GWS with filter query goes wrong in C# program

I created GWS and the URI is like:

/sap/opu/odata/sap/ZGW_GET_PO_SRV/PurchaseOrdersSet?$filter=PURCHORD eq 'XXXX' and PURCHORD eq 'XXXX'

I have 5 filters like PURCHORD ,VENDORNO ,Material... , meanwhile I set PURCHORD ,VENDORNO as the Key.

URL testing looks good.

But when this GWS goes to C# program(it's invoked as a service), and some coding like below to simulate filter value:

var uri = newUri(ConfigurationManager.AppSettings["PO"]);

var container = new PO.ZGW_GET_PO_SRV_Entities(uri);

try

{

var po = container.PurchaseOrdersSet.Where(p => p.PURCHORD == "XXXX"&& p.VENDORNO == "XXXX");

At the C# debug point I found the URL comes out is like below:

XXXXXXX/sap/opu/odata/sap/ZGW_GET_PO_SRV/PurchaseOrdersSet(PurchOrd='XXXX',VendorNo='XXXX')

That is Read not the Query so error happened. I know the reason is "the filter fields are the same as the key".

And if I change the key of the GWS(to avoid equal to filter fields), C# will come out right URL.

Should the C# program be changed? and How? Thanks.