegate1();
delegate void Delegate2();

static void method(Delegate1 d, Delegate2 e, System.Delegate f)
{
    // Compile-time error.
    //Console.WriteLine(d == e);

    // OK at compile-time. False if the run-time type of f 
    // is not the same as that of d.
    System.Console.WriteLine(d == f);
}