正确使用实体框架中的导航属性

Question

我正在制作一个表单，将新库存添加到我设计的库存跟踪数据库中。我已经完成了到 EF 的映射，并且正在使用 LINQ to EF 来查询数据。

设备表有一个导航属性EquipmentInventories。考虑以下代码片段：

public partial class Content_AddInventoryItems : System.Web.UI.Page
{
    public Equipment equipment;

    protected void Page_Load(object sender, EventArgs e)
    {
        using (MSSInvDBEntities myEntities = new MSSInvDBEntities())
        {
            var manuPop = (from equipment in myEntities.Equipments
                           select equipment.equipmentManu).Distinct();
            ManuList.DataSource = manuPop;
            ManuList.DataBind();
        }
        using (MSSInvDBEntities myEntities = new MSSInvDBEntities())
        {
            var modelPop = from equipment in myEntities.Equipments
                           select equipment.equipmentModel;
            ModelList.DataSource = modelPop;
            ModelList.DataBind();
        }

    }

    private void DisplayEquipmentData()
    {
        ManuList.SelectedValue = equipment.equipmentManu;
        ModelList.SelectedValue = equipment.equipmentModel;
        tboSerial.Text = equipment.EquipmentInventories.serialNumber;

    }
}

但是，当我尝试使用 EquipmentInventories 导航属性引用 EquipmentInventories 对象的 serialNumber 属性时，我不断收到错误equipment 对象的。

有什么想法我错在哪里吗？

Answer 1

我没有看到你在哪里实例化你的公共字段设备。 （equipment in from Equipment in... 是另一个变量，即 LINQ 查询的范围变量。）查看您的代码，我期望出现 NullReferenceException< /code> 因为设备 为null。

您应该有类似的内容：

using (MSSInvDBEntities myEntities = new MSSInvDBEntities())
{
    equipment = (from e in myEntities.Equipments
                 select e)
                .FirstOrDefault();
}

但这也会导致异常，因为您没有加载 equipment.EquipmentInventories 属性，并且延迟加载在您的 DisplayEquipmentData 方法中不起作用因为您已经处理了上下文（自动在 using 块的末尾）。延迟加载需要尚未释放的上下文。

在您的情况下，我会使用急切加载：

using (MSSInvDBEntities myEntities = new MSSInvDBEntities())
{
    equipment = (from e in myEntities.Equipments.Include("EquipmentInventories")
                 select e)
                .FirstOrDefault();
}

然后使用此查询立即加载导航属性，您可以安全地处置上下文。

Answer 2

斯劳玛说的话，或者……

private void DisplayEquipmentData() 
{ 
    ManuList.SelectedValue = equipment.equipmentManu; 
    ModelList.SelectedValue = equipment.equipmentModel; 
    if (!equipment.EquipmentInventoriesReference.IsLoaded)
        equipment.EquipmentInventoriesReference.Load();
    tboSerial.Text = equipment.EquipmentInventories.serialNumber; 

}