LINQ 到实体 - 空引用

发布于 2024-08-25 00:38:03 字数 757 浏览 2 评论 0原文

我可以发誓这在前几天有效：

var resultSet =
    (from o in _entities.Table1
     where o.Table2.Table3.SomeColumn == SomeProperty
     select o
    ).First();
SelectedItem = resultSet.Table2.SomeOtherColumn;

我在最后一行收到空引用异常：resultSet.Table2 为空。
我不仅确定所有外键等都具有正确的值，而且我不明白 Table2 怎么可能 为空，因为 o.Table2.Table3.SomeColumn == SomeProperty。

resultSet 具有所有正确值，但 Table2 为空。

[编辑] 这有效：

~~SelectedItem = _entities.Table2.First( o => o.Table2.SomeColumn == SomeProperty).SomeOtherColumn;~~

~~而且，上面的 resultSet 具有所有正确的值，因此这不是数据问题在数据库中； LINQ-to-entities 只是做错了事。~~

原文

I could swear this was working the other day:

var resultSet =
    (from o in _entities.Table1
     where o.Table2.Table3.SomeColumn == SomeProperty
     select o
    ).First();
SelectedItem = resultSet.Table2.SomeOtherColumn;

I am getting a null reference exception on the last line: resultSet.Table2 is null.
Not only am I sure that all the foreign keys and whatnot have the correct values, but I don't see how Table2 could be null, since o.Table2.Table3.SomeColumn == SomeProperty.

resultSet has all the correct values, with the exception that Table2 is null.

[Edit] This works:

SelectedItem = _entities.Table2.First(
    o => o.Table2.SomeColumn == SomeProperty).SomeOtherColumn;

And, above, resultSet has all the correct values, so it is not a problem with the data in the database; LINQ-to-entities is simply doing something wrong.

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著墨染雨君画夕 2024-09-01 00:38:03

LINQ-to-entities 只是做错了什么。

不，它正在按设计工作。当您强制执行时，L2E 只会在表中JOIN。这提高了性能。只要你在L2E，你就可以引用任何关系。这就是它起作用的原因：

SelectedItem = _entities.Table2.First(
    o => o.Table2.SomeColumn == SomeProperty).SomeOtherColumn;

此处的 lambda 表达式将由 LINQ to Entities 解释，并转换为 SQL。另一方面，这：

var resultSet =
    (from o in _entities.Table1
     where o.Table2.Table3.SomeColumn == SomeProperty
     select o
    ).First();
SelectedItem = resultSet.Table2.SomeOtherColumn;

...给出 Table1 类型的结果。您现在处于对象空间中。由于您的查询不会强制加载 Table2，因此 L2E 不会为其生成 SQL 列。当您不需要 Table2 时，这会导致更高效的 SQL。当你这样做时，你必须这样说：

var resultSet =
    (from o in _entities.Table1.Include("Table2")
     where o.Table2.Table3.SomeColumn == SomeProperty
     select o
    ).First();

SelectedItem = resultSet.Table2.SomeOtherColumn;

这会起作用。但是，上面的 First(lambda) 方法是更好的解决方案。（比较 SQL。）

LINQ-to-entities is simply doing something wrong.

No, it's working as designed. L2E will only JOIN in tables when you force it to. This improves performance. As long as you are in L2E, you can reference any relationship. That's why this works:

SelectedItem = _entities.Table2.First(
    o => o.Table2.SomeColumn == SomeProperty).SomeOtherColumn;

Your lambda expression here will be interpreted by LINQ to Entities, and converted to SQL. On the other hand, this:

var resultSet =
    (from o in _entities.Table1
     where o.Table2.Table3.SomeColumn == SomeProperty
     select o
    ).First();
SelectedItem = resultSet.Table2.SomeOtherColumn;

...gives a result of type Table1. You are now in object space. Since your query does not force Table2 to be loaded, L2E won't generate SQL columns for it. This results in more efficient SQL when you don't need Table2. When you do, you have to say so:

var resultSet =
    (from o in _entities.Table1.Include("Table2")
     where o.Table2.Table3.SomeColumn == SomeProperty
     select o
    ).First();

SelectedItem = resultSet.Table2.SomeOtherColumn;

This will work. However, your First(lambda) method above is a better solution. (Compare the SQL.)

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