Uploading Invoice details

Hi Experts
I want upload(add) Invoice detail by using Web service.
but i am not able to add it.
Please can any body suggest to how to do it.
what is the code should i write.
to add the invoice .I try following code it is not working

public void addinv()
        {
            this.login(true);
            RecordRef custenty = new RecordRef();
            custenty.internalId = "31203";
           Invoice inv = new Invoice();
           inv.entity = custenty;
           RecordRef item1 = new RecordRef();
           item1.internalId = "1539";
        
            InvoiceItemList list = new InvoiceItemList();
          // list.item =item1;
          // list.item = 1539;
          //cust.entityID= " xyx xx" ;   
            try
            {
                WriteResponse response = _service.add(inv);

                Console.Write("Succes invoice");
            }
            catch (Exception e)