Custom Collection Serialization

[akilin]

hi, I am trying to read/write a collection of objects with next structure

public class Contact
{
    public int Id { get; set; }
    public string Name { get; set; }
    public List<string> Pets { get; set; }
    public List<string> PhoneNumbers { get; set; }
}

but I have a specific requirement about how collections of Pets/PhoneNumbers should be serialised.
1 column per pet/phone number. user can have 0..100+ pets/phone numbers.

csv file should look something like

Id,Name,Pet1,Pet2,Phone1,Phone2
1,TestName,FirstPet,SecondPet,+3801,+3802
2,SecondUser,FirstPet,,+1234,

I managed to write a correct Map that is able to read this file

public class ContactMap : ClassMap<Contact>
    {
        public ContactMap()
        {
            Map(m => m.Id).Name("Id");
            Map(m => m.Name).Name("Name");
            Map(m => m.Pets).Convert(row =>
                row.Row.HeaderRecord.Where(header => header.StartsWith("Pet"))
                     .Select(header => row.Row.GetField<string>(header))
                     .Where(value => !string.IsNullOrWhiteSpace(value))
                     .ToList()
            );

            Map(m => m.PhoneNumbers).Convert(row =>
                row.Row.HeaderRecord.Where(header => header.StartsWith("Phone"))
                     .Select(header => row.Row.GetField<string>(header))
                     .Where(value => !string.IsNullOrWhiteSpace(value))
                     .ToList()
            );
        }
    }

but I am having issues creating a write part for it.
(there can be a variable number of pets/phones. on average it will be in range of 1..100. I can't just always use 100 columns for each collection).

could you please advise on a best way to handle write operation for such a structure?