[DelimitedRecord(",")]
public partial class Person : INotifyWrite<Person>
{
    public string FirstName;
    public string LastName;
    [FieldOptional]
    public string Optional1;
    [FieldOptional]
    public string Optional2;
    [FieldOptional]
    public string Optional3;

    public void BeforeWrite(BeforeWriteEventArgs<Person> e)
    {
    }

    public void AfterWrite(AfterWriteEventArgs<Person> e)
    {
        // count the non-optional fields
        var numberOfNonOptionalFields = typeof(Person).GetFields()
            .Where(f => !f.GetCustomAttributes(false).Any(a => a is FieldOptionalAttribute))
            .Count();

        // take only the first n tokens
        e.RecordLine = String.Join(",", e.RecordLine.Split(',').Take(numberOfNonOptionalFields));
    }
}

class Program
{
    private static void Main(string[] args)
    {
        var engine = new FileHelperEngine<Person>();
        var export = engine.WriteString(
                     new Person[] {
                       new Person() {
                           FirstName = "Joe",
                           LastName = "Bloggs",
                           Optional1 = "Option 1",
                           Optional2 = "Option 2",
                           Optional3 = "Option 3"
                       }
                     });
        Assert.AreEqual("Joe,Bloggs" + Environment.NewLine, export);
        Console.WriteLine("Export was as expected");
        Console.ReadLine();
    }
}