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How does the Repository pattern abstract data access, and what interface typically defines its contract? How would you implement the Decorator pattern in C# to add behavior to an object without modifying its original class? 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 Builder pattern using method chaining (a fluent API) to construct a complex object step by step? How would you implement the Chain of Responsibility pattern in C# to process a request through a series of independent handlers? 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 CQRS (Command Query Responsibility Segregation) pattern's basic structure in a C# application using MediatR-style handlers?

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Answer

CQRS separates WRITE operations (Commands, which change state and typically return little/nothing) from READ operations (Queries, which return data without side effects) into distinct request/handler pairs — often implemented with a mediator pattern where each Command/Query has its own dedicated Handler class, keeping business logic focused and independently testable.
public record CreateOrderCommand(int CustomerId, List<OrderItem> Items) : IRequest<int>;

public class CreateOrderHandler : IRequestHandler<CreateOrderCommand, int> {
  public async Task<int> Handle(CreateOrderCommand command, CancellationToken ct) {
    var order = new Order(command.CustomerId, command.Items);
    await _repository.AddAsync(order);
    return order.Id;
  }
}
Real-world example Structuring a complex domain's write and read operations into clearly separated, independently-scalable handler classes.

Common follow-ups: What benefits does CQRS provide specifically in systems with very different read versus write scaling requirements?

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All 11 Design Patterns in C# questions