public class UserController : Controller
{
    private IMediator _mediator;
    public UserController(IMediator mediator)
    {
        _mediator = mediator;
    }

    public async Task<IActionResult> GetById(int id)
    {
        var user = await _mediator.Send(new GetUserById());
        return Ok(user);
    }
}

public class UserHandler : IRequestHandler<GetUserById, User>
{
    private readonly IUserRepository _userRepository;
    public UserHandler(IUserRepository userRepository)
    {
        _userRepository = userRepository;
    }

    public async Task<User> Handle(GetUserById request, CancellationToken cancellationToken)
    {
        return _userRepository.GetUserById(request.Id);
    }
}

public interface IUserRepository
{
    User GetUserById(int id);
}

public class GetUserById : IRequest<User>
{
    public int Id { get; set; }
}