Why should you care?

Controllers stay thin. Same use case can run from HTTP, RabbitMQ consumer, or unit test.

See it live — copy this example

Create a Web API project (dotnet new webapi), paste the code, then run dotnet run.

public record RegisterUserCommand(string Name, string Email);

public class RegisterUserHandler
{
    private readonly IUserRepository _repo;
    public RegisterUserHandler(IUserRepository repo) => _repo = repo;

    public async Task<Guid> HandleAsync(RegisterUserCommand cmd, CancellationToken ct = default)
    {
        var user = UserProfile.Create(cmd.Name, cmd.Email);
        await _repo.AddAsync(user, ct);
        return user.Id;
    }
}

Run Example »

Edit the code and click Run — like W3Schools Try it Yourself.

What happened?

• Handler calls domain factory then repository.
• Validation attributes on command or FluentValidation in this layer.

Try it yourself

1. Add RegisterUserHandler and GetUserByIdHandler.
2. Register handlers in AddApplication() extension.
3. Unit test handler with fake IUserRepository.
4. Change a string or route in the example and save — watch Swagger or the RabbitMQ Management UI update.
5. Break the code on purpose (remove a semicolon), read the error message, then fix it.

Remember

Application = use cases. Handlers orchestrate domain + repo. Testable without Kestrel.