/// <summary>
 /// Context类，维护一个ConcreteState子类的实例，这个实例定义当前的状态。
 /// </summary>
 public class Context
 {
  private State state;
  /// <summary>
  /// 定义Context的初始状态
  /// </summary>
  /// <param name="state"></param>
  public Context(State state)
  {
   this.state = state;
  }

  /// <summary>
  /// 可读写的状态属性，用于读取和设置新状态
  /// </summary>
  public State State
  {
   get { return state; }
   set { state = value; }
  }

  /// <summary>
  /// 对请求做处理，并设置下一个状态
  /// </summary>
  public void Request()
  {
   state.Handle(this);
  }
 }

 /// <summary>
 /// 抽象状态类，定义一个接口以封装与Context的一个特定状态相关的行为
 /// </summary>
 public abstract class State
 {
  public abstract void Handle(Context context);
 }

 /// <summary>
 /// 具体状态类，每一个子类实现一个与Context的一个状态相关的行为
 /// </summary>
 public class ConcreteStateA : State
 {
  /// <summary>
  /// 设置ConcreteStateA的下一个状态是ConcreteStateB
  /// </summary>
  /// <param name="context"></param>
  public override void Handle(Context context)
  {
   Console.WriteLine("当前状态是 A.");
   context.State = new ConcreteStateB();
  }
 }

 public class ConcreteStateB : State
 {
  /// <summary>
  /// 设置ConcreteStateB的下一个状态是ConcreteSateA
  /// </summary>
  /// <param name="context"></param>
  public override void Handle(Context context)
  {
   Console.WriteLine("当前状态是 B.");
   context.State = new ConcreteStateA();
  }
 }

 class Program
 {
  static void Main(string[] args)
  {
   // 设置Context的初始状态为ConcreteStateA
   Context context = new Context(new ConcreteStateA());

   // 不断地进行请求，同时更改状态
   context.Request();
   context.Request();
   context.Request();
   context.Request();

   Console.Read();
  }
 }

 /// <summary>
 /// 电灯类，对应模式中的Context类
 /// </summary>
 public class Light
 {
  private LightState state;

  public Light(LightState state)
  {
   this.state = state;
  }

  /// <summary>
  /// 按下电灯开关
  /// </summary>
  public void PressSwich()
  {
   state.PressSwich(this);
  }

  public LightState State
  {
   get { return state; }
   set { state = value; }
  } 
 }

 /// <summary>
 /// 抽象的电灯状态类，相当于State类
 /// </summary>
 public abstract class LightState
 {
  public abstract void PressSwich(Light light);
 }

 /// <summary>
 /// 具体状态类, 开
 /// </summary>
 public class On : LightState
 {
  /// <summary>
  /// 在开状态下，按下开关则切换到关的状态。
  /// </summary>
  /// <param name="light"></param>
  public override void PressSwich(Light light)
  {
   Console.WriteLine("Turn off the light.");

   light.State = new Off();
  }
 }

 /// <summary>
 /// 具体状态类，关
 /// </summary>
 public class Off: LightState
 {
  /// <summary>
  /// 在关状态下，按下开关则打开电灯。
  /// </summary>
  /// <param name="light"></param>
  public override void PressSwich(Light light)
  {
   Console.WriteLine("Turn on the light.");

   light.State = new On();
  }
 }

 class Program
 {
  static void Main(string[] args)
  {
   // 初始化电灯，原始状态为关
   Light light = new Light(new Off());

   // 第一次按下开关，打开电灯
   light.PressSwich();
   // 第二次按下开关，关闭电灯
   light.PressSwich();

   Console.Read();
  }
 }