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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 t

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