如何在带有LINQ的c＃Windows窗体中显示结果值甚至为null

Question

[嗨，我正在使用Entity Framwork winform，并且我试图在dataGridView中显示具有部门名称和Roomnr的订单，甚至在不具有Roomnr且Roomnr为null的订单中显示订单，但是，仅显示具有Department和RoomId的对象不为null。如何显示RoomId == null的那些部门？这是我的代码：

var Customerorders = (from u in db.Orders
                      join s in db.Employee on u.Staffid equals s.EmployeeId
                      join d in db.Department on u.DepartmentId equals d.DepartmentId
                      join r in db.Room on u.RoomId equals r.RoomId
                      where u.OrderNr == 1005 || u.OrderNr == 1005 && u.RoomId == null
                      select new
                      {
                         CheckinoutId = u.CheckInAndOutId,
                         CheckinDate = u.CheckInDate,
                         Checkout = u.CheckOutDate,
                         Department = d.DepartmentName,
                         RoomNr = r.RoomNr,
                         Personal = s.FirstName+" "+s.LastName
                         }).ToList();
                         if(Customerorders != null) // I Tried even with out if condition
                         {

                          dgvOrders.DataSource = Customerorders;
                         }

即使没有房间也无法显示订单？请帮助

Answer 1

这是LEFT JOIN中DbSet<Room>的LINQ

var Customerorders = (from u in db.Orders
                      join s in db.Employee on u.Staffid equals s.EmployeeId
                      join d in db.Department on u.DepartmentId equals d.DepartmentId
                      join r in db.Room on u.RoomId equals r.RoomId into r2
                      from r3 in r2.DefaultIfEmpty()
                      where u.OrderNr == 1005
                      select new
                      {
                          CheckinoutId = u.CheckInAndOutId,
                          CheckinDate = u.CheckInDate,
                          Checkout = u.CheckOutDate,
                          Department = d.DepartmentName,
                          RoomNr = r3.RoomNr,
                          Personal = s.FirstName + " " + s.LastName
                      }).ToList();

这只是您的请求的解决方案
var Customerorders = db.Orders.Where(
    item => item.OrderNr == 1005
).Select(
    item => new
    {
        item.CheckInAndOutId,
        item.CheckInDate,
        item.CheckOutDate,
        item.Department.DepartmentName,
        Personal = item.Employee.FirstName + " " + item.Employee.LastName,
        RoomNr = item.RoomId.HasValue ? item.Room.RoomNr : (int?)null
    }
);