Is there a linq-y way to union collection properties of a collection of objects?

Question

Sorry, that title just hurts. I'm wondering if there is a Linq to collections extension method that collapses the following code segment into a single line:

public IEnumerable<Child> GetAllChildren(IEnumerable<Parent> parents){

  var result = new List<Child>();

  foreach(Parent parent in parents)
    foreach(Child child in parent.Children)
      result.Add(child);

  return result;
}

If you can collapse that into a single statement, try it on insane difficulty:

public IEnumerable<Child> GetAllChildren(IEnumerable<Grandparent> nanas){

  var result = new List<Child>();

  foreach(Grandparent papa in nanas)
    foreach(Parent parent in papa.Children)
      foreach(Child child in parent.Children)
        result.Add(child);

  return result;
}

Answer 1

This will work:

public IEnumerable<Child> GetAllChildren(IEnumerable<Parent> parents)
{
    return from parent in parents
           from child in parent.Children
           select child;
}

and then this:

public IEnumerable<Child> GetAllChildren(IEnumerable<Grandparent> nanas)
{
    return from papa in nanas
           from parent in papa.Children
           from child in parent.Children
           select child;
}

Note, in this example I'm not actually returning a list, I'm returning an IEnumerable data source that until you start to foreach over it, or similar, won't actually do any processing.

If you need to return a list, modify each return statement as follows:

    return (from .....
            ...
            select child).ToList();

Answer 2

Here's the obligatory method form.

return parents
  .SelectMany(p => p.Children);

And for two levels:

return oldies
  .SelectMany(grand => grand.Children)
  .SelectMany(parent => parent.Children);