how to merge header columns of gridview in c#

protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
       {
           if (e.Row.RowType == DataControlRowType.DataRow)
           {
               Button btn = new Button();
               //btn.Text = "preview";
               e.Row.Cells[5].Controls.Add(btn);
               btn.CausesValidation = false;
               btn.Text = "preview";

               Button btn1 = new Button();
               //btn.Text = "preview";
               e.Row.Cells[6].Controls.Add(btn1);
               btn1.CausesValidation = false;
               btn1.Text = "edit";


               Button btn2 = new Button();
               //btn.Text = "preview";
               e.Row.Cells[7].Controls.Add(btn2);

               btn2.CausesValidation = false;
               btn2.Text = "close";



           }

         }