C#扩展一个现有的类

做个记录，写个示例

using System;

class Rubbish
{
    public void Say()
    {
        Console.Write("Hello");
    }
}
static class RubbishExtensions
{
    public static void Say(this Rubbish argRubbish, string argString)
    {
        Console.Write(argString);
    }
}
class Program
{
    static void Main(string[] args)
    {
        Rubbish theRubbish = new Rubbish();
        theRubbish.Say("Hey");
        Console.ReadKey(false);
    }
}