// tìm kiếm kho hàng
        public List <khohang> SearchKhoHang(string ma, string ten)
        {
            List <khohang> list  = new List <khohang>();
            string         query = string.Format("select * from khohang a where a.ma like N'%{0}%' and a.ten like N'%{1}%'", ma, ten);
            DataTable      data  = DataProvider.Instance.ExecuteQuery(query);

            foreach (DataRow i in data.Rows)
            {
                khohang mh = new khohang(i);
                list.Add(mh);
            }
            return(list);
        }
        //xem kho hàng
        public List <khohang> XemKhoHang()
        {
            List <khohang> list  = new List <khohang>();
            string         query = "select ma, ten, diachi from khohang";
            DataTable      data  = DataProvider.Instance.ExecuteQuery(query);

            foreach (DataRow item in data.Rows)
            {
                khohang kh = new khohang(item);
                list.Add(kh);
            }
            return(list);
        }