Generate some readable IDs

IdGenerator.cs

    public static class IdGenerator
    {
        private static readonly ConcurrentDictionary<string, IEnumerator<string>> _generators = new ConcurrentDictionary<string, IEnumerator<string>>();
        public static string Next(string prefix)
        {
            var generator = _generators.GetOrAdd(prefix, x => MakeSequence(prefix).GetEnumerator());

            generator.MoveNext();
            return generator.Current;
        }

        public static IEnumerable<string> MakeSequence(string prefix)
        {
            var letters = Enumerable.Range(0, 26)
               .Select(i => new string((char)(i + 'A'), 1))
               .ToList();

            var doubleLetters =
                from x in letters
                from y in letters
                select $"{x}{y}";

            var tripleLetters =
                from x in letters
                from y in letters
                from z in letters
                select $"{x}{y}{z}";

            var sequence = letters.Concat(doubleLetters).Concat(tripleLetters);

            foreach (var item in sequence)
            {
                yield return $"{prefix}{item}";
            }
        }
    }