How to find the DocEntry of a new Invoice?

I can create an invoice using the DI API SAPbobsCOM ok but can anyone tell me how to obtain the unique reference number [DocEntry] of the new OINV record?

After oInvoice.Add it would be nice if oInvoice.DocEntry were set so that I could retrieve it, but it is not.

Thanks for any help offered.

Mal.