策略模式定义了一系列的算法，并将每一个算法封装起来，使它们可以相互替换。策略模式让算法的变化独立于使用算法的客户。

// 策略接口  
public interface IStrategy  
{  void Execute();  
}  // 具体策略A  
public class ConcreteStrategyA : IStrategy  
{  public void Execute()  {  Console.WriteLine("Executing strategy A");  }  
}  // 具体策略B  
public class ConcreteStrategyB : IStrategy  
{  public void Execute()  {  Console.WriteLine("Executing strategy B");  }  
}  // 上下文类，接收客户的请求并使用某种策略  
public class Context  
{  private IStrategy _strategy;  public Context(IStrategy strategy)  {  _strategy = strategy;  }  public void SetStrategy(IStrategy strategy)  {  _strategy = strategy;  }  public void ExecuteStrategy()  {  _strategy.Execute();  }  
}  // 客户端代码  
class Program  
{  static void Main(string[] args)  {  Context context = new Context(new ConcreteStrategyA());  context.ExecuteStrategy();  context.SetStrategy(new ConcreteStrategyB());  context.ExecuteStrategy();  }  
}