• The Chain of Responsibility design pattern offers an opportunity to build a collection of loosely coupled objects by relieving a client from having to know which objects in a collection can satisfy a request by arranging these objects in a chain.
• It avoids 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.
• This pattern requires a way to order the search for an object that can handle the request.
  • This search is usually modeled according to the specific needs of the application domain.
  • Note that Chain-of-Responsibility is not commonly used in business application development.
• Occasionally you may run into a Chain of Responsibility implementation in which a chain of objects process a message between a sender and a receiver, in which each object does some processing on the message as it travels through the chain from the sender to the receiver.
  • This is slightly different from the GoF definition in which just one object in a chain decides to handle the request.

Participants

• **Handler** (IApprover)
  • Defines an interface for handling the requests.
  • (Optional) implements the successor link.
• **ConcreteHandler** (Director, VicePresident, President)
  • 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.

var larry = new Director();
var sam = new VicePresident();
var tammy = new President();

larry.Successor = sam;
sam.Successor = tammy;

var purchase = new Purchase(2034, 350.00, "Assets");
larry.ProcessRequest(purchase);

purchase = new Purchase(2035, 32590.10, "Project X");
larry.ProcessRequest(purchase);

purchase = new Purchase(2036, 122100.00, "Project Y");
larry.ProcessRequest(purchase);

public interface IApprover
{
    IApprover Successor { get; set; }
    void ProcessRequest(Purchase purchase);
}

public class Director : IApprover
{
    public IApprover Successor { get; set; }
    public void ProcessRequest(Purchase purchase)
    {
        if (purchase.Amount < 10000.0)
            Console.WriteLine("{0} approved request# {1}", GetType().Name, purchase.Number);
        else if (Successor != null)
            Successor.ProcessRequest(purchase);
    }        
}

public class VicePresident : IApprover
{
    public IApprover Successor { get; set; }
    public void ProcessRequest(Purchase purchase)
    {
        if (purchase.Amount < 25000.0)
            Console.WriteLine("{0} approved request# {1}", GetType().Name, purchase.Number);
        else if (Successor != null)
            Successor.ProcessRequest(purchase);
    }
}

public class President : IApprover
{
    public IApprover Successor { get; set; }
    public void ProcessRequest(Purchase purchase)
    {
        if (purchase.Amount < 100000.0)
            Console.WriteLine("{0} approved request# {1}", GetType().Name, purchase.Number);
        else Console.WriteLine("Request# {0} requires an executive meeting!", purchase.Number);
    }
}

public class Purchase
{
    public int Number { get; set; }
    public double Amount { get; set; }
    public string Purpose { get; set; }

    public Purchase(int number, double amount, string purpose)
    {
        Number = number;
        Amount = amount;
        Purpose = purpose;
    }        
}

Rules of Thumb

• Chain of Responsibility, Command, Mediator, and Observer, address how you can decouple senders and receivers, but with different trade-offs.
• Chain of Responsibility passes a sender request along a chain of potential receivers.
• Chain of Responsibility can use Command to represent requests as objects.
• Chain of Responsibility is often applied in conjunction with Composite.
  • There, a component’s parent can act as its successor.