爲什麼實體框架返回null？

所以我有這些類：爲什麼實體框架返回null？

[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; } 
}

，當我使用下面的代碼，它拋出NullReferenceException我，當調試空是OrderInfo項目部分我無法弄清楚如何解決這一問題，雖然！

下面

是調用數據的代碼：

  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();

來源

2014-04-22 user2125348

什麼樣的應用程序，你的目標是WinForm/WPF網站？ – Habib

你確定'Item'已加載嗎？你可能必須明確包含它。 – germi

@Habib這是一個WinForm應用程序 – user2125348

我能夠通過改變查詢來解決這個問題，使用我的導航屬性也有與項目表中Include：

所以我改變這條線

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

要

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

然後它一切正常。

來源

2014-04-22 14:58:38 user2125348

實體框架默認工作在懶惰狀態。加載訂單時，您可能需要Include您的Item。這樣的事情：

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

來源

2014-04-22 15:00:43 germi

爲什麼實體框架返回null？

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