Definition

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Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it.

UML class diagram

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Participants

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

• Handler

defines an interface for handling the requests

(optional) implements the successor link

• ConcreteHandler

handles requests it is responsible for

can access its successor

if the ConcreteHandler can handle the request, it does so; otherwise it forwards the request to its successor

• Client - initiates the request to a ConcreteHandler object on the chain

Sample code in C#

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  /// <summary>
  /// MainApp startup class for Structural
  /// Chain of Responsibility Design Pattern.
  /// </summary>
  class MainApp
  {
    /// <summary>
    /// Entry point into console application.
    /// </summary>
    static void Main()
    {
      // Setup Chain of Responsibility
      Handler h1 = new ConcreteHandler1();
      Handler h2 = new ConcreteHandler2();
      Handler h3 = new ConcreteHandler3();
      h1.SetSuccessor(h2);
      h2.SetSuccessor(h3);
 
      // Generate and process request
      int[] requests = { 2, 5, 14, 22, 18, 3, 27, 20 };
 
      foreach (int request in requests)
      {
        h1.HandleRequest(request);
      }
 
      // Wait for user
      Console.ReadKey();
    }
  }
 
  /// <summary>
  /// The 'Handler' abstract class
  /// </summary>
  abstract class Handler
  {
    protected Handler successor;
 
    public void SetSuccessor(Handler successor)
    {
      this.successor = successor;
    }
 
    public abstract void HandleRequest(int request);
  }
 
  /// <summary>
  /// The 'ConcreteHandler1' class
  /// </summary>
  class ConcreteHandler1 : Handler
  {
    public override void HandleRequest(int request)
    {
      if (request >= 0 && request < 10)
      {
        Console.WriteLine("{0} handled request {1}",
          this.GetType().Name, request);
      }
      else if (successor != null)
      {
        successor.HandleRequest(request);
      }
    }
  }
 
  /// <summary>
  /// The 'ConcreteHandler2' class
  /// </summary>
  class ConcreteHandler2 : Handler
  {
    public override void HandleRequest(int request)
    {
      if (request >= 10 && request < 20)
      {
        Console.WriteLine("{0} handled request {1}",
          this.GetType().Name, request);
      }
      else if (successor != null)
      {
        successor.HandleRequest(request);
      }
    }
  }
 
  /// <summary>
  /// The 'ConcreteHandler3' class
  /// </summary>
  class ConcreteHandler3 : Handler
  {
    public override void HandleRequest(int request)
    {
      if (request >= 20 && request < 30)
      {
          Console.WriteLine("{0} handled request {1}",
          this.GetType().Name, request);
      }
      else if (successor != null)
      {
        successor.HandleRequest(request);
      }
    }
  }

 

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