This is my code in c#

protected void ibtCopiar_Click(object sender, ImageClickEventArgs e) {

           GridViewRow grdrows = DGPlanilla.Rows[0];
            TextBox tb1 = (TextBox)grdrows.FindControl("TextBox1");
            string valor = tb1.Text;

                if (valor != null || valor != "0")
                {
                    foreach (GridViewRow grdrow in DGPlanilla.Rows)
                    {
                        grdrow.Cells[5].Text = valor;
                    }
               }

    }

this my code for the button

when i debug i see that the value i have in the firts box is pass to the other textboxes in the column, but when it dysplay the page onle the first box show the value i have enter. the other texboxes don´t change.