How to maintain all the values even if i visited any pages in the GridView

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VB

ASP.NET

Hi,

When i go to next page in the gridview, the previous values such as checkbox values and Textbox values are cleared.

I want to maintain all the values even if i visited any pages.

What is the problem and how to maintain the values...?

see my code.
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ASPX Code
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<asp:GridView ID="GridView1" runat="server" align="center" AllowPaging="True"
                                AutoGenerateColumns="False" class="tabulardata" DataSourceID="SqlDataSource1"
                                HorizontalAlign="Center" Width="100%" OnRowCommand="GridView1_RowCommand"
                                OnRowDataBound="GridView1_RowDataBound" OnRowDeleted="GridView1_RowDeleted" OnRowDeleting="GridView1_RowDeleting">
                                <columns>
                                    <asp:TemplateField HeaderText="Sl.No" ItemStyle-HorizontalAlign="Center"
                                        ItemStyle-Width="4%">
                                        <itemtemplate>
                                            <asp:Label ID="Sno" runat="server">
                                        </itemtemplate>
                                        <itemstyle horizontalalign="Center" />

                                   <asp:TemplateField HeaderText="CheckAll">
                                       <HeaderTemplate>
                                           <asp:CheckBox ID="chkSelectAll" runat="server" AutoPostBack="true" OnCheckedChanged="chkSelectAll_CheckedChanged"/>
                                       </HeaderTemplate>
                                       <itemtemplate>
                                               <asp:CheckBox ID="chkSelect" runat="server" AutoPostBack="true" OnCheckedChanged="chkSelect_CheckedChanged"/>
                                       </itemtemplate>

                                    <asp:BoundField HeaderText="Department" Visible="true" DataField="Department"
                                        ItemStyle-HorizontalAlign="center" ReadOnly="true" SortExpression="Department">
                                        <itemstyle horizontalalign="Center" width="20%" />

                                     <asp:BoundField HeaderText="Cost Centre" Visible="true" DataField="CostCentre"
                                        ItemStyle-HorizontalAlign="center" ReadOnly="true" SortExpression="CostCentre">
                                        <itemstyle horizontalalign="Center" width="20%" />

                                     <asp:BoundField HeaderText="Work Service Code" DataField="WorkServiceCode"
                                        ItemStyle-HorizontalAlign="center" ReadOnly="true" SortExpression="WorkServiceCode">
                                        <itemstyle horizontalalign="Center" width="30%" />

                                     <asp:BoundField HeaderText="Prof.Role Family ID" DataField="ProfRoleFamilyID"
                                        ItemStyle-HorizontalAlign="center" ReadOnly="true" SortExpression="ProfRoleFamilyID">
                                        <itemstyle horizontalalign="Center" width="28%" />

                                     <asp:TemplateField HeaderText="Budget ManHours"  ItemStyle-Width="65%">
                                       <itemtemplate>
                                               <asp:TextBox ID="txtManHours" runat="server" ForeColor="Blue"  Width="75%" ReadOnly="true">
                                       </itemtemplate>
                                        <itemstyle horizontalalign="Center" />

                                </columns>
                                <alternatingrowstyle backcolor="#FFF5F5" />

ASPX.VB Code
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Protected Sub GridView1_RowDataBound(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.GridViewRowEventArgs) Handles GridView1.RowDataBound
      Dim sno As New Label
      Dim SrNo As Integer
      If e.Row.RowType = DataControlRowType.DataRow Then
          SrNo = (GridView1.PageIndex + 1) * 10 - 9
          sno = e.Row.FindControl("Sno")
          sno.Text = e.Row.RowIndex + SrNo
          sno.DataBind()
      End If
      'Dim chkNewActive As CheckBox = DirectCast(GridView1.FooterRow.FindControl("chkNewActive"), CheckBox)
  End Sub
  Protected Sub chkSelect_CheckedChanged(ByVal sender As Object, ByVal e As EventArgs)
      Dim chkTest As CheckBox = DirectCast(sender, CheckBox)
      Dim grdRow As GridViewRow = DirectCast(chkTest.NamingContainer, GridViewRow)
      Dim txtMHrs As TextBox = DirectCast(grdRow.FindControl("txtManHours"), TextBox)
      'Dim txtlocation As TextBox = DirectCast(grdRow.FindControl("txtLocation"), TextBox)
      If chkTest.Checked Then
          txtMHrs.[ReadOnly] = False
          txtMHrs.ForeColor = System.Drawing.Color.Black
      Else
          txtMHrs.[ReadOnly] = True
          txtMHrs.ForeColor = System.Drawing.Color.Blue
      End If
  End Sub

  Protected Sub chkSelectAll_CheckedChanged(ByVal sender As Object, ByVal e As EventArgs)
      Dim chkAll As CheckBox = DirectCast(GridView1.HeaderRow.FindControl("chkSelectAll"), CheckBox)
      If chkAll.Checked = True Then
          For Each gvRow As GridViewRow In GridView1.Rows
              Dim chkSel As CheckBox = DirectCast(gvRow.FindControl("chkSelect"), CheckBox)
              chkSel.Checked = True
              Dim txtMHrs As TextBox = DirectCast(gvRow.FindControl("txtManHours"), TextBox)
              txtMHrs.[ReadOnly] = False
              txtMHrs.ForeColor = System.Drawing.Color.Black
          Next
      Else
          For Each gvRow As GridViewRow In GridView1.Rows
              Dim chkSel As CheckBox = DirectCast(gvRow.FindControl("chkSelect"), CheckBox)
              chkSel.Checked = False
              Dim txtMHrs As TextBox = DirectCast(gvRow.FindControl("txtManHours"), TextBox)
              txtMHrs.[ReadOnly] = True
              txtMHrs.ForeColor = System.Drawing.Color.Blue
          Next
      End If
  End Sub