Linq，如何 select 即使连接表中存在空值，C# windows 形式

Question

我有车 table 和车主 table。我想在 DataGridView 中显示它们。但我的代码仅在 Owner 字段不为空时显示。我想显示所有这些这是我的代码，也许你看到代码会更好地理解：

 var cars = (from u in db.cars
             join o in db.Owner on u.OwnerId equals o.OwnerId
             order by u.carId
             select new
             {
                 CarId = u.CarId,
                 CarName = u.CarName,
                 OwnerName = o.FirstName +" "+ o.FirstName, // Here I want to display owner name or Empty if there is no owner
             }).ToList();

                dgvCars.DataSource = cars;

我哪里做错了？我应该如何编写代码？ u.OwnerId 在我的 table 中可以为空。现在只显示拥有车主的汽车。但我什至想显示没有车主且车主姓名为 emptu 的汽车。提前致谢！

Answer 1

将查询更改为此，您需要外连接：

var cars = (from u in db.cars
     join o in db.Owner on u.OwnerId equals o.OwnerId into oj
     from o in oj.DefaultIfEmpty()
      order by u.carId
      select new
      {
    CarId = u.CarId,
   CarName = u.CarName,
   OwnerName = o.FirstName==null || o.LastName==null ?"": o.FirstName +" "+ o.LastName, 
       }).ToList();

Answer 2

var cars = (from u in db.cars
             join o in db.Owner on u.OwnerId equals o.OwnerId into r2
from r3 in r2.DefaultIfEmpty()
             order by u.carId
             select new
             {
                 CarId = u.CarId,
                 CarName = u.CarName,
                 OwnerName = r3.FirstName +" "+ r3.FirstName,  owner name or Empty if there is no owner
             }).ToList();

                dgvCars.DataSource = cars;

Linq，如何 select 即使连接表中存在空值，C# windows 形式

Linq, How to select even where there is null in joined tables , C# windows form

c#

linq

nullable

winforms