LINQ to SQL 左连接优于左连接 c#

Question

我正在尝试对现有的左连接执行左连接，因为这是一个查找表。

类似：

(from m in maintablerepository.AsQueryable()
join st1 in subtable1repository.AsQuerable on 
m.id = s1.id into grps1
from s in grps1.DefaultIfEmpty()
join lt1 in lookuptablerepository.AsQuerable on 
s.lkpid = lt1.id into grplt1
from lkp in grplt1.DefaultIfEmpty()

select new {

prop1 = m.prop1,
prop2 = s.prop2,
prop3 = lkp.prop3

}).ToList();

问题是当我删除查找表上的左联接时，此查询运行良好，尽管子表中没有匹配的值。

但是，将其放回原处会导致“未设置对象引用”错误。

我什至尝试过以下方法，但不行：

(from m in maintablerepository.AsQueryable()
join st1 in subtable1repository.AsQuerable on 
m.id = s1.id into grps1
from s in grps1.DefaultIfEmpty()
join lt1 in lookuptablerepository.AsQuerable on 
s.lkpid = lt1.id into grplt1
from lkp in grplt1.DefaultIfEmpty()
where s!=null && lkp!=null
select new {

prop1 = m.prop1,
prop2 = (s==null)?string.Empty:s.prop2,
prop3 = (lkp==null)?string.Empty:lkp.prop3

}).ToList();

如果第一个左连接结果为空，如何对其进行另一个连接？

Answer 1

问题在于 s.lkpid = lt1.id 而 s 为空。尝试像下面那样放置 s != null

(from m in maintablerepository.AsQueryable()
join st1 in subtable1repository.AsQuerable on 
m.id = s1.id into grps1
from s in grps1.DefaultIfEmpty()
where s!=null
join lt1 in lookuptablerepository.AsQuerable on 
s.lkpid = lt1.id into grplt1
from lkp in grplt1.DefaultIfEmpty()
where lkp!=null
select new {

prop1 = m.prop1,
prop2 = (s==null)?string.Empty:s.prop2,
prop3 = (lkp==null)?string.Empty:lkp.prop3

}).ToList();

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