我可以在Where()中使用FirstOrDefault()或First()吗

发布于 2024-11-04 06:36:36 字数 480 浏览 1 评论 0原文

我正在尝试实现此查询：

 data.Where(d => d.ObjectsA != null && 
   d.ObjectsA.First().ObjectsB != null && 
   d.ObjectsA.First().ObjectsB().First().Nr == 1)

in Nhibernate.Linq，但出现错误。当我从Where() 中删除First() 时，一切正常。我尝试这个解决方案，但这没有得到我所需要的。

 data.Where(d => d.ObjectsA.Where(a.ObjectsB.Where(b=>b.Nr == 1).Any()).Any());

我可以在Where()中使用FirstOrDefault()或First()吗？

编辑：在我的数据库表中，本示例中的所有行都没有空值。

原文

I am trying to implement this query:

 data.Where(d => d.ObjectsA != null && 
   d.ObjectsA.First().ObjectsB != null && 
   d.ObjectsA.First().ObjectsB().First().Nr == 1)

in Nhibernate.Linq, but I have an error.
When I delete First() from Where() all work.
I try this solution but this don't get what I need.

 data.Where(d => d.ObjectsA.Where(a.ObjectsB.Where(b=>b.Nr == 1).Any()).Any());

Can I use FirstOrDefault() or First() inside Where() ?

EDIT : In my DB tables all rows in this example don't have null values.

分享到QQ

分享到微博

如果你对这篇内容有疑问，欢迎到本站社区发帖提问参与讨论，获取更多帮助，或者扫码二维码加入 Web 技术交流群。

发布评论

需要登录才能够评论，你可以免费注册一个本站的账号。

最终幸福 2024-11-11 06:36:36

如果集合为空，First() 将引发异常。在 Where()Any() > call 确实应该可以解决问题，但是您必须编写如下内容：

data.Where(d => d.ObjectsA != null && d.ObjectsA.Any()
    && d.ObjectsA.First().ObjectsB != null && d.ObjectsA.First().ObjectsB.Any()
    && d.ObjectsA.First().ObjectsB.First().Nr == 1);

这不太好，因为 First() 最终会对相同的数据调用多次。我建议向 lambda 表达式添加主体，并使用 FirstOrDefault() 的中间变量：

data.Where(d => {
    if (d.ObjectsA != null) {
        var firstA = d.ObjectsA.FirstOrDefault();
        if (firstA != null && firstA.ObjectsB != null) {
            var firstB = firstA.ObjectsB.FirstOrDefault();
            if (firstB != null) {
                return (firstB.Nr == 1);
            }
        }
    }
    return false;
});

编辑：上面的第二个代码片段显然不适用于 LINQ to NHibernate。如果您可以使用查询语法并且不必检查 ObjectsA 和 ObjectsB 是否为 null，您可以编写：

from d in data
let firstA = d.ObjectsA.FirstOrDefault()
let firstB = (firstA != null ? firstA.ObjectsB.FirstOrDefault() : null)
where (firstB != null && firstB.Nr == 1)
select d;

First() will throw an exception if the collection is empty. Using Any() in your Where() call should indeed solve the problem, but you would have to write something like:

data.Where(d => d.ObjectsA != null && d.ObjectsA.Any()
    && d.ObjectsA.First().ObjectsB != null && d.ObjectsA.First().ObjectsB.Any()
    && d.ObjectsA.First().ObjectsB.First().Nr == 1);

That's not very nice, because First() ends up being called several times on the same data. I'd suggest adding a body to your lambda expression and using intermediate variables with FirstOrDefault():

data.Where(d => {
    if (d.ObjectsA != null) {
        var firstA = d.ObjectsA.FirstOrDefault();
        if (firstA != null && firstA.ObjectsB != null) {
            var firstB = firstA.ObjectsB.FirstOrDefault();
            if (firstB != null) {
                return (firstB.Nr == 1);
            }
        }
    }
    return false;
});

EDIT: The second code fragment above apparently does not work with LINQ to NHibernate. If you can use query syntax and don't have to check if ObjectsA and ObjectsB are null, you can write:

from d in data
let firstA = d.ObjectsA.FirstOrDefault()
let firstB = (firstA != null ? firstA.ObjectsB.FirstOrDefault() : null)
where (firstB != null && firstB.Nr == 1)
select d;

回复收藏 0 原文

秋意浓 2024-11-11 06:36:36

如果条件中使用的集合可以为空，则可以使用默认变量和 ?? nullif 运算符与 FirstOrDefault()。

var defaultA = new ObjectA();
var defaultB = new ObjectB();
var defaultBList = new List<ObjectB>();

data.Where(d => d.ObjectsA != null && 
   ((d.ObjectsA.FirstOrDefault() ?? defaultA).ObjectsB ?? defaultBList)
   .FirstOrDefault() ?? defaultB).Nr == 1)

If the collections used in the condition can be empty, you can use a default variable and the ?? nullif operator with FirstOrDefault().

var defaultA = new ObjectA();
var defaultB = new ObjectB();
var defaultBList = new List<ObjectB>();

data.Where(d => d.ObjectsA != null && 
   ((d.ObjectsA.FirstOrDefault() ?? defaultA).ObjectsB ?? defaultBList)
   .FirstOrDefault() ?? defaultB).Nr == 1)

回复收藏 0 原文

~没有更多了~