using System;namespace 扩展方法
{
    class Program
    {
        static void Main(string[] args)
        {
            string str = null;
            str.SayHello();
            string str1 = "not hello";
            str1.SayHello();
            Console.ReadKey();
        }
    }

    public static class Extension   //  必须是一个静态类
    {
        public static void SayHello(this string str)    //必须为public static 类型，且参数使用this关键字
        {
            if (str == null)
            {
                Console.WriteLine("Hello");
            }
            else
                Console.WriteLine("{0}", str);
        }
    }
}

using System;

namespace 扩展方法
{
    class Program
    {
        static void Main(string[] args)
        {
            MyClass mc = new MyClass(3,4);
            Console.WriteLine("avg is {0}", mc.getAvg());
            Console.ReadKey();
        }
    }

    class MyClass
    {
        private double width, height;

        public MyClass(double val1, double val2)
        {
            width = val1;
            height = val2;
        }

        public double getSum()
        {
            return width + height;
        }
    }

    static class ExtendMyClass
    {
        public static double getAvg(this MyClass mc)
        {
            return mc.getSum() / 2;
        }
    }
}

using System;
using System.Collections.Generic;

namespace 扩展方法
{
    class Program
    {
        static void Main(string[] args)
        {
            MyClass<int> mc = new MyClass<int>(6);
            mc.Print();
            Console.ReadKey();
        }
    }

    class MyClass<T>
    {
        private T data;

        public MyClass(T val)
        {
            data = val;
        }

        public T getData()
        {
            return data;
        }
    }

    static class ExtendMyClass
    {
        public static void Print(this MyClass<int> mc)
        {
            Console.WriteLine("Data is {0}",mc.getData());
        }
    }
}