LINQ:如何将内部列表中的项目放入一个列表中？

Question

拥有以下课程(高度简化):

public class Child
{
    public string Label;
    public int CategoryNumber;
    public int StorageId;
}

public class Parent
{
    public string Label;
    public List<Child> Children = new List<Child>();
}

并拥有以下数据:

var parents = new List<Parent>();

var parent = new Parent() {Label="P1"};
parent.Children.Add(new Child() {Label="C1", CategoryNumber=1, StorageId=10});
parent.Children.Add(new Child() {Label="C2", CategoryNumber=2, StorageId=20});
parents.Add(parent);

parent = new Parent() {Label="P2"};
parent.Children.Add(new Child() {Label="C3", CategoryNumber=1, StorageId=10});
parent.Children.Add(new Child() {Label="C4", CategoryNumber=2, StorageId=30});
parents.Add(parent);

parent = new Parent() {Label="P3"};
parent.Children.Add(new Child() {Label="C5", CategoryNumber=3, StorageId=10});
parent.Children.Add(new Child() {Label="C6", CategoryNumber=2, StorageId=40});
parents.Add(parent);

现在,我如何从包含至少一个CategoryNumber = 1的孩子的父母列表中获取子列表(CategoryNumber = 2)？

我可以执行以下操作,但它似乎不是最佳的:

var validParents = from p in parents
                   where p.Children.Any (c => c.CategoryNumber==1)
                   select p;
var selectedChildren = validParents.Select(p => from c in p.Children 
                                                where c.CategoryNumber == 2
                                                select c);

这是我为selectedChildren获得的:

• IEnumerable<IEnumerable<Child>>
  • IEnumerable<Child>
    • C2 2 20
  • IEnumerable<Child>
    • C4 2 30

是否可以只有一个包含两个子元素的平面列表而不是两个子列表？如何在LINQ中翻译？

Answer 1

您可以使用SelectMany和Where将一对查询串在一起.

var selectedChildren = (from p in parents
                       where p.Children.Any (c => c.CategoryNumber==1)
                       select p)
                       .SelectMany(p => p.Children)
                       .Where(c => c.CategoryNumber == 2);

// or...

var selectedChildren = parents
                         .Where(p => p.Children.Any(c => c.CategoryNumber == 1))
                         .SelectMany(p => p.Children)
                         .Where(c => c.CategoryNumber == 2);

Answer 2

斯科特的回答很棒; 我只想指出你实际上可以使用查询延续语法来执行此查询:

from parent in parents 
where parent.Children.Any (c => c.CategoryNumber==1)
select parent into p
from child in p.Children
where child.CategoryNumber == 2
select child

注意"into"如何让您将一个查询的结果传递给下一个查询.漂亮,嗯？