private static char[] base26Chars =  "abcdefghijklmnopqrstuvwxyz".ToCharArray();
 
public static string Base26Encode(Int64 value) {
    string returnValue = null;
    do {
        returnValue = base26Chars[value % 26] + returnValue;
        value /= 26;
    } while (value-- != 0);
 
    return returnValue;