using System;

using System.Collections.Generic;

using System.Linq;

public class Program {

  private static IEnumerable<T[]> SolveMe<T>(int maxLength, IEnumerable<T> chars, IEqualityComparer<T> comparer = default) {

    if (maxLength < 0)

      throw new ArgumentOutOfRangeException(nameof(maxLength));

​

    if (chars is null)

      throw new ArgumentNullException(nameof(chars));

​

    var letters = chars.Distinct(comparer ?? EqualityComparer<T>.Default).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;

      }

    }

  } 

  public static void Main() {

    List<char[]> result = SolveMe(3, "ab").ToList();

​

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

​

    Console.Write(report);  

  }

}