PurchaseStockAudit.aspx.cs 中代码

/// <summary>
    /// 入库确认
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    protected void btnAudit_Click(object sender, EventArgs e)
    {
        int cnt = 0;
        foreach (GridViewRow row in dgvInfo.Rows)   //遍历GridView所有的行
        {
            bool chk = ((CheckBox)row.Cells[1].FindControl("chkAuditFlag")).Checked;
            if (chk)    //判断该行是行被选中
            {
                cnt++;
                //取GridView行主键值
                string tempId = dgvInfo.DataKeys[row.RowIndex].Value.ToString();
                string billCode = row.Cells[2].Text;   //取GridView行单据号内容
                if (!string.IsNullOrEmpty(tempId))
                {
                    try
                    {
                        //调用业务层入库单审核方法
                        [font color=#FF0000]PurchaseService.Instance.AuditPurchase(Convert.ToInt32(tempId), true, Context.User.Identity.Name);
[/font]                        ((CheckBox)row.Cells[1].FindControl("chkAuditFlag")).Checked = false;
                    }
                    catch
                    {
                        throw new Exception("单据号" + billCode + "审核失败");
                    }
                }
            }
        }


调用 PurchaseService.cs 中 
 /// <summary>
        /// 入库单审核确认
        /// </summary>
        /// <param name="id"></param>
        /// <param name="isAudit"></param>
        /// <param name="operatorCode"></param>
        /// <returns></returns>
        public bool AuditPurchase(int id, bool isAudit, string operatorCode)
        {.......


 Store storeParam = new Store(); 
                    storeParam.Warehouse = tPur.Content.WareHouse;
                    storeParam.ProductCode = tPurDetail.DetDetail.ProductCode;
                    Store sto = null;
                    //根据仓库、货品代码取库存表中该货品信息
                    [font color=#FF0000]sto = daoStore.GetStore(storeParam);
[/font]                  
                   
                    if (sto == null)//库存中没有该货品
                    {.......

调用 StoreDao.cs 中的 
/// <summary>
        /// 根据仓库,商品代码获得库存实体
        /// </summary>
        /// <param name="obj">只需传入仓库代码,商品代码即可</param>
        /// <returns>库存实体</returns>
        public Store GetStore(Store obj)
        {
            [font color=#00FF00]return dc.Store.FirstOrDefault(itm => itm.Warehouse == obj.Warehouse && itm.ProductCode == obj.ProductCode);
[/font]        }

这里报错：从查询中返回了结果后，将无法修改数据上下文选项。

这个怎么解决？？？？？？？？？？