C# Extension Method

C#可以为类创建扩展方法，不过必须是Static类和Static方法，用this关键字加上Class来确定为哪个Class扩展方法。

// Define an interface named IMyInterface. namespace DefineIMyInterface{    using System;    public interface IMyInterface    {        // Any class that implements IMyInterface must define a method         // that matches the following signature.         void MethodB();    }}// Define extension methods for IMyInterface. namespace Extensions{    using System;    using DefineIMyInterface;    // The following extension methods can be accessed by instances of any      // class that implements IMyInterface.     public static class Extension    {        public static void MethodA(this IMyInterface myInterface, int i)        {            Console.WriteLine                ("Extension.MethodA(this IMyInterface myInterface, int i)");        }        public static void MethodA(this IMyInterface myInterface, string s)        {            Console.WriteLine                ("Extension.MethodA(this IMyInterface myInterface, string s)");        }        // This method is never called in ExtensionMethodsDemo1, because each          // of the three classes A, B, and C implements a method named MethodB         // that has a matching signature.         public static void MethodB(this IMyInterface myInterface)        {            Console.WriteLine                ("Extension.MethodB(this IMyInterface myInterface)");        }    }}// Define three classes that implement IMyInterface, and then use them to test // the extension methods. namespace ExtensionMethodsDemo1{    using System;    using Extensions;    using DefineIMyInterface;    class A : IMyInterface    {        public void MethodB() { Console.WriteLine("A.MethodB()"); }    }    class B : IMyInterface    {        public void MethodB() { Console.WriteLine("B.MethodB()"); }        public void MethodA(int i) { Console.WriteLine("B.MethodA(int i)"); }    }    class C : IMyInterface    {        public void MethodB() { Console.WriteLine("C.MethodB()"); }        public void MethodA(object obj)        {            Console.WriteLine("C.MethodA(object obj)");        }    }    class ExtMethodDemo    {        static void Main(string[] args)        {            // Declare an instance of class A, class B, and class C.            A a = new A();            B b = new B();            C c = new C();            // For a, b, and c, call the following methods:             //      -- MethodA with an int argument             //      -- MethodA with a string argument             //      -- MethodB with no argument.             // A contains no MethodA, so each call to MethodA resolves to              // the extension method that has a matching signature.            a.MethodA(1);           // Extension.MethodA(object, int)            a.MethodA("hello");     // Extension.MethodA(object, string)             // A has a method that matches the signature of the following call             // to MethodB.            a.MethodB();            // A.MethodB()             // B has methods that match the signatures of the following             // method calls.            b.MethodA(1);           // B.MethodA(int)            b.MethodB();            // B.MethodB()             // B has no matching method for the following call, but              // class Extension does.            b.MethodA("hello");     // Extension.MethodA(object, string)             // C contains an instance method that matches each of the following             // method calls.            c.MethodA(1);           // C.MethodA(object)            c.MethodA("hello");     // C.MethodA(object)            c.MethodB();            // C.MethodB()        }    }}/* Output:    Extension.MethodA(this IMyInterface myInterface, int i)    Extension.MethodA(this IMyInterface myInterface, string s)    A.MethodB()    B.MethodA(int i)    B.MethodB()    Extension.MethodA(this IMyInterface myInterface, string s)    C.MethodA(object obj)    C.MethodA(object obj)    C.MethodB() */

参考Link

http://msdn.microsoft.com/library/bb383977.aspx