i have some problem when i pass values to my crystal report page

basically, on page load first time i can bind  and show the report

but the problem is here...

when i click on export, print, next/previous page, zoom, navigate

my page told me "Missing parameter value"

here is all of my code

1    using System;
2    using System.Data;
3    using System.Data.SqlClient;
4    using System.Web.Configuration;
5    using System.Configuration;
6    using System.Web;
7    using System.Web.Security;
8    using System.Web.UI;
9    using System.Web.UI.WebControls;
10   using System.Web.UI.WebControls.WebParts;
11   using System.Web.UI.HtmlControls;
12   using CrystalDecisions.CrystalReports.Engine;
13   using CrystalDecisions.Shared;
14   using dsShippingSummByProductTableAdapters;
15   
16   public partial class Admin_AdminCrShippingSummByProduct : System.Web.UI.Page
17   {
18       string fDate;
19       string tDate;
20       string viewBy;
21       string fullName;
22       ReportDocument report = new ReportDocument();
23       ParameterFields pfields = new ParameterFields();
24   
25       protected void Page_Load(object sender, EventArgs e)
26       {
27           fDate = Session["FDATE"].ToString();
28           tDate = Session["TDATE"].ToString();
29           viewBy = Session["VIEWBY"].ToString();
30   
31           string staffID = "0000000001";              //Integrate remove this line 
32           Session["staffID"] = staffID;               //Integrate remove this line 
33           Session["StaffID"] = Session["staffID"];
34   
35           string strConn = "Data Source=MIXAR-PC;Initial Catalog=StockProductInfo;Integrated Security=True";
36   
37           SqlConnection Conn = new SqlConnection(strConn);
38           if (Conn.State == ConnectionState.Open)
39           {
40               Conn.Close();
41           }
42           Conn.ConnectionString = strConn;
43           Conn.Open();
44   
45           string sqlStaffName = "select Staff_FName, Staff_LName ";
46           sqlStaffName += "from masStaff ";
47           sqlStaffName += "where Staff_ID = " + Session["StaffID"].ToString();
48   
49           SqlDataAdapter daStaff = new SqlDataAdapter(sqlStaffName, Conn);
50           DataSet dsStaff = new DataSet();
51           daStaff.Fill(dsStaff, "Staff");
52   
53           string fName = dsStaff.Tables["Staff"].Rows[0]["Staff_FName"].ToString();
54           string lName = dsStaff.Tables["Staff"].Rows[0]["Staff_LName"].ToString();
55           fullName = fName + " " + lName;
56   
57           ParameterField pfieldFDATE = new ParameterField();
58           ParameterDiscreteValue disValFDATE = new ParameterDiscreteValue();
59           disValFDATE.Value = DateTime.Parse(fDate);	
60           pfieldFDATE.Name = "FDate";		
61           pfieldFDATE.CurrentValues.Add(disValFDATE);
62           
63           ParameterField pfieldTDATE = new ParameterField();
64           ParameterDiscreteValue disValTDATE = new ParameterDiscreteValue();
65           disValTDATE.Value = DateTime.Parse(tDate);	
66           pfieldTDATE.Name = "TDate";
67           pfieldTDATE.CurrentValues.Add(disValTDATE);
68   
69           ParameterField pfieldVIEWBY = new ParameterField();
70           ParameterDiscreteValue disValVIEWBY = new ParameterDiscreteValue();
71           disValVIEWBY.Value = viewBy;	
72           pfieldVIEWBY.Name = "ViewBy";
73           pfieldVIEWBY.CurrentValues.Add(disValVIEWBY);
74   
75           ParameterField pfieldSTAFF = new ParameterField();
76           ParameterDiscreteValue disValSTAFF = new ParameterDiscreteValue();
77           disValSTAFF.Value = fullName;
78           pfieldSTAFF.Name = "StaffName";
79           pfieldSTAFF.CurrentValues.Add(disValSTAFF);
80   
81           pfields.Add(pfieldFDATE);
82           pfields.Add(pfieldTDATE);
83           pfields.Add(pfieldVIEWBY);
84           pfields.Add(pfieldSTAFF);
85           
86           CrystalReportViewer1.ParameterFieldInfo = pfields;	
87   
88           crvBinding();
89       }
90       private void crvBinding()
91       {
92           report.Load(Server.MapPath("../SalesReport/ShippingSummByProductReport.rpt"));
93   
94           SPShipSummByProductTableAdapter adapter = new SPShipSummByProductTableAdapter();
95           CrystalReportManager crManager = new CrystalReportManager(report, adapter.GetData(fDate,tDate));
96           
97           CrystalReportViewer1.ReportSource = crManager.Report;
98           CrystalReportViewer1.Visible = true;
99       }
100      protected void CrystalReportViewer1_ViewZoom(object source, CrystalDecisions.Web.ZoomEventArgs e)
101      {
102          crvBinding();
103      }
104      protected void CrystalReportViewer1_Search(object source, CrystalDecisions.Web.SearchEventArgs e)
105      {
106          crvBinding();
107      }
108      protected void CrystalReportViewer1_Navigate(object source, CrystalDecisions.Web.NavigateEventArgs e)
109      {
110          crvBinding();
111      }
112  }
113  

how can i pass values to parameter fields 

when i click on export, print, previous/next page, navigate zoom

can anyone help me to correct my code

to support all functions(export, print, previous/next page, navigate zoom) ?

or has another way to pass value