asp.net - ASP Cookie "Object reference not set to an instance of an object"

Question

好的，我将遵循 Cristian Darie 所著《电子商务中的 ASP.NET 入门》一书中的示例。该示例构建了一个名为 BalloonShop 的在线商店。我一直忙到第 17 章左右，此时我的网站由于此错误而不再启动:

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:             {

描述:执行当前 Web 请求期间发生未处理的异常。请查看堆栈跟踪以获取有关错误及其在代码中的来源的更多信息。

异常详细信息:System.NullReferenceException:未将对象引用设置为对象的实例。

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

就我有限的知识而言，我的代码似乎没问题(如下所示，来 self 的 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();
            }
        }
    }
}

我在编程学习的这个阶段非常无助。据我所知，我的程序正在寻找一个 cookie，如果它没有看到它，它就会创建一个，但不知何故，现在它都出错了。在第 16 章之前我都做得很好，但现在我有点搞砸了，因为我不知道如何解决它。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;

根据错误，唯一可能的情况是:

• HTTP 上下文不可用(因为您位于用户控件中，所以这种情况不太可能发生)。
• HTTP 请求或 cookies 集合为空(不可能以获取 HTTP 上下文的方式)
• Cookies["BalloonShop_CartID"] 返回 null，当您访问 .Value(最有可能)时，此错误会爆发

这些是对象引用异常的唯一可能原因，并且很可能是最后一项，cookie 返回 null。查看整个代码示例，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
{
     .
     .