// tìm kiếm xuất kho
        public List <phieuxuatkho> SearchXuatKho(string ma, string makh, string manv, string maquay)
        {
            List <phieuxuatkho> list = new List <phieuxuatkho>();
            string    query          = string.Format("select * from phieuxuatkho a where a.ma like N'%{0}%' and a.makhohang like N'%{1}%' and a.manhanvien like N'%{2}%' and a.maquay like N'%{3}%'", ma, makh, manv, maquay);
            DataTable data           = DataProvider.Instance.ExecuteQuery(query);

            foreach (DataRow i in data.Rows)
            {
                phieuxuatkho xk = new phieuxuatkho(i);
                list.Add(xk);
            }
            return(list);
        }
        // xem xuất kho
        public List <phieuxuatkho> XemXuatKho()
        {
            List <phieuxuatkho> list = new List <phieuxuatkho>();
            string    query          = "select ma,makhohang,ngayxuat,tonggiatri,maquay,manhanvien from phieuxuatkho ";
            DataTable data           = DataProvider.Instance.ExecuteQuery(query);

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