public static IEnumerable<T> Traverse<T>(this IEnumerable<T> source, Func<T, IEnumerable<T>> fnRecurse)
{
    foreach (T item in source)
    {
        yield return item;

        IEnumerable<T> seqRecurse = fnRecurse(item);
        if (seqRecurse != null)
        {
            foreach (T itemRecurse in Traverse(seqRecurse, fnRecurse))
            {
                yield return itemRecurse;
            }
        }
    }
}

http://stackoverflow.com/questions/12318372/linq-nested-list-contains

Credit: Tim Schmelter

Traverse nested lists (IEnumerable)

c#

C#