How to AddRange many lists of IEnumerable<IEnumerable<T>> into IEnumerable<List<int>>

Question

I have those functions

    public static IEnumerable<IEnumerable<T>> GetPermutations<T>(IEnumerable<T> items, int count)
    {
        int i = 0;
        foreach (var item in items)
        {
            if (count == 1)
                yield return new T[] { item };
            else
            {
                foreach (var result in GetPermutations(items.Skip(i + 1), count - 1))
                    yield return new T[] { item }.Concat(result);
            }

            ++i;
        }
    }

    public static List<List<int>> GetAllValidCombinations(List<int> items)
    {
        var finalList = new List<List<int>>();
        switch (items.Count)
        {
            case 1:
                finalList.Add(items);
                break;
            case 3:
                finalList.AddRange(GetPermutations(items, 2));
                finalList.AddRange((List<List<int>>)GetPermutations(items, 3));
                break;
        }
        return finalList;
    }

and i want to get an List<List> from GetAllValidCombinations.

In the case 3 of GetAllValidCombinationsin the first line im getting: Error CS1503 Argument 1: cannot convert from 'System.Collections.Generic.IEnumerable<System.Collections.Generic.IEnumerable>' to 'System.Collections.Generic.IEnumerable<System.Collections.Generic.List>'

and if i try the second line im getting error Specified cast is not valid

How i can do this cast in one line?

Answer 1

Sweepers answer is on the money, also you could refactor it a little by only using lists in the inner collection and making it completely generic.

public static IEnumerable<List<T>> GetPermutations<T>(List<T> items, int count)
{
   for (var index = 0; index < items.Count; index++)
      if (count == 1)
         yield return new List<T> { items[index] };
      else
         foreach (var result in GetPermutations(items.Skip(index + 1).ToList(), count - 1))
            yield return new List<T> { items[index] }
               .Concat(result)
               .ToList();
}

public static IEnumerable<List<T>> GetAllValidCombinations<T>(List<T> items)
{
   if (items.Count == 1)
       return new List<List<T>> {items};

   if (items.Count == 3)
      return GetPermutations(items, 2)
         .Concat(GetPermutations(items, 3));

   return Enumerable.Empty<List<T>>();
}

Usage

var results = GetAllValidCombinations(new List<int>() {1, 2, 3});

foreach (var result in results)
   Console.WriteLine(string.Join(",",result));

Output

1,2
1,3
2,3
1,2,3

Full Demo Here