How to generate Permutations with repetition recursively in C#

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// 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)}]");
    }
}

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