CQRS Pattern with MediatR in ASP.NET Core

Introduction

CQRS separates reads from writes — ShopNest Order Management uses MediatR so controllers send one command/query object and handlers contain the logic.

After this article you will

• Implement commands and queries with IRequest
• Build handlers and pipeline behaviors
• Publish domain events as INotification
• Combine CQRS with Clean Architecture layers
• Understand read vs write model trade-offs

Prerequisites

• Article 44 — Clean Architecture
• ShopNest API, EF Core, and DI from prior modules

Concept deep-dive

public record CreateOrderCommand(string CustomerId, List<OrderLineDto> Lines)
    : IRequest<Guid>;

public class CreateOrderHandler : IRequestHandler<CreateOrderCommand, Guid>
{
    public async Task<Guid> Handle(CreateOrderCommand req, CancellationToken ct)
    {
        var order = Order.Create(req.CustomerId, req.Lines);
        await _repo.AddAsync(order, ct);
        await _publisher.Publish(new OrderCreatedEvent(order.Id), ct);
        return order.Id;
    }
}

public record GetOrderQuery(Guid OrderId) : IRequest<OrderDetailDto?>;

// Pipeline behavior — validation for all commands
public class ValidationBehavior<TReq, TRes> : IPipelineBehavior<TReq, TRes>
    where TReq : IRequest<TRes>
{
    public async Task<TRes> Handle(TReq req, RequestHandlerDelegate<TRes> next, CancellationToken ct)
    {
        await _validator.ValidateAndThrowAsync(req, ct);
        return await next();
    }
}

Same DB vs separate: Start with one database; split read models when read scale demands it. Event sourcing is optional advanced topic.

Hands-on — ShopNest Order Management System

1. Install MediatR; register from Application assembly.
2. CreateOrderCommand + GetOrderQuery handlers.
3. LoggingBehavior and ValidationBehavior pipeline.
4. OrdersController: await _mediator.Send(command).

Common errors & best practices

• Fat handlers with 200 lines — extract domain services.
• CQRS for every CRUD — overkill on simple modules.
• Forgetting async all the way through handlers.

Interview questions

Q: CQRS benefit?
A: Independent scaling and models for reads vs writes; clearer intent per operation.

Q: MediatR purpose?
A: In-process mediator decoupling controllers from handlers + pipeline behaviors.

Q: IRequest vs INotification?
A: IRequest returns value (command/query); INotification is fire-and-forget event.

Summary

• Commands mutate; queries read — separate handlers
• MediatR pipeline cross-cuts validation and logging
• Domain events decouple side effects (email, inventory)
• Fits Clean Architecture Application layer perfectly

Previous: Clean Architecture
Next: Repository Pattern — Deep Dive

FAQ