LINQ: How to get items from an inner list into one list?

Question

Having the following classes (highly simplified):

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>(); } 

And having the following data:

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); 

Now, how would I get a list of children (with CategoryNumber=2) from the list of parents containing at least one child with CategoryNumber = 1 ?

I can do the following but it does not appear to be optimal:

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); 

Here's what I get for selectedChildren:

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

Is it possible to only have one flat list containing the two children elements instead of two sub-list? How would it translate in LINQ ?

Answer 1

You can string a couple queries together, using SelectMany and Where.

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);