c# - Update textbox for each row in gridview on One Button Click -

in gridview have given textbox each row has corresponding button fetches value textbox , updates rows value in database.

but want add 1 button on top in header template fetch value of textboxes , update rows.

i have tried following code null reference exception @ "instance_id cell[0]"

my code below.

`

                <asp:templatefield>                     <headertemplate>                         <asp:button id="button1" runat="server" text="button" onclick="button1_click" />                     </headertemplate>                 <itemtemplate>                         <asp:textbox id="textbox1" runat="server"></asp:textbox>                 </itemtemplate>             </asp:templatefield>             <asp:buttonfield text="update attendance" commandname="select" />              </columns>         </asp:gridview>` 

and on button click fetch data textboxes:

protected void button1_click(object sender, eventargs e) {     foreach (gridviewrow row in gridview1.rows)     {         vo.instance_id = gridview1.selectedrow.cells[0].text;          vo.project_id = gridview1.selectedrow.cells[1].text;          vo.volunteer_id = gridview1.selectedrow.cells[6].text;          vo.campus_id = gridview1.selectedrow.cells[5].text;          vo.attendance_hours = (gridview1.selectedrow.findcontrol("textbox1") textbox).text;          convert.toint32(vo.attendance_hours);          try         {             int success = bl.fn_updateattendance(vo);             if (success == 2)             {                 scriptmanager.registerclientscriptblock(this, this.gettype(), "alertmessage", "alert('attendance updated successfully')", true);                 // label2.text = "attendance updated successfully";             }             else             {                 scriptmanager.registerclientscriptblock(this, this.gettype(), "alertmessage", "alert('attendance not updated')", true);                 //  label2.text = "attendance not updated";             }         }         catch (exception ex)         {             response.write(ex.message);         }     } } 

my friend suggested using row.cells[0]; inside foreach , worked!

thank everyone.