problem overriding connection string from web.config file

Question

hi all,

i want use my web.config file connectionstring to my crystal report.my code is given below.

protected void Page_Load(object sender, EventArgs e)

    {

        TextBox1.Text = Session["Selected"].ToString();

        TextBox2.Text = Session["ReportBy"].ToString();

        TextBox3.Text = Session["FromDate"].ToString();

        TextBox4.Text = Session["ToDate"].ToString();

       ReportDocument repDoc = new ReportDocument();

       string MyconnectionString = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;

     SqlConnectionStringBuilder SConn = new SqlConnectionStringBuilder(MyconnectionString);

      repDoc.DataSourceConnections[0].SetConnection(SConn.DataSource, SConn.InitialCatalog, SConn.UserID, SConn.Password);

       repDoc.Load(Server.MapPath("Report/RptExpiryListByitem.rpt"));

ParameterDiscreteValue objDiscreteValue;

        ParameterField objParameterField;

        objDiscreteValue = new ParameterDiscreteValue();

        objParameterField = new ParameterField();

        objDiscreteValue.Value =TextBox1.Text;

        objParameterField = CrystalReportViewer1.ParameterFieldInfo["@strIDs"];

        objParameterField.CurrentValues.Add(objDiscreteValue);

        CrystalReportViewer1.ParameterFieldInfo.Add(objParameterField);

        objDiscreteValue.Value = TextBox2.Text;

        objParameterField = CrystalReportViewer1.ParameterFieldInfo["@intFilterType"];

        objParameterField.CurrentValues.Add(objDiscreteValue);

        CrystalReportViewer1.ParameterFieldInfo.Add(objParameterField);

        objDiscreteValue.Value = TextBox3.Text;

        objParameterField = CrystalReportViewer1.ParameterFieldInfo["@dtFromDate"];

        objParameterField.CurrentValues.Add(objDiscreteValue);

        CrystalReportViewer1.ParameterFieldInfo.Add(objParameterField);

        objDiscreteValue.Value = TextBox4.Text;

        objParameterField = CrystalReportViewer1.ParameterFieldInfo["@dtToDate"];

        objParameterField.CurrentValues.Add(objDiscreteValue);

        CrystalReportViewer1.ParameterFieldInfo.Add(objParameterField);

        CrystalReportViewer1.ReportSource =repDoc;

    }

when i run my apllication and pass the parameter to the report its give the error ..

System.NullReferenceException: Object reference not set to an instance of an object.

at line 

 string MyconnectionString = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;

please anyone provide me the solution for this problem..

thanks