How to Toggle String Case in .NET

string ToggleStringCase(string strToToggle)
{
   StringBuilder sb = new StringBuilder();
   foreach(char ch in strToToggle.ToCharArray())
   {
      if((int)ch>=65&&(int)ch<91)
      {
        sb.Append((char)((int)ch+32));
      }
      else if((int)ch>=97&&(int)ch<123)
      {
        sb.Append((char)((int)ch-32))
      }
      else
      {
        sb.Append(ch);
      }
    }
   return sb.ToString();
}