Engineering Note

Mastering CQRS with MediatR in .NET

May 21, 2025

Mastering CQRS with MediatR in .NET

“Separation of concerns isn’t just a principle — it’s a power tool.”

In this guide, you’ll see how to implement CQRS using MediatR to build clean, scalable architecture for modern .NET applications.

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Why CQRS?

CQRS (Command Query Responsibility Segregation) helps you:

• Split read and write logic
• Increase scalability and maintainability
• Enable event sourcing and microservices

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Project Structure

/Features
   /Products
     CreateProductCommand.cs
     CreateProductHandler.cs
     GetProductByIdQuery.cs
     GetProductByIdHandler.cs

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1. Define the Command

// CreateProductCommand.cs
public record CreateProductCommand(string Name, decimal Price) : IRequest<Guid>;

2. Handle the Command

// CreateProductHandler.cs
public class CreateProductHandler : IRequestHandler<CreateProductCommand, Guid>
{
    private readonly IProductRepository _repo;

    public CreateProductHandler(IProductRepository repo)
    {
        _repo = repo;
    }

    public async Task<Guid> Handle(CreateProductCommand request, CancellationToken cancellationToken)
    {
        var product = new Product { Name = request.Name, Price = request.Price };
        await _repo.AddAsync(product);
        return product.Id;
    }
}

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3. Define the Query

// GetProductByIdQuery.cs
public record GetProductByIdQuery(Guid Id) : IRequest<ProductDto>;

4. Handle the Query

// GetProductByIdHandler.cs
public class GetProductByIdHandler : IRequestHandler<GetProductByIdQuery, ProductDto>
{
    private readonly IProductRepository _repo;

    public GetProductByIdHandler(IProductRepository repo)
    {
        _repo = repo;
    }

    public async Task<ProductDto> Handle(GetProductByIdQuery request, CancellationToken cancellationToken)
    {
        var product = await _repo.GetByIdAsync(request.Id);
        return new ProductDto(product.Id, product.Name, product.Price);
    }
}

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5. Register MediatR in Program.cs

builder.Services.AddMediatR(cfg => cfg.RegisterServicesFromAssembly(typeof(Program).Assembly));

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6. Trigger Commands/Queries in Controllers

[HttpPost]
public async Task<IActionResult> Create([FromBody] CreateProductCommand cmd)
{
    var id = await _mediator.Send(cmd);
    return CreatedAtAction(nameof(Get), new { id }, id);
}

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Bonus Tip

Use Pipeline Behaviors in MediatR for cross-cutting concerns:

• Logging
• Validation (FluentValidation)
• Caching
• Retry logic

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Final Thoughts

CQRS + MediatR helps you write cleaner, decoupled, testable code. It scales well with complex domains and high-load systems.

What’s your favorite CQRS pattern tweak or MediatR trick? Share below.

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#dotnet #CSharp #CQRS #MediatR #CleanArchitecture #SoftwareEngineering #DevTips #AsyncProgramming