Need help, I am trying to make an array list which will add all of the invoices in the database to a drop down list, however, I am getting errors

//this function is used to populate the invoice class
       Int32 DisplayInvoices()
       {
           //create an instance of the county class
           clsInvoice Invoice = new clsInvoice();
           //var to store the invoice no primary key
           string InvoiceNo;
           //var to store the name of the invoice
           string Invoice;
           //var to store the index for the loop
           Int32 Index = 0;
           //while the index is less that the number of records to process
           while (Index < Invoices.Count)
           {
               //get the invoice number from the database
               InvoiceNo = Convert.ToString(Invoices.AllInvoices[Index].InvoiceNo);
               //get the invoice name from the database
               Invoice = Invoices.AllInvoices[Index].Invoice;
               //set up the new row to be added to the list
               ListItem NewInvoice = new ListItem(Invoice, InvoiceNo);
               //add the new row to the list
               ddlInvoice.Items.Add(NewInvoice);
               //increment the index to the next record
               Index++;
           }
           //return the number of records found
           return Invoices.Count;

       }