State -- Design Patterns

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

Participants

    The classes and/or objects participating in this pattern are:

• Context  (Account)

     Defines the interface of interest to clients

     Maintains an instance of a Concrete State subclass that defines the current state.

• State  (State)

     Defines an interface for encapsulating the behavior associated with a particular state of the Context.

• Concrete State  (Red State, Silver State, Gold State)

 Each subclass implements a behavior associated with a state of Context

Example for State Design Pattern:

using System;  namespace State.Structural {   /// <summary>   /// MainApp startup class for Structural   /// State Design Pattern.   /// </summary>   class MainApp   {     /// <summary>     /// Entry point into console application.     /// </summary>     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.ReadKey();     }   }    /// <summary>   /// The 'State' abstract class   /// </summary>   abstract class State   {     public abstract void Handle(Context context);   }    /// <summary>   /// A 'ConcreteState' class   /// </summary>   class ConcreteStateA : State   {     public override void Handle(Context context)     {       context.State = new ConcreteStateB();     }   }    /// <summary>   /// A 'ConcreteState' class   /// </summary>   class ConcreteStateB : State   {     public override void Handle(Context context)     {       context.State = new ConcreteStateA();     }   }    /// <summary>   /// The 'Context' class   /// </summary>   class Context   {     private State _state;     // Constructor     public Context(State state)     {       this.State = state;     }      // Gets or sets the state     public State State     {       get { return _state; }       set       {         _state = value;         Console.WriteLine("State: " +           _state.GetType().Name);       }     }      public void Request()     {       _state.Handle(this);     }   } }

Output: State: ConcreteStateA State: ConcreteStateB State: ConcreteStateA State: ConcreteStateB State: ConcreteStateA