ASP Cookie“对象引用未设置为对象的实例”

Question

好的，所以我在Cristian Darie的一本书中以电子商务中的Beginning ASP.NET开头的一本书中关注了这个示例。该示例构建了一个名为BalloonShop的在线商店。我沿着左右，直到第17章，由于这个错误crusing当我的站点不再推出：

Object reference not set to an instance of an object 

Line 27:    HttpContext context = HttpContext.Current; 
Line 28:    // try to retrieve the cart ID from the user cookie 
Line 29:    string cartId = context.Request.Cookies["BalloonShop_CartID"].Value; 
Line 30:    // if the cart ID isn't in the cookie... 
Line 31:    { 

说明：在当前Web请求的执行过程中发生未处理的异常。请查看堆栈跟踪以获取有关该错误的更多信息以及源代码的位置。

异常详细信息：System.NullReferenceException：对象不设置为一个对象的一个​​实例。

我的堆栈跟踪如下：

[NullReferenceException: Object reference not set to an instance of an object.] ShoppingCartAccess.get_shoppingCartId() in f:\TheGate\App_Code\ShoppingCartAccess.cs:29 ShoppingCartAccess.GetItems() in f:\TheGate\App_Code\ShoppingCartAccess.cs:188 UserControls_CartSummary.PopulateControls() in f:\TheGate\UserControls\CartSummary.ascx.cs:25 UserControls_CartSummary.Page_PreRender(Object sender, EventArgs e) in f:\TheGate\UserControls\CartSummary.ascx.cs:18 System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, >EventArgs e) +14 System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) +35 System.Web.UI.Control.OnPreRender(EventArgs e) +8998946 System.Web.UI.Control.PreRenderRecursiveInternal() +103 System.Web.UI.Control.PreRenderRecursiveInternal() +175 System.Web.UI.Control.PreRenderRecursiveInternal() +175 System.Web.UI.Control.PreRenderRecursiveInternal() +175 System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +2496

我的代码似乎还好我有限的知识（如下，从我ShoppingCartAccess.cs类）：

private static string shoppingCartId 
{ 
    get 
    { 

     // get the current HttpContext 
     HttpContext context = HttpContext.Current; 
     // try to retrieve the cart ID from the user cookie 
     string cartId = context.Request.Cookies["BalloonShop_CartID"].Value; 
     // if the cart ID isn't in the cookie... 
     { 
      // check if the cart ID exists as a cookie 
      if (context.Request.Cookies["BalloonShop_CartID"] != null) 
      { 
       // return the id 
       return cartId; 
      } 
      else 
      // if the cart ID doesn't exist in the cookie as well, generate a new ID 
      { 
       // generate a new GUID 
       cartId = Guid.NewGuid().ToString(); 
       // create the cookie object and set its value 
       HttpCookie cookie = new HttpCookie("BalloonShop_CartID", cartId); 
       // set the cookie's expiration date 
       int howManyDays = TheGateConfiguration.CartPersistDays; 
       DateTime currentDate = DateTime.Now; 
       TimeSpan timeSpan = new TimeSpan(howManyDays, 0, 0, 0); 
       DateTime expirationDate = currentDate.Add(timeSpan); 
       cookie.Expires = expirationDate; 
       // set the cookie on the client's browser 
       context.Response.Cookies.Add(cookie); 
       // return the CartID 
       return cartId.ToString(); 
      } 
     } 
    } 
} 

我敢在我的编程学习的这个阶段无奈。据我所知，我的程序正在寻找一个cookie，如果它没有看到它创建一个cookie，但现在它不知何故全部停止。直到第16章为止，我的表现都不错，但现在我有点不舒服，因为我不知道如何解决这个问题。有任何想法吗？谢谢！！

Answer 1

由于第29行是这样的：

string cartId = context.Request.Cookies["BalloonShop_CartID"].Value; 

基于错误，唯一的东西，它可能是有：

• 的HTTP上下文不可用（因为你是在用户控制，这是不太可能的）。
• HTTP请求或Cookies集合为空（不是可能的方式，你所得到的HTTP上下文）
• Cookies["BalloonShop_CartID"]返回null，当你访问.Value（最有可能）

那些是在炸毁对象引用异常的唯一可能原因，并且很可能是最后一项，该cookie将返回null。在查看整个代码示例时，奇怪的是它检查cookie，然后进行空检查以查看cookie是否不为空;它应该执行以下操作（删除第29行）。

HttpContext context = HttpContext.Current; 
// check if the cart ID exists as a cookie 
if (context.Request.Cookies["BalloonShop_CartID"] != null) 
{ 
    // return the id 
     return context.Request.Cookies["BalloonShop_CartID"].Value; 
} 
else 
// if the cart ID doesn't exist in the cookie as well, generate a new ID 
{ 
    . 
    . 

Answer 2

你也可以做这样的事情...

string cartId = context.Request.Cookies["BalloonShop_CartID"] != null 
    ? context.Request.Cookies["BalloonShop_CartID"].Value 
    : ""; 

另外，我只注意到你的代码是有点儿messsed起来，我重写了它在这里，我真的不能测试，但希望它让你更接近你的答案：

private static string GetShoppingCartId() { 

    HttpContext context = HttpContext.Current; 
    string cartId = context.Request.Cookies["BalloonShop_CartID"] != null 
     ? context.Request.Cookies["BalloonShop_CartID"].Value 
     : ""; 

    if (cartId == "") 
    { 
      // generate a new GUID 
      cartId = Guid.NewGuid().ToString(); 
      int cartPersistDays = TheGateConfiguration.CartPersistDays; 
      // create the cookie object and set its value 
      context.Response.Cookies.Add(new HttpCookie("BalloonShop_CartID", cartId) { 
       Expires = DateTime.Now.AddDays(cartPersistDays) 
      }); 
    } 

    return cartId; 
}