how to add selected record of gridview to textbox

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ASP.NET

Ajax

in my project i need to select the record from grid.from that selected record only id of that particular record should enter in textbox.for that i m using griedview's templetfield property.
and ajax's modelpopup to display grid
on click of ok button i required above mentioned functionality.
here is my code.

//----grid code-----
<cc1:ModalPopupExtender ID="btn_add_student_ModalPopupExtender" runat="server" 
    BackgroundCssClass="modalBackground" CancelControlID="btn_cancel" DynamicServicePath="" 
    Enabled="True" PopupControlID="Panel1" TargetControlID="btn_add_student">
</cc1:ModalPopupExtender>
                
<asp:Panel ID="Panel1" runat="server" BackColor="#C3C3C3" BorderStyle="Outset" BorderWidth="4px">
       <asp:Label ID="Label10" runat="server" Font-Bold="True" Font-Names="Cambria" 
        Font-Size="Large" ForeColor="White" Text="STUDENTS LIST"></asp:Label>
        <asp:GridView ID="GridView1"  runat="server" DataKeyNames="pk_student_id" AutoGenerateColumns="false">
             <Columns>
              <asp:TemplateField>
               <ItemTemplate>
                 <asp:CheckBox ID="ch1" runat="server" />
               </ItemTemplate>
              </asp:TemplateField>
              <asp:TemplateField>
                <HeaderTemplate> STUDENT NAME </HeaderTemplate>
                  <ItemTemplate>
                     <asp:Label ID="lbl_student_name"  runat="server" Text='<%#Eval("student_firstname") %>'></asp:Label>
                  </ItemTemplate>
                </asp:TemplateField> 
             </Columns>
       </asp:GridView>

//-----cs code------
foreach (GridViewRow row1 in GridView1.Rows)
        {
            CheckBox ch= (CheckBox)row1.Cells[0].FindControl("ch1");
            if (ch.Checked == true)
            {
                string sid = GridView1.DataKeys[row1.RowIndex].Value.ToString();
                txt_all_id.Text = sid;
            }

        }
    }