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What is MediatR and how does it relate to Clean Architecture?

ICSM Computer 600 16 Jun 2025

Ask by ICSM Computer

Updated 16 Jun 2025

1 Answers

ICSM Computer 16 Jun 2025

🧭 What is MediatR?

MediatR is a simple and lightweight .NET library that implements the Mediator Pattern, which helps reduce tight coupling between components by centralizing communication.

In MediatR:

Requests (Commands or Queries) are sent to a mediator instead of calling services or logic directly.
The mediator then invokes the appropriate handler.

NuGet Package:

Install-Package MediatR.Extensions.Microsoft.DependencyInjection

Basic MediatR Flow

Controller → MediatR.Send(request) → Handler → Business Logic → Response

Example

1. Define a Command (write action)

public record CreateStudentCommand(string Name, string Email) : IRequest<bool>;

2. Define a Handler

public class CreateStudentHandler : IRequestHandler<CreateStudentCommand, bool>
{
    private readonly AppDbContext _db;
    public CreateStudentHandler(AppDbContext db) => _db = db;

    public async Task<bool> Handle(CreateStudentCommand request, CancellationToken ct)
    {
        _db.Students.Add(new Student { Name = request.Name, Email = request.Email });
        await _db.SaveChangesAsync();
        return true;
    }
}

3. Send from Controller

public class StudentsController : ControllerBase
{
    private readonly IMediator _mediator;
    public StudentsController(IMediator mediator) => _mediator = mediator;

    [HttpPost]
    public async Task<IActionResult> CreateStudent(CreateStudentCommand command)
    {
        var result = await _mediator.Send(command);
        return Ok(result);
    }
}

MediatR + Clean Architecture

Clean Architecture separates the system into layers:

[ Presentation ]
[ Application  ] ← Business rules (Application layer, UseCases)
[ Domain       ] ← Core business models/rules
[ Infrastructure ] ← EF Core, Email, External APIs, etc.

How MediatR Fits In:

Clean Architecture Layer	Role of MediatR
Application Layer	Hosts MediatR requests and handlers (CQRS)
Presentation Layer	Sends requests via `IMediator.Send(...)`
Infrastructure	Injected dependencies consumed by handlers

Benefits:

Encourages Single Responsibility per handler
Keeps Controllers thin
Promotes decoupling of business logic from delivery (controller/UI)
Facilitates unit testing of handlers
Natural fit for CQRS (Command Query Responsibility Segregation)

CQRS + MediatR + Clean Architecture (Typical Pattern)

[ API Controller ]
        ↓
[ IMediator.Send() ]
        ↓
[ Command / Query Handler ]
        ↓
[ Application Services / Repositories ]

Summary

Aspect	MediatR
Pattern	Mediator
Used for	Decoupling request handling
Works well with	Clean Architecture & CQRS
Pros	Decouples logic, testable, modular
Common Use	Handle Commands & Queries

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