How to pass parameter to Crystal reports.

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C#

C#4.0

Hi,
I am getting Error as follows. any one can tell me why i am getting this error i am new in crystal reports.

System.NullReferenceException was caught
Message=Object reference not set to an instance of an object.
Source=StaticData
StackTrace:
     at StaticData.View.AbstractFormUpdate.GetReport(Int32 rptID) in F:\25Dec2011\CMSWorldApp-5-7-2011\StaticData\View\AbstractFormUpdate.cs:line 505
InnerException:

I want to pass parameter at run time to crystal report i have wrote this code for that
CODE:

ParameterFieldDefinitions varParameterFieldDefinitions;
            ParameterFieldDefinition varParameterFieldDefinition;
            ParameterValues varParameterValues=null;
            ParameterDiscreteValue varParameterDiscreteValue = new    ParameterDiscreteValue();
            try
            {
                dr = GetParameterAndValue(rptID);
                Temp = GetRptTempName(rptID);// geting path of Report Template
                varParameter=dr["ParameterName"].ToString();
                varParameterValue = dr["ParamValue"].ToString();


                if (System.IO.File.Exists(Temp))
                {
                    O_Report.Load(Temp);
                    O_Report.SetDatabaseLogon(userid, password, serverName, databaseName);
                }
                else
                    DXMessageBox.Show("The source file does not exist.");

                varParameterDiscreteValue.Value = varParameterValue;
                varParameterFieldDefinitions = O_Report.DataDefinition.ParameterFields;
                varParameterFieldDefinition=varParameterFieldDefinitions[varParameter];

               // varParameterValues.Clear();
                varParameterValues.Add(varParameterDiscreteValue);
              varParameterFieldDefinition.ApplyCurrentValues(varParameterValues);

                RptViewer.ReportSource = O_Report;

Thanks in advanced.