Frequency Analysis on a text (Caesar cipher + key search)

gistfile1.cs

static Dictionary<char, double> AnalyseFrequency(string text)
{
    if (text == null)
        return null;

    Dictionary<char, double> frequencies = new Dictionary<char, double>();
    int textLength = text.Length;

    for (int i = 0; i < textLength; i++)
    {
        char c = text[i];
        char key = '#';

        //ignore chars that are not letters
        if ((c >= 'a' && c <= 'z'))
            key = c;
        if (c >= 'A' && c <= 'Z')
            key = (char)(c + 'a' - 'A');

        if (frequencies.Keys.Contains(key))
            frequencies[key] = frequencies[key] + 1;
        else
            frequencies[key] = 1;
    }
    //cannot enumerate throught the dictionnay keys directly.
    List<char> keys = frequencies.Keys.ToList();

    foreach (char c in keys)
    {
        frequencies[c] /= textLength;
    }

    return frequencies;
}
frequencies = AnalyseFrequency(caesarCryptogram);
double maxFreq = frequencies.Values.Max();
char maxChar = frequencies.Keys.Where(c => frequencies[c] == maxFreq).FirstOrDefault();

//'E' is the maxChar
Console.WriteLine(string.Format("E is probably: {0} - {1}", maxChar, maxFreq));
Console.WriteLine(string.Format("Key would be: {0}", (char) (maxChar - 'e' + 'A')));