private void btnAdd_Click(object sender, EventArgs e)
        {

            //校验
            if ((txtprice.Text == "") || (txtquantity.Text == "")
                 || (txtdiscount.Text == "") || (txtgid.Text == "")
                 || (txtname.Text == "") || (txtdate.Text == "") || (txtcolor.Text == "") || (txtsize.Text == "")
                || (txtshuxing.Text == "") || (txtbrand.Text == ""))
            {
                MessageBox.Show("不能为空。");
                return;
            }

            string ConStr = @"Provider=Microsoft.Jet.OLEDB.4.0;Data Source="
                    + System.Environment.CurrentDirectory + @"\Sale.mdb";
            myOleConn = new OleDbConnection(ConStr);
            //检索 txtgid 当前库存数
            String sel_str = "select Gquantity from goods where gid = '" + txtgid.Text + "';";
            OleDbCommand myconn2 = new OleDbCommand(sel_str, myOleConn);
            myOleConn.Open();

            int selled = Convert.ToInt32(txtquantity.Text);//卖出商品的数量
            System.Diagnostics.Debug.Write(sel_str);
            int count = 6;//查询出商品的库存

            int tar_stock = count - selled;
            if (tar_stock < 0)
            {
                //无法完成销售
                MessageBox.Show("当前库存不足，无法完成销售");
            }
            else
            {
                //update
                String str_up = "update goods set Gquantity='" + tar_stock + "' where gid = '" + txtgid.Text + "';";
                OleDbCommand myconn = new OleDbCommand(str_up, myOleConn);
                myconn2.ExecuteNonQuery();
                myOleConn.Open();
                MessageBox.Show("库存更新成功");
                //可以进行销售
                string sql = "insert into xiaoshoubiao (Sprice,Squantity,discount,gid,Sname,Sdate,Scolor,Ssize,Sshuxing,Sbrand) values ('" + txtprice.Text
                + "','" + txtquantity.Text + "','" + txtdiscount.Text
                + "','" + txtgid.Text
                + "','" + txtname.Text + "','" + txtdate.Text + "','" + txtcolor.Text + "','" + txtsize.Text
                + "','" + txtshuxing.Text + "','" + txtbrand.Text + "')";
                try
                {
                    OleDbCommand myconn1 = new OleDbCommand(sql, myOleConn);
                    myconn.ExecuteNonQuery();
                    RefreshDG();
                    MessageBox.Show("销售成功");
                }
                catch (Exception ee)
                {
                    MessageBox.Show(ee.Message);
                }

            }

            txtprice.Text = "";
            txtquantity.Text = "";
            txtdiscount.Text = "";
            txtgid.Text = "";
            txtname.Text = "";
            txtdate.Text = "";
            txtcolor.Text = "";
            txtsize.Text = "";
            txtshuxing.Text = "";
            txtbrand.Text = "";
            myOleConn.Close();
        }

系统提示这“ myconn.ExecuteNonQuery();”出现得错误是“标准表达式中数据类型不匹对”