How to generate Permutations with repetition recursively in C#

// 1: [6, 1, 1, 1, 1, 1, 1, 1, 1]
// 2: [1, 6, 1, 1, 1, 1, 1, 1, 1]
// 3: [1, 1, 6, 1, 1, 1, 1, 1, 1]
// 4: [1, 1, 1, 6, 1, 1, 1, 1, 1]
// 5: [1, 1, 1, 1, 6, 1, 1, 1, 1]
// 6: [1, 1, 1, 1, 1, 6, 1, 1, 1]
// 7: [1, 1, 1, 1, 1, 1, 6, 1, 1]
// 8: [1, 1, 1, 1, 1, 1, 1, 6, 1]
// 9: [1, 1, 1, 1, 1, 1, 1, 1, 6]

using System;

public static class PermutationsWithRep
{
    private static int numberOfCombos;

    public static void Main()
    {
        var collection = new[] { 6, 1, 1, 1, 1, 1, 1, 1, 1 };
        PermuteRep(collection);
    }

    private static void PermuteRep<T>(T[] workArray, int? end = null, int start = 0)
        where T : IComparable<T>
    {
        if (end == null)
        {
            end = workArray.Length - 1;
        }

        PrintPerm(workArray);

        for (int left = end.Value - 1; left >= start; left--)
        {
            for (int right = left + 1; right <= end; right++)
                if (workArray[left].CompareTo(workArray[right]) != 0)
                {
                    Swap(ref workArray[left], ref workArray[right]);
                    PermuteRep(workArray, end, left + 1);
                }

            var firstElement = workArray[left];

            for (int i = left; i <= end.Value - 1; i++)
            {
                workArray[i] = workArray[i + 1];
            }

            workArray[end.Value] = firstElement;
        }
    }

    private static void Swap<T>(ref T a, ref T b)
    {
        var temp = a;
        a = b;
        b = temp;
    }

    private static void PrintPerm<T>(T[] arr)
    {
        Console.WriteLine($"{++numberOfCombos}: [{string.Join(", ", arr)}]");
    }
}