NoNull.cs

public interface IOrderReadService
{
    Task<OrderResponse> GetOrderByIdAsync(int orderId);
}

public class OrderReadService : IOrderReadService
{
    private readonly OrderContext _context;

    public OrderReadService(OrderContext context)
    {
        _context = context;
    }
    
    public async Task<OrderResponse> GetOrderByIdAsync(int orderId)
    {
        var orderResponse = await _context.Orders
            .Where(x => x.OrderId == orderId)
            .Select(x => new OrderResponse
            {
                OrderDate = x.OrderDate,
                OrderItems = x.OrderItems.Select(oi => new OrderItemResponse
                {
                    PictureUrl = oi.PictureUrl,
                    ProductId = oi.ProductId,
                    ProductName = oi.ProductName,
                    UnitPrice = oi.UnitPrice,
                    Units = oi.Units
                }).ToList(),
                OrderNumber = x.OrderNumber,
                ShippingAddress = x.ShippingAddress,
                Total = x.Total()
            })
            .SingleAsync();

        return orderResponse;
    }
}

public class GetOrderQueryHandler : IRequestHandler<GetOrderQuery, OrderResponse>
{
    private readonly IOrderReadService _orderReadService;

    public GetOrderQueryHandler(IOrderReadService orderReadService)
    {
        _orderReadService = orderReadService;
    }
    
    public Task<OrderResponse> Handle(GetOrderQuery request, CancellationToken cancellationToken)
    {
        return _orderReadService.GetOrderByIdAsync(request.OrderId);
    }
}

public class OrderController : Controller
{
    private readonly IMediator _mediator;

    public OrderController(IMediator mediator)
    {
        _mediator = mediator;
    }
    
    public async Task<IActionResult> Detail(int orderId)
    {
        var viewModel = await _mediator.Send(new GetOrderQuery(orderId));
        return View(viewModel);
    }
}