public string ToggleCase(string input)
{
  string result = string.Empty;
  char[] inputArray = input.ToCharArray();
  foreach (char c in inputArray)
  {
    if (char.IsLower(c))
      result += c.ToString().ToUpper();
    else if (char.IsUpper(c))
      result += c.ToString().ToLower();
    else
      result += c.ToString();
  }
  return result;
}

public string ToggleCase(string input)
{
  string result = string.Empty;
  char[] inputArray = input.ToCharArray();
  foreach (char c in inputArray)
  {
    if (char.IsLower(c))
      result += Char.ToUpper(c);
    else
      result += Char.ToLower(c);
  }
  return result;
}