Crystal report viewer toolbar not working in asp.net c#

Hi All,

I am using sap crystal reports in visual studio 2010

report is loading properly.

when i click any thing on toolbar(zoom,search,page navigation,export) it says no valid report source is available

I am using following code

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using CrystalDecisions.CrystalReports.Engine;
using CrystalDecisions.Shared;
using System.Data;

public partial class Reports_ClassWise : System.Web.UI.Page
{
    ReportDocument reportdocument;
    protected void Page_Init(object sender, EventArgs e)
    {
        if (reportdocument == null)
        {
            reportdocument = new ReportDocument();
        }
        if (!Page.IsPostBack)
        {            
            loadClasses();
            ddlClass.Focus();
            
        }
    }
    protected void Page_Load(object sender, EventArgs e)
    { 
    }
    protected void Page_Unload(object sender, EventArgs e)
    {
        if (reportdocument != null)
        {
            reportdocument.Close();
            reportdocument.Dispose();
        }
    }
  
    private void SearchStudent()
    {
        
        ParameterField paramField = new ParameterField();
        ParameterField paramField1 = new ParameterField();
        ParameterFields paramFields = new ParameterFields();
        ParameterDiscreteValue paramDiscreteValue = new ParameterDiscreteValue();
        ParameterDiscreteValue paramDiscreteValue1 = new ParameterDiscreteValue();


        paramField.Name = "@classid";
        paramDiscreteValue.Value = Convert.ToInt64(ddlClass.SelectedValue.ToString());

        paramField.CurrentValues.Add(paramDiscreteValue);
        paramFields.Add(paramField);

        paramField1.Name = "@ClassName";        
        paramDiscreteValue1.Value = ddlClass.SelectedItem.Text;

        paramField1.CurrentValues.Add(paramDiscreteValue1);
        paramFields.Add(paramField1);

        CrystalReportViewer1.ParameterFieldInfo = paramFields;
        reportdocument.Load(Server.MapPath("~/CrystalReports/Student/ClassWise.rpt"));
        reportdocument.FileName = Server.MapPath("~/CrystalReports/Student/ClassWise.rpt");
        reportdocument.SetDatabaseLogon(Global.User, Global.Pwd, Global.Server, Global.Db);
        CrystalReportViewer1.ReportSource = reportdocument;

    }
    private void loadClasses()
    {
        ClassBAL ob = new ClassBAL();
        DataTable dt = ob.getAllClasses();
        ddlClass.DataSource = dt;
        ddlClass.DataTextField = "ClassName";
        ddlClass.DataValueField = "ClassId";
        ddlClass.DataBind();
        ddlClass.Items.Insert(0, new ListItem("All", "0"));

    }
    protected void LbSearch_Click(object sender, EventArgs e)
    {
        SearchStudent();
        ddlClass.Focus();
    }
}

how can i use toolbar options?

Thanks,