All combinations and permutations of a set of characters to a given length in c#

I have a set of characters: "ab", I need every combinations and permutations of it to given length.
What I need sets like these if I give the length of 3:
a,
b,
aa,
ab,
ba,
bb,
aaa,
aab,
aba,
abb,
baa,
bab,
bba,
bbb

and so on.

I found a sort of good solution (https://stackoverflow.com/a/3640224/18535919), but I think it is a bit complicated. I have found a good solution in the past but I can not find it again.

static int maxlength = 3;
static string ValidChars = "abc";
static void Dive(string prefix, int level)
{
       level += 1;
       foreach (char c in ValidChars)
       {
            Console.WriteLine(prefix + c);
            if (level < maxlength)
            {
                Dive(prefix + c, level);
            }
      }
}

Your current code looks simple enough, however if you are looking for generalized approach which doesn't use recursion you can try something like this (frankly speaking it's more complicated, but can be used in general case):

Fiddle

using System.Linq;

...

private static IEnumerable<T[]> SolveMe<T>(int maxLength, IEnumerable<T> chars) {
  if (maxLength < 0)
    throw new ArgumentOutOfRangeException(nameof(maxLength));

  if (chars is null)
    throw new ArgumentNullException(nameof(chars));

  var letters = chars.Distinct().ToArray();

  if (letters.Length == 0)
     yield break;

  for (var agenda = new Queue<T[]>(new[] { Array.Empty<T>() }); 
           agenda.Peek().Length < maxLength;) {
    var current = agenda.Dequeue();

    foreach (var letter in letters) {
      var next = current.Append(letter).ToArray();

      agenda.Enqueue(next);

      yield return next;
    }
  }
}

Demo:

// All solutions as a list
List<char[]> result = SolveMe(3, "ab").ToList();

var report = string.Join(", ", result.Select(item => string.Concat(item)));

Console.Write(report);

Output:

a, b, aa, ab, ba, bb, aaa, aab, aba, abb, baa, bab, bba, bbb

You can achieve this using recursive techniques to generate all the combinations and permutations of the given characters. Check the below code:

class Program
{
    static void Main(string[] args)
    {
        string characters = "ab";
        int maxLength = 3;
        var output = GenerateCombinations(characters, maxLength, "");
        Console.WriteLine(string.Join(", ", output.Select(item => string.Concat(item))));
    }

    static IEnumerable<string> GenerateCombinations(string characters, int maxLength, string current)
    {
        if (current.Length > 0)
        {
            yield return current;
        }

        if (current.Length < maxLength)
        {
            foreach (char c in characters)
            {
                foreach (string result in GenerateCombinations(characters, maxLength, current + c))
                {
                    yield return result;
                }
            }
        }
    }
}

Output: a, aa, aaa, aab, ab, aba, abb, b, ba, baa, bab, bb, bba, bbb