How to get all the unique n-long combinations of a set of duplicatable elements?

Question

I have found many solutions giving a collection elements combined in all possible orders but they all use every element just once in every result while I need them to be tr

Answer 1

You can use a depth first search:

class Program
{
    private static string[] letters = {"a", "b", "c"};
    private static void dfs(string accum, int depth)
    {
        if (depth == 0)
        {
            System.Console.WriteLine(accum);
            return;
        }
        foreach (string c in letters)
            dfs(accum + c, depth - 1);
    }
    public static void Main()
    {
        int depth = 2; //Number of letters in each result
        dfs("", depth);
        System.Console.ReadLine();
    }
}

Output:

aa
ab
ac
ba
bb
bc
ca
cb
cc