To Order By Descending sub-elements using LINQ or Lambda Expression

Question

I had a problem to order sub-elements in List and found the topic to discuss List<T> vs IEnumerable<T> in foreach .I wonder how to order child List by using LINQ or Lambda Expression. Can someone recommend me? the code example as follows:

public class Parent
{
    // Other properties...
    public IList<Child> Children { get; set; }
}

public IEnumerable<Parent> DoStuff()
{
    var result = DoOtherStuff() // Returns IEnumerable<Parent>
        .OrderByDescending(SomePredicate) 
        .ThenBy(AnotherPredicate); // This sorting works as expected in the return value.

    foreach (Parent parent in result)
    {
        parent.Children = parent.Children.OrderBy(YetAnotherPredicate).ToList();
        // When I look at parent.Children here in the debugger, it's sorted properly.
    }

    return result;
    // When I look at the return value, the Children are not sorted.
}

Answer 1

You enumerate result each time that you enumerate the return IEnumerable of DoStuff , plus one additional time inside the loop in DoStuff itself. Modifications you make inside the loop do not stay, though, because of deferred execution : next time you enumerate DoStuff() there is another call to DoOtherStuff etc.

You can fix this in several ways:

• Convert result to list before sorting children, ie

  DoOtherStuff() // Returns IEnumerable<Parent> .OrderByDescending(SomePredicate) .ThenBy(AnotherPredicate) .ToList();

• Add child sorting in a Select :

   DoOtherStuff() // Returns IEnumerable<Parent> .Select(parent => { parent.Children = parent.Children.OrderBy(YetAnotherPredicate).ToList(); return parent; }) .OrderByDescending(SomePredicate) .ThenBy(AnotherPredicate) .ToList();

• Use yield return result in the loop (this is a variation of the Select solution).