C# implementation for a combination with repetition. Implemented as an extension method for IEnumerable<char>.

CombineWithRepetition.cs

public static IEnumerable<IList<char>> CombineWithRepetitions(this IEnumerable<char> input, int take)
{
    ICollection<IList<char>> output = new Collection<IList<char>>();
    IList<char> item = new char[take];

    CombineWithRepetitions(output, input, item, 0);

    return output;
}

private static void CombineWithRepetitions(ICollection<IList<char>> output, IEnumerable<char> input, IList<char> item, int count)
{
    if (count < item.Count)
    {
        var enumerable = input as IList<char> ?? input.ToList();
        foreach (var symbol in enumerable)
        {
            item[count] = symbol;
            CombineWithRepetitions(output, enumerable, item, count + 1);
        }
    }
    else
    {
        output.Add(new List<char>(item));
    }
}