Quotes should be added if field contains QuoteChar for QuoteMode.OptionalForWrite and QuoteMode.OptionalForBoth

[mordio]

In FileHelpers/FileHelpers/Fields/DelimitedField.cs

            // Add Quotes If:
            //     -  optional == false
            //     -  is optional and contains the separator
            //     -  is optional and contains a new line

            if ((QuoteChar != '\0') &&
                (QuoteMode == QuoteMode.AlwaysQuoted ||
                 QuoteMode == QuoteMode.OptionalForRead ||
                 ((QuoteMode == QuoteMode.OptionalForWrite || QuoteMode == QuoteMode.OptionalForBoth)
                  && mCompare.IndexOf(field, Separator, CompareOptions.Ordinal) >= 0) || hasNewLine))
                StringHelper.CreateQuotedString(sb, field, QuoteChar);
            else
                sb.Append(field);

should be changed to

            // Add Quotes If:
            //     -  optional == false
            //     -  is optional and contains the separator
            //     -  is optional and contains the quote character
            //     -  is optional and contains a new line

            if ((QuoteChar != '\0') &&
                (QuoteMode == QuoteMode.AlwaysQuoted ||
                 QuoteMode == QuoteMode.OptionalForRead ||
                 ((QuoteMode == QuoteMode.OptionalForWrite || QuoteMode == QuoteMode.OptionalForBoth) && 
                  (mCompare.IndexOf(field, Separator, CompareOptions.Ordinal) >= 0 || 
                   mCompare.IndexOf(field, QuoteChar, CompareOptions.Ordinal) >= 0)) || 
                 hasNewLine))
                StringHelper.CreateQuotedString(sb, field, QuoteChar);
            else
                sb.Append(field);

according to RFC 4180:

Each field may or may not be enclosed in double quotes (however some programs, such as Microsoft Excel, do not use double quotes at all). If fields are not enclosed with double quotes, then double quotes may not appear inside the fields.

If the user inputs a sole double quote:
1.1,",,",13772/8681,100,100,1,0,,,,,100,100

In that case two fields are omitted by the reading application. My workaround is to strip sole double quotes because in my case I don't need them. I just wanted to add this here since I stumbled on it today.