获取带有子列表条件的列表 [英] Get list with condition on sub list

问题描述

我有两个列表listA是MyClassA的列表，而listB是MyClassB

I have two list listA is a list of MyClassA and listB of MyClassB

MyClassA具有属性:

• 编号
• 名称

MyClassB具有以下属性:

• 编号
• 名称
• 活动
• MyClassAId

项目MyClassA可以具有一个或多个项目MyClassB

An item MyClassA can have one or more item MyClassB

- MyClassA
    |
    | MyClassB
    | MyClassB
- MyClassA
    |
    | MyClassB
    | MyClassB
    | MyClassB
    | MyClassB

我想获取MyClassA的列表，其中MyClassB与active = true的项数> 1

I'd like get a list of MyClassA where the number of items MyClassB with active = true is > 1

我尝试过这样的事情:

listA.Where(b => listB.Any(a => a.MyClassAId == b.Id && a.Active == true));

推荐答案

Any将为>0.对于> 1，您可以执行以下操作:

Any will be > 0. For > 1 you could do something like:

listA.Where(a => listB.Count(b => b.MyClassAId == a.Id && b.Active) > 1);

或具有更好的短路性(但对于LINQ-to-Objects可能很好，但对于EF则不行)

or with better short-circuiting (but probabably fine for LINQ-to-Objects, but not EF etc):

listA.Where(
    a => listB.Where(b => b.MyClassAId == a.Id && b.Active).Skip(1).Any());

另一种方法是预先计算计数:

Another approach would be to pre-compute the counts:

var include = new HashSet<int>(listB.Where(b => b.Active)
                  .GroupBy(b => b.MyClassAId)
                  .Where(grp => grp.Count() > 1)
                  .Select(grp => grp.Key));

然后:

listA.Where(a => include.Contains(a.Id));

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