Flatten list problem - cannot be inferred from usage

[Techno]

So I have a List<> of items.
each item has a list of other items (grandparent > parent > child > children etc...)

I want to now flatten the list so all the items are in just one list.

how can I do this?

I tried:

List<Items> flat = structuredItems.SelectMany(m => m);

but I get a compiler error:

Error	202	The type arguments for method 'System.Linq.Enumerable.SelectMany<TSource,TResult>(System.Collections.Generic.IEnumerable<TSource>, System.Func<TSource,System.Collections.Generic.IEnumerable<TResult>>)' cannot be inferred from the usage. Try specifying the type arguments explicitly.