结构
意图	允许一个对象在其内部状态改变时改变它的行为。对象看起来似乎修改了它的类。
适用性	一个对象的行为取决于它的状态, 并且它必须在运行时刻根据状态改变它的行为。 一个操作中含有庞大的多分支的条件语句，且这些分支依赖于该对象的状态。这个状态通常用一个或多个枚举常量表示。通常, 有多个操作包含这一相同的条件结构。S t a t e模式将每一个条件分支放入一个独立的类中。这使得你可以根据对象自身的情况将对象的状态作为一个对象，这一对象可以不依赖于其他对象而独立变化。

 

1 using System; 2  3     abstract class State  4     { 5         protected string strStatename;         6  7         abstract public void Pour(); 8         // do something state-specific here 9     }10 11     class OpenedState : State 12     {        13         public OpenedState ()14         {15             strStatename = "Opened";16         }17         override public void Pour()18         {19             Console.WriteLine("...pouring...");20             Console.WriteLine("...pouring...");21             Console.WriteLine("...pouring...");22         }23     }24     25     class ClosedState : State 26     {        27         public ClosedState()28         {29             strStatename = "Closed";30         }31         override public void Pour()32         {33             Console.WriteLine("ERROR - bottle is closed - cannot pour");34         }35     }36 37     class ContextColaBottle 38     {39         public enum BottleStateSetting {40             Closed,41             Opened42         };43 44         // If teh state classes had large amounts of instance data,45         // we could dynamically create them as needed - if this demo46         // they are tiny, so we just  create them as data members47         OpenedState openedState = new OpenedState();48         ClosedState closedState = new ClosedState();49 50         public ContextColaBottle ()51         {52             // Initialize to closed53             CurrentState = closedState;54         }55 56         private State CurrentState;57         58         public void SetState(BottleStateSetting newState)59         {60             if (newState == BottleStateSetting.Closed)61             {62                 CurrentState = closedState;63             }64             else 65             {66                 CurrentState = openedState;67             }68         }69 70         public void Pour()71         {72             CurrentState.Pour();73         }    74     }75 76       /// 
77     ///    Summary description for Client.78     /// 
79     public class Client80     {81         public static int Main(string[] args)82         {83             ContextColaBottle contextColaBottle = new ContextColaBottle();84 85             Console.WriteLine("initial state is closed");86 87             Console.WriteLine("Now trying to pour");88               contextColaBottle.Pour();89 90             Console.WriteLine("Open bottle");91             contextColaBottle.SetState(ContextColaBottle.BottleStateSetting.Opened);92 93             Console.WriteLine("Try to pour again");94             contextColaBottle.Pour();95 96             return 0;97         }98     }