LINQ函数中可空类型的问题

Question

Parent_ObjectiveID并且identity是int?数据类型.在我的程序中应该返回一个对象,但它给出了一个错误:Sequence contains no elements.

int? identity = null;

Objective currentObjective = (from p in cd.Objective
                              where p.Parent_ObjectiveID == identity
                              select p).Single();

虽然,如果我将identity变量替换为null.它有效,但我不明白.

currentObjective = (from p in cd.Objective
                    where p.Parent_ObjectiveID == null
                    select p).Single();

发生了什么？

更新1:

我这样做了:

if (identity == null)
{
     currentObjective = (from p in cd.Objective
                         where p.Parent_ObjectiveID == null
                         select p).Single();
}
else
{
     currentObjective = (from p in cd.Objective
                         where p.Parent_ObjectiveID == identity
                         select p).Single();
}

但我真的不喜欢它.

Answer 1

LINQ 似乎不支持 where 子句中的这种情况。

这个问题是关于同样的问题。另请参阅此线程。

你可以尝试：

Objective currentObjective = (from p in cd.Objective
                                  where p.Parent_ObjectiveID == (identity ?? null)
                                  select p).Single();

编辑：如果这不起作用，请尝试与 object.Equals 进行比较：

Objective currentObjective = (from p in cd.Objective
                                  where object.Equals(p.Parent_ObjectiveID, identity)
                                  select p).Single();