public class User
        {
//使用省缺参数，一般不需要再为多态做各种静态重载了
public User( string name = "anonym", string type = "user" )
                {
this.UserName = name;
this.UserType = type;
                 }
public UserName { private set; get; }
public UserType { private set; get; }
        }

User user = new User();  //不带任何参数实例化
Console.WriteLine( user.UserName );  //输出 anonym
Console.WriteLine( user.UserType );    //输出 user

// dynamic 关键字可以绕过静态语言的强类型检查
dynamic d = user;
Console.WriteLine( d.UserName );      //输出 anonym
Console.WriteLine( d.UserType );        //输出 user

// ExpandoObject 是一个特殊对象,包含运行时可动态添加或移除的成员
dynamic eo = new System.Dynamic.ExpandoObject();
        eo.Description = "this is a dynamic property";
Console.WriteLine( eo.Description );    // 输出 this is a dynamic property

//继承 DynamicObject 类，重写 TryInvokeMember 虚拟方法
public class Animal : DynamicObject
        {
//尝试执行成员，成功返回 true，并从第二个参数输出执行后的返回值
public override bool TryInvokeMember( InvokeMemberBinder binder, 
object[] args, out object result)
               {
bool success = base.TryInvokeMember( binder, args, out result);
//基类尝试执行方法，返回 false，表示方法不存在，out 输出 null 值
//if (! success)  result = null;
//若返回 false 将抛出异常
return true;
               }
        }

//派生动态类
public class Duck : Animal
        {
public string Quack()
              {
return "Quack!!";
              }
        }

//派生动态类
public class Human : Animal
        {
public string Talk()
              {
return "Talk!!";
              }
         }

//调用动态方法
public static string DoQuack( dynamic animal )
        {
string result = animal.Quack();
return result ?? "Null";  //(result == null)? "Human" : result;
         }

var duck = new Duck();
var human = new Human();
Console.WriteLine( DoQuack( duck ) );      //输出 Quack
Console.WriteLine( DoQuack( human ) );   //输出 Null , Human类不包含Quack方法，执行失败

C# 4 dynamic 动态对象 动态类型转换