/// <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();
        }
    }