天天向上

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace csrefKeywordsOperators
{
     class Base
     {
         public override string ToString()
         {
             return "Base";
         }
     }
     class Derived : Base { }
     //-------------------------------------------------------
     class classA { }
     class classB { }

    class Program
     {
         static void Main(string[] args)
         {
             Derived d = new Derived();
             Base b = d as Base;
             if (b != null)
             {
                 Console.WriteLine(b.ToString());
                 Console.ReadLine();
             }
             //--------------------------------------------------------
             object[] objArray = new object[6];
             objArray[0] = new classA();
             objArray[1] = new classB();
             objArray[2] = "hello";
             objArray[3] = 123;
             objArray[4] = 123.4;
             objArray[5] = null;
             /*The as operator is like a cast operation.
              * However, if the conversion isn't possible,
              * as returns null instead of raising an exception.
             */
             for (int i = 0; i < objArray.Length; i++)
             {
                 string s = objArray[i] as string;  // cast operation
                 Console.WriteLine("{0}:", i);
                 if (s != null)
                 {
                     Console.WriteLine("" + s + "");
                 }
                 else
                 {
                     Console.WriteLine("not a string");
                 }
             }
             Console.ReadLine();
         }
     }
}