filebuilder

private void AutoGenEntryDetailSequenceNumbers()
{
  AchFile.Batches.ToList().ForEach(batch =>
  {
    var currentMaxSequenceNumber = batch.EntryDetails.Max(d => d.EntryDetailSequenceNumber);
    batch.EntryDetails.OrderBy(d => d.EntryDetailSequenceNumber).ToList()
      .ForEach(d => d.EntryDetailSequenceNumber = d.EntryDetailSequenceNumber ?? ++currentMaxSequenceNumber);
  });
}


-----------------------
new:

private void AutoGenEntryDetailSequenceNumbers()
    {
      AchFile.Batches.ToList().ForEach(batch =>
      {
        var currentMaxSequenceNumber = 1;
        batch.EntryDetails.ToList()
          .ForEach(d => 
            {
              d.EntryDetailSequenceNumber = d.EntryDetailSequenceNumber ?? currentMaxSequenceNumber;
              ++currentMaxSequenceNumber;
            });
      });
    }