AlphabetVariationGenerator

private IEnumerable<string> GetAlphabetVariations()
{
    char[] alphabet = "abcdefghijklmnopqrstuvwxyz".ToCharArray();

    foreach (var first in alphabet)
    {
        foreach (var second in alphabet)
        {
            yield return "" + first + second;
        }
    }
    // Use recursion for longer strings.
}