C#的扩展方法

using System;
using System.Collections;
using System.Collections.Generic;
using System.IO;
using System.Reflection;
using System.Text.RegularExpressions;
using System.Linq;

namespace codeTest
{
    class Program
    {
        static void Main(string[] args)
        {
            string str = "12312312 123123 123123";
            Console.WriteLine(str.WordCount());
            MyClass mylClass = new MyClass() { str = "" };
            Console.WriteLine(mylClass.WordCount());
            Console.ReadLine();
        }

    }

    /// <summary>
    /// 扩展方法必须在静态类里面，扩展方法也必须是静态的，首参数为this
    /// </summary>
    public static class stringExtension
    {
        public static int WordCount(this string str)
        {
            return str.Count();
        }

        public static int WordCount(this MyClass mylClass)
        {
            return mylClass.str.Count();
        }
    }

    public class MyClass
    {
        public string str;
    }

}