C# Chain of Responsibility

The Chain of Responsibility design pattern avoids coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. This pattern chains the receiving objects and passes the request along the chain until an object handles it.

Frequency of use:
medium-low
C# Design Patterns

UML class diagram

A visualization of the classes and objects participating in this pattern.


Participants

The classes and objects participating in this pattern include:

  • Handler   (Approver)
    • 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
  • Client   (ChainApp)
    • initiates the request to a ConcreteHandler object on the chain

Structural code in C#

This structural code demonstrates the Chain of Responsibility pattern in which several linked objects (the Chain) are offered the opportunity to respond to a request or hand it off to the object next in line.

using System;

namespace Chain.Structural
{
    /// <summary>
    /// Chain of Responsibility Design Pattern
    /// </summary>

    public class Program
    {
        public static void Main(string[] args)
        {
            // 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>

    public 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>

    public 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>

    public 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>

    public 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);
            }
        }
    }
}
Output
ConcreteHandler1 handled request 2
ConcreteHandler1 handled request 5
ConcreteHandler2 handled request 14
ConcreteHandler3 handled request 22
ConcreteHandler2 handled request 18
ConcreteHandler1 handled request 3
ConcreteHandler3 handled request 27
ConcreteHandler3 handled request 20

Real-world code in C#

This real-world code demonstrates the Chain of Responsibility pattern in which several linked managers and executives can respond to a purchase request or hand it off to a superior. Each position has can have its own set of rules which orders they can approve.

using System;

namespace Chain.RealWorld
{
    /// <summary>
    /// Chain of Responsibility Design Pattern
    /// </summary>

    public class Program
    {
        public static void Main(string[] args)
        {
            // Setup Chain of Responsibility

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

            larry.SetSuccessor(sam);
            sam.SetSuccessor(tammy);

            // Generate and process purchase requests

            Purchase p = new Purchase(2034, 350.00, "Supplies");
            larry.ProcessRequest(p);

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

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

            // Wait for user

            Console.ReadKey();
        }
    }
    /// <summary>
    /// The 'Handler' abstract class
    /// </summary>

    public abstract class Approver
    {
        protected Approver successor;

        public void SetSuccessor(Approver successor)
        {
            this.successor = successor;
        }

        public abstract void ProcessRequest(Purchase purchase);
    }

    /// <summary>
    /// The 'ConcreteHandler' class
    /// </summary>

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

    /// <summary>
    /// The 'ConcreteHandler' class
    /// </summary>

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

    /// <summary>
    /// The 'ConcreteHandler' class
    /// </summary>

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

    /// <summary>
    /// Class holding request details
    /// </summary>

    public class Purchase
    {
        int number;
        double amount;
        string purpose;

        // Constructor

        public Purchase(int number, double amount, string purpose)
        {
            this.number = number;
            this.amount = amount;
            this.purpose = purpose;
        }

        // Gets or sets purchase number

        public int Number
        {
            get { return number; }
            set { number = value; }
        }

        // Gets or sets purchase amount

        public double Amount
        {
            get { return amount; }
            set { amount = value; }
        }

        // Gets or sets purchase purpose

        public string Purpose
        {
            get { return purpose; }
            set { purpose = value; }
        }
    }
}
Output
Director Larry approved request# 2034
President Tammy approved request# 2035
Request# 2036 requires an executive meeting!

.NET Optimized code in C#

The .NET optimized code demonstrates the same real-world situation as above but uses modern, built-in .NET features, such as, generics, reflection, LINQ, lambda functions, etc. You can find an example on our Singleton pattern page.

All other patterns (and much more) are available in our Dofactory .NET product.


Not only does Dofactory .NET cover the Gang of Four and Enterprise patterns, it also includes pattern architectures, low-code, and RAD (Rapid Application Development) techniques. Accelerate development to where you can write entire solutions in just 33 days!.

This unique package will change your developer lifestyle.  Here's what is included:



#1 .NET Success Platform
  • 69 gang-of-four pattern projects
  • 46 head-first pattern projects
  • Fowler's enterprise patterns
  • Multi-tier patterns
  • Convention over configuration
  • Active Record and CQRS patterns
  • Repository and Unit-of-Work patterns
  • MVC, MVP, & MVVM patterns
  • REST patterns with Web API
  • SparkTM Rapid App Dev (RAD) data access
  • Complete Art Shop, Ecommerce App
  • Complete Analytics, Dashboard App
  • Complete Art Shop, Ecommerce App
  • Complete SaaS, Multi-Tenant App
  • Everything 100% source code


Guides


vsn 3.1