Controller.cs

public class OrderController : Controller
{
    private readonly IMediator _mediator;

    public OrderController(IMediator mediator)
    {
        _mediator = mediator;
    }
    
    public async Task<IActionResult> Detail(int orderId)
    {
        try
        {
            var viewModel = await _mediator.Send(new GetOrderQuery(orderId));
            return View(viewModel);
        }
        catch (OrderNotFoundException ex)
        {
            return NotFound($"Order ({ex.OrderId}) not found.");
        }
    }
}