在Linq中使用空值的自连接c#

本文关键字：自连接 空值 Linq | 更新日期: 2023-09-27 18:11:34

我怎么能得到有空的行用于自连接的列?我的代码是:

IEnumerable<ListClassView> list = from l in lists join lp in lists on 
l.ParentListId equals lp.Id 
select new ListClassView()
{
    Id = l.Id,
    ListName = l.ListName,
    Description = l.Description,
    ParentName = lp.ListName,
    IsActive = l.IsActive
};

我无法获取ParentListId=null。是否有一种方法，我可以得到所有的行?

在Linq中使用空值的自连接c#

备选语法:

var list = lists.GroupJoin(
  lists,
  l => l.ParentListId,
  lp => lp.Id,
  (l, lp) => new ListClassView
            { 
               Id = l.Id, 
               ListName = l.ListName, 
               Description = l.Description,
               ParentName = lp.FirstOrDefault() == null ? null : lp.First().ListName,
               IsActive = l.IsActive
            });

我从这里通过应用左连接找到了解决方案:

IEnumerable<ListClassView> list = from l in lists join lp in lists on 
l.ParentListId equals lp.Id into lpp
from p in lpp.DefaultIfEmpty()
select new ListClassView()
{
    Id = l.Id,
    ListName = l.ListName,
    Description = l.Description,
    ParentName = p.ListName,
    IsActive = l.IsActive
};

现在，它得到所有行，包括ParentListId = null

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