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How would you implement the Observer pattern in C# using events, versus using the IObservable<T>/IObserver<T> interfaces from System? How would you implement the Unit of Work pattern alongside the Repository pattern to coordinate multiple repository changes within a single transaction? How would you implement the CQRS (Command Query Responsibility Segregation) pattern's basic structure in a C# application using MediatR-style handlers? How would you implement the Builder pattern using method chaining (a fluent API) to construct a complex object step by step? What are design patterns, and how are the Singleton, Adapter, and Strategy patterns typically implemented in C#?
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How would you implement the Chain of Responsibility pattern in C# to process a request through a series of independent handlers?

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Answer

Each handler holds a reference to the NEXT handler in the chain; it either processes the request itself and stops, or passes it along to the next handler if it can't handle it — commonly implemented in ASP.NET Core's own middleware pipeline, which is essentially a real-world Chain of Responsibility implementation.
public abstract class ValidationHandler {
  protected ValidationHandler? Next;
  public ValidationHandler SetNext(ValidationHandler next) { Next = next; return next; }
  public abstract bool Handle(Order order);
}
public class CreditCheckHandler : ValidationHandler {
  public override bool Handle(Order order) =>
    CheckCredit(order) && (Next?.Handle(order) ?? true);
}
Real-world example Building a multi-step validation pipeline for an order (credit check, inventory check, fraud check) where each step can short-circuit the chain.

Common follow-ups: How does ASP.NET Core's own middleware pipeline (app.Use(...)) directly mirror this exact pattern?

Interfaces & Abstract Classes
All 11 Design Patterns in C# questions