System.Net.Mail.MailAddressParser.ParseLocalPart C# (CSharp) Method

        private static string ParseLocalPart(string data, ref int index, bool expectAngleBracket,
            bool expectMultipleAddresses)
        {
            // Skip comments and whitespace
            index = ReadCfwsAndThrowIfIncomplete(data, index);

            // Mark the start of the local-part
            int startingIndex = index;

            // Is the local-part component in quoted-string format or dot-atom format?
            if (data[index] == MailBnfHelper.Quote)
            {
                index = QuotedStringFormatReader.ReadReverseQuoted(data, index, true);
            }
            else
            {
                index = DotAtomReader.ReadReverse(data, index);

                // Check that the local-part is properly separated from the next component. It may be separated by a 
                // comment, white space, an expected angle bracket, a quote for the display-name, or an expected comma 
                // before the next address.
                if (index >= 0 &&
                        !(
                            MailBnfHelper.IsAllowedWhiteSpace(data[index]) // < local@domain >
                            || data[index] == MailBnfHelper.EndComment // <(comment)local@domain>
                            || (expectAngleBracket && data[index] == MailBnfHelper.StartAngleBracket) // <local@domain>
                            || (expectMultipleAddresses && data[index] == MailBnfHelper.Comma) // local@dom,local@dom
                                                                                               // Note: The following condition is more lax than the RFC.  This is done so we could support 
                                                                                               // a common invalid formats as shown below.
                            || data[index] == MailBnfHelper.Quote // "display"local@domain
                        )
                    )
                {
                    throw new FormatException(SR.Format(SR.MailHeaderFieldInvalidCharacter, data[index]));
                }
            }

            string localPart = data.Substring(index + 1, startingIndex - index);

            index = WhitespaceReader.ReadCfwsReverse(data, index);

            return NormalizeOrThrow(localPart);
        }