Chain of Responsibility

Context: 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.

public abstract class Handler
{
    protected Handler _next;
    public void SetNext(Handler next) => _next = next;
    public abstract void HandleRequest(int request);
}

public class ConcreteHandlerA : Handler
{
    public override void HandleRequest(int request)
    {
        if (request < 10) Console.WriteLine("Handler A handled");
        else _next?.HandleRequest(request);
    }
}

Real-world usage example

Logging framework: Loggers with different levels (INFO, DEBUG, ERROR) form a chain. If a logger cannot handle a level, it passes to the next.

Example: ASP.NET Core middleware pipeline is a chain of responsibility. Each middleware decides to process or pass to the next. Also, ValidationHandler in FluentValidation.

Related sections

• 5.3 Behavioural patterns
• 5.3.3 Observer
• 5.3.5 Mediator