private void BtnThemDocGia_Click(object sender, EventArgs e)
 {
     if (checkNullForm() == false)
     {
         if (txtTienNo.Text.Trim().Equals(""))
         {
             txtTienNo.Text = "0";
         }
         if (checkDateTime() == false)
         {
             DocGia dg = new DocGia();
             dg.ThemDocGia(txtNameDG.Text, datePickerNSDG.Value, txtAddressDG.Text, txtEmailDG.Text, datePickerNgayLapThe.Value, datePickerNgayHetHan.Value, Int16.Parse(txtTienNo.Text));
             loadDocGia();
         }
     }
 }