private void BtnExport_Click(object sender, EventArgs e)
{
ExportToExcel excel = new ExportToExcel();
string query = "SELECT * FROM dbo.KhachHang";
System.Data.DataTable data = DataProvider.Instance.ExecuteQuery(query);
excel.ExportKhachHang(data, "Danh sach KH", "DANH SÁCH KHÁCH HÀNG");
}