Passing Parameter to crystal report is not working

Hello everyone. I have developed a web based inventory and accounts system using c# in VS2010, Sql server 2008 and Crystal Reports for VS2010. I am Passing parameters to a test crystal report as under the following which seems to work fine only for the first time but don't update the parameter value subsequent times.

privatevoidSetDate1AndDate2Parameters(ReportDocumentRptDoc)
    {
        ParameterFieldDefinition PFD =RptDoc.DataDefinition.ParameterFields["ParamDate1"];
        ParameterValues PV =newParameterValues();
        ParameterDiscreteValue PDV=newParameterDiscreteValue();

        PDV.Value=Convert.ToDateTime(TextBox1.Text);
        PV.Add(PDV);
        PFD.ApplyCurrentValues(PV);
}

When i change the value for the parameter that changed value is not reflected in the parameter on crystal reports. There is another way of passing parameters i have come to know after googling i.e. to use crystalreportviewer.ParameterFieldsInfo i.e.

privatevoidSetDate1AndDate2Parameters(ReportDocumentRptDoc)
    {

        ParameterFieldsPFs=newParameterFields();
        ParameterFieldPFDate1=newParameterField();
        ParameterDiscreteValuePDVDate1=newParameterDiscreteValue();
        PFDate1.Name="ParamDate1";
        PDVDate1.Value=Convert.ToDateTime(Date1TextBox.Text.Trim());
        PFDate1.CurrentValues.Add(PDVDate1);
        PFs.Add(PFDate1);

        ParameterFieldPFDate2=newParameterField();
        ParameterDiscreteValuePDVDate2=newParameterDiscreteValue();
        PFDate2.Name="ParamDate2";
        PDVDate2.Value=Convert.ToDateTime(Date2TextBox.Text.Trim());
        PFDate2.CurrentValues.Add(PDVDate2);
        PFs.Add(PFDate2);

        // Method
        ParameterFieldPFMethod=newParameterField();
        ParameterDiscreteValuePDVMethod=newParameterDiscreteValue();
        PFMethod.Name="ParamMethod";
        PDVMethod.Value=MethodDDL.SelectedValue;
        PFMethod.CurrentValues.Add(PDVMethod);
        PFs.Add(PFMethod);

        // for Currency.

        ParameterFieldPFCurrency=newParameterField();
        ParameterDiscreteValuePDVCurrency=newParameterDiscreteValue();
        PFCurrency.Name="ParamCurrency";
        PDVCurrency.Value=CurrencyDDL.SelectedValue;
        PFCurrency.CurrentValues.Add(PDVCurrency);
        PFs.Add(PFCurrency);

        // for currency rates
        ParameterFieldPFCurrencyRate=newParameterField();
        ParameterDiscreteValuePDVCurrencyRate=newParameterDiscreteValue();
        PFCurrencyRate.Name="ParamCurrencyRate";
        PDVCurrencyRate.Value=GetCurrentCurrencyRate();
        PFCurrencyRate.CurrentValues.Add(PDVCurrencyRate);
        PFs.Add(PFCurrencyRate);

        CrystalReportViewer1.ParameterFieldInfo=PFs;
       
    }

By using CrystalReportViewer1.ParameterFieldInfo is working for me but when the page is postback by pressing any other button e.g. a Button to show up Modal Box to change the parameters or Refresh button that will cause the report to get refresh is causing Error to display on CrystalReport viewer. I am unable to figure out what event i do require to handle so that this Error thing doesnot come.

I am desperately waiting for your guidance.