public class CaseAlternatorTask
{
    public static List<string> AlternateCharCases(string lowercaseWord)
    {
        var result = new List<string>();
        AlternateCharCases(lowercaseWord.ToCharArray(), 0, result);
        return result;
    }

    static void AlternateCharCases(char[] word, int startIndex, List<string> result)
    {
        if(startIndex == word.Length)
        {
            result.Add(new string(word));
            return;
        }

        if (Char.IsLetter(word[startIndex]))
        {
            var word1 = word.ToArray();
            var word2 = word.ToArray();

            word1[startIndex] = Char.ToLower(word1[startIndex]);
            word2[startIndex] = Char.ToUpper(word2[startIndex]);

            AlternateCharCases(word1, startIndex+1, result);
            if(word1[startIndex] != word2[startIndex])
                AlternateCharCases(word2, startIndex+1, result);
        }else
            AlternateCharCases(word, startIndex+1, result);
    }
}