How to generate Combinations with repetition recursively in C#

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//  1: [0, 0]
//  2: [0, 1]
//  3: [0, 2]
//  4: [0, 3]
//  5: [1, 1]
//  6: [1, 2]
//  7: [1, 3]
//  8: [2, 2]
//  9: [2, 3]
// 10: [3, 3]
 
using System;
 
public static class CombinationsWithRep
{
    private static int numberofCombos;
    private static int n;
    private static int k;
    private static int[] storageArr;
 
    public static void Main()
    {
        n = 4;
        k = 2;
        storageArr = new int[k];
 
        GenCombinationsWithRep();
    }
 
    private static void GenCombinationsWithRep(int index = 0, int element = 0)
    {
        if (index >= storageArr.Length)
        {
            PrintCombo();
            return;
        }
 
        for (int i = element; i < n; i++)
        {
            storageArr[index] = i;
            GenCombinationsWithRep(index + 1, i);
        }
    }
 
    private static void PrintCombo()
    {
        Console.WriteLine(
            "{0,3}: [{1}]",
            ++numberofCombos,
            string.Join(", ", storageArr));
    }
}