How to Toggle String Case in .NET

public static string ToggleCase(this string str)
{
     if (string.IsNullOrEmpty(str)) return str;
     return string.Join("", (from i in str
                             select (char.IsLetter(i) ?
                                  (char.IsUpper(i) ?
                                        char.ToLower(i) :
                                        char.ToUpper(i)) :
                                   i).ToString()
                             ).ToArray());
}