c# - LINQ 函数中可空类型的问题

Question

Parent_ObjectiveID 和 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，如果我将标识变量替换为 null。它有效，但我不明白。

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

发生了什么事？

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();
}

但我不是很喜欢。

Answer 1

LINQ 似乎不支持 where 子句中的这种情况。

This question是关于同样的问题。另见 this thread .

你可以试试:

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();