Getting Empty Parameter Value on DP Class

Dear All,

Below is my Code

[CONTRACT CLASS]

[DataContractAttribute]
public class ARMPurchContract

{

PurchId pid;

}

[DataMemberAttribute(identifierStr(PurchId))]
public PurchId parmPurchId(PurchId _PurchId = pid)
{
pid = _PurchId;
return pid;
}

[CONTROLLER CLASS]

class ArmPurchController extends SrsReportRunController
{
}

protected void prePromptModifyContract()
{
PurchTable PurchTableLocal;
ARMPurchContract contract;
PurchId pid;

;
super();

contract = this.parmReportContract().parmRdpContract() as ARMPurchContract;
PurchTableLocal = this.parmArgs().record() as PurchTable;

pid = PurchTableLocal.PurchId;

info(strFmt("id..%1",pid));

contract.parmPurchId(pid);

}

public static client void main(Args args)

{

ArmPurchController purchcontroller = new ArmPurchController();

purchcontroller.parmArgs(args);

purchcontroller.parmReportName(ssrsReportStr(PurchReport,PrecisionDesign1));

purchcontroller.parmShowDialog(false);
purchcontroller.startOperation();

}

[DP CLASS]

[
SRSReportParameterAttribute(classStr(ARMPurchContract))
]
class ArmPurchRDP extends SRSReportDataProviderBase
{
PurchReportTempdb purchReportTempdb;
}

[SRSReportDataSetAttribute(tablestr('PurchReportTempdb'))]
public PurchReportTempdb getPurchReportRDPTmp()
{
//select data from table
select * from purchReportTempdb;
//return the table
return purchReportTempdb;
}

[SysEntryPointAttribute]
public void processReport()
{
PurchId purchid;
PurchTable PurchTableLocal;
PurchLine PurchLineLocal;

ARMPurchContract ARMPurchContractLocal;

ARMPurchContractLocal = this.parmDataContract() as ARMPurchContract;
purchid = ARMPurchContractLocal.parmPurchId();    //Not getting value here

while select * from PurchLineLocal order by LineNumber where PurchLineLocal.PurchId == purchid
{
purchReportTempdb.clear();

purchReportTempdb.PurchId=PurchLineLocal.PurchId;
purchReportTempdb.ItemId=PurchLineLocal.ItemId;

purchReportTempdb.insert();
}

}