Implementing CQRS bases in .NET

Doru Bulubasa

09 July 2025

After understanding the basic concepts and the recommended structure, it's time to move on to implementation. At this stage, we will configure MediatR and create the first commands and queries in a .NET application, using a simple example: Post.

3️⃣ Installing MediatR

To separate responsibilities and avoid direct ties between layers, we use MediatR – a library that facilitates sending commands and queries to dedicated handlers.

🛠️ Installation

In the MyApp.Application project, add MediatR:

dotnet add package MediatR

dotnet add package MediatR.Extensions.Microsoft.DependencyInjection

In MyApp.API (or the UI/API project), configure the services in Program.cs:

builder.Services.AddMediatR(cfg =>

cfg.RegisterServicesFromAssembly(typeof(MyApp.Application.AssemblyReference).Assembly));

Add a reference class in MyApp.Application:

namespace MyApp.Application;

public static class AssemblyReference { }

4️⃣ Creating a simple command: CreatePostCommand

🧾 Command: CreatePostCommand.cs

using MediatR;

public record CreatePostCommand(string Title, string Content) : IRequest<Guid>;

IRequest<Guid> means that this command will return a Guid (the ID of the created post)

🧠 Handler: CreatePostCommandHandler.cs

public class CreatePostCommandHandler : IRequestHandler<CreatePostCommand, Guid>

{

private readonly IApplicationDbContext _context;

public CreatePostCommandHandler(IApplicationDbContext context)

{

_context = context;

}

public async Task<Guid> Handle(CreatePostCommand request, CancellationToken cancellationToken)

{

var post = new Post

{

Id = Guid.NewGuid(),

Title = request.Title,

Content = request.Content,

CreatedAt = DateTime.UtcNow

};

_context.Posts.Add(post);

await _context.SaveChangesAsync(cancellationToken);

return post.Id;

}

5️⃣ Complete separation for a query: GetPostByIdQuery

🔍 Query: GetPostByIdQuery.cs

using MediatR;

public record GetPostByIdQuery(Guid Id) : IRequest<PostDto>;

PostDto is a display-only object – without business logic.

🔄 Handler: GetPostByIdQueryHandler.cs

public class GetPostByIdQueryHandler : IRequestHandler<GetPostByIdQuery, PostDto?>

{

private readonly IApplicationDbContext _context;

public GetPostByIdQueryHandler(IApplicationDbContext context)

{

_context = context;

}

public async Task<PostDto?> Handle(GetPostByIdQuery request, CancellationToken cancellationToken)

{

var post = await _context.Posts

.AsNoTracking()

.Where(p => p.Id == request.Id)

.Select(p => new PostDto(p.Id, p.Title, p.Content, p.CreatedAt))

.FirstOrDefaultAsync(cancellationToken);

return post;

}

📦 PostDto.cs (in Application)

public record PostDto(Guid Id, string Title, string Content, DateTime CreatedAt);

6️⃣ Summary: IRequest and IRequest

Request type	Interface	Returns
Command (write)	IRequest<Unit>	Nothing (Unit.Value)
Command with result	IRequest<Guid>	For example: the created ID
Query (read)	IRequest<PostDto>	A DTO type result

🧪 Usage example from Controller / UI

[HttpPost]

public async Task<IActionResult> Create(CreatePostCommand command)

{

var id = await _mediator.Send(command);

return CreatedAtAction(nameof(GetById), new { id }, null);

}

[HttpGet("{id}")]

public async Task<ActionResult<PostDto>> GetById(Guid id)

{

var post = await _mediator.Send(new GetPostByIdQuery(id));

return post is not null ? Ok(post) : NotFound();

}

🧠 Conclusion

At this stage, we have implemented the first essential pieces of a CQRS system:

✅ Commands that modify the application state
✅ Separate queries for reading
✅ Dedicated and independent handlers
✅ MediatR as a central intermediary

🏁 What's next

In the next article we will add:

✅ Validations with FluentValidation
✅ Mapping between DTOs and entities
✅ Clear separation between WriteModel and ReadModel