为什么实体框架返回null？

Question

所以我有这些课：

[Table("OrderInfo")]
public class OrderInfo
{
    public long ID {get; set;}
    public long OrderID {get; set;}
    public virtual Order Order { get; set; }
    public long ItemID {get; set;}
    public double Qty { get; set; }
    public virtual Item Item { get; set; }
}

[Table("Items")]
public class Item
{

    public Item()
    {
        this.Orders = new List<OrderInfo>();
    }
    #region Strings
    public string Color { get; set; }
    public string FullName { get; set; }
    [Column(@"Sheet/Roll")]
    public string Type { get; set; }
    public string PrimaryMachine { get; set; }
    public string Alias { get; set; }
    public string Brand { get; set; }

    #endregion

    #region Long
    public long ID { get; set; }
    public long? Weight { get; set; }
    #endregion

    #region Doubles
    public double? Size1 { get; set; }
    public double? Size2 { get; set; }
    public double? Size3 { get; set; }
    #endregion.

    public virtual ICollection<OrderInfo> Orders { get; set; }
}

当我使用以下代码时，它向我抛出NullReferenceException ，而在调试中，null是OrderInfo的Item部分，但我不知道如何解决此问题！

以下是调用数据的代码：

            int ID = Convert.ToInt32(dgv["ID", dgv.SelectedRows[0].Index].Value);
            List<OrderInfo> OrderInfo = new List<OrderInfo>();
            OrderInfo = c.OrderInfo.Where(x => x.OrderID == ID).ToList();

             if(OrderInfo.Count <= 0)
             {
                MessageBox.Show("No Info Found For This Order!");
                ClearForm();
                return;
             }

            lblPO.Text = "P.O. # " + OrderInfo[0].ID.ToString();
            lblRequestedBy.Text = "Requested By: " + OrderInfo[0].Order.RequestedBy;
            lblOrderDate.Text = "Ordered On: " + OrderInfo[0].Order.Date.ToShortDateString();

            dgvOrderItems.DataSource = OrderInfo.Select(x => new { x.ItemID, x.Qty, x.Item.FullName, x.Item.Brand, x.Item.Color }).ToList();

Answer 1

实体框架默认情况下是惰性的。 您可能希望在加载订单时Include Item 。 像这样：

// by using 'Include' you tell EF to fetch Item with your OrderInfo
OrderInfo = c.OrderInfo.Include("Item").Where [...] 

Answer 2

我可以通过使用“导航属性”将查询更改为在items表中Include一个Include来解决此问题：

所以我改变了这一行

OrderInfo = c.OrderInfo.Where(x => x.OrderID == ID).ToList();

至

OrderInfo = c.OrderInfo.Include("Item").Where(x => x.OrderID == ID).ToList();

然后一切正常。