State Design Pattern in C#

Allows an object to alter its behavior when its internal state changes. The object will appear to change its class.

This structural code demonstrates the State pattern which allows an object to behave differently depending on its internal state. The difference in behavior is delegated to objects that represent this state.

using System;

  class MainApp
  {
    static void Main()
    {
      // Setup context in a state 
      Context c = new Context(new ConcreteStateA());

      // Issue requests, which toggles state 
      c.Request();
      c.Request();
      c.Request();
      c.Request();

      // Wait for user 
      Console.Read();
    }
  }

  // "State" 
  abstract class State
  {
    public abstract void Handle(Context context);
  }

  // "ConcreteStateA" 
  class ConcreteStateA : State
  {
    public override void Handle(Context context)
    {
      context.State = new ConcreteStateB();
    }
  }

  // "ConcreteStateB" 
  class ConcreteStateB : State
  {
    public override void Handle(Context context)
    {
      context.State = new ConcreteStateA();
    }
  }

  // "Context" 
  class Context
  {
    private State state;

    // Constructor 
    public Context(State state)
    {
      this.State = state;
    }

    // Property 
    public State State
    {
      get{ return state; }
      set
      { 
        state = value; 
        Console.WriteLine("State: " + 
          state.GetType().Name);
      }
    }

    public void Request()
    {
      state.Handle(this);
    }
  }