internal void AppendUnicodeHexadecimal(int charCode)
{
if (charCode < 0 || charCode > 0xFFFF)
throw new ArgumentOutOfRangeException(nameof(charCode));
AppendBackslash('u');
AppendDirect(charCode.ToString("X4", CultureInfo.InvariantCulture).PadLeft(4, '0'));
}