Linq to SQL C# linq join example

             try{
            LinqDataObjectsDataContext dc = new LinqDataObjectsDataContext();

            /*  select * from invoice inner join AccountInvoices on 
             *  invoice.cust= AccountInvoices.cust and invoice.sub = AccountInvoices.sub             
             */

            StringBuilder sb = new StringBuilder();
            sb.AppendFormat("<Invoices customer='{0}' sub='{1}'>", cust, sub);

            var ret  =  from invoice in dc.Invoices
                          join accountInvoice in  dc.AccountInvoices on
                          invoice.InvoiceID equals accountInvoice.InvoiceID
                          where accountInvoice.SubID ==sub && accountInvoice.CustomerID ==cust
                          select invoice;

            foreach (Invoice i in ret){
                sb.Append(CXMLConv.toXML(i));
            }

            sb.AppendLine("</Invoices>");
            return OK(sb.ToString());
            } catch (Exception ex) {
                 return RegisterError(ex, "getInvoicesXML","cust/sub"+cust+"/"+sub);
            }

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