protected void Button1_Click(object sender, EventArgs e)
    {
        switch (DropDownList1.SelectedValue)
        {
        case "1":
            GridView2.DataSource = admin.bookAll();
            GridView2.DataBind();
            break;

        case "2":
            GridView2.DataSource = admin.bookNameSel(TextBox12.Text);
            GridView2.DataBind();
            break;

        case "3":
            GridView2.DataSource = admin.bookISBNSel(TextBox12.Text);
            GridView2.DataBind();
            break;

        case "4":
            GridView2.DataSource = admin.bookAmountSel(Convert.ToInt32(TextBox12.Text));
            GridView2.DataBind();
            break;
        }
    }