在异步上下文中添加到Dictionary时的NullReferenceException

Question

我有一个非常奇怪的行为,如下图所示:

正如您在Watch窗口中看到的那样,可能的一切null都不是null.

这是函数的完整代码:

    public LogInTokenCache GetUserIdFromToken(string token)
    {
        LogInTokenCache item = null;

        if (this.TokenCache.ContainsKey(token))
        {
            item = this.TokenCache[token];

            if (item.ExpirationTime < DateTime.Now)
            {
                this.TokenCache.Remove(item.Token);
                return null;
            }
        }
        else
        {
            LogInTokenBusiness tokenBusiness = new LogInTokenBusiness();

            var entity = tokenBusiness.FindToken(token);
            if (entity != null && entity.Token != null)
            {
                item = new LogInTokenCache()
                {
                    Token = entity.Token,
                    UserID = entity.UserId,
                    ExpirationTime = entity.ExpirationTime,
                };

                this.TokenCache.Add(item.Token, item);
            }
        }

        return item;
    }

我使用了查找所有引用功能,这是我使用构造函数和声明的唯一地方(我将它用于整个Web应用程序):

public class IdentityController : Controller
{

    private static EmailAdpater EmailAdapter = new EmailAdpater();
    private static UserIdentityTokenShortener TokenShortener = new UserIdentityTokenShortener();
    public static LoginTokenManager LoginTokenManager = new LoginTokenManager();
    ...

有谁遇到过这个问题？我做错了什么？

编辑:添加StackTrace和详细信息 EDIT2:编辑标题,以便未来的人可以搜索此主题,以防他们遇到相同的问题.

   at System.Collections.Generic.Dictionary12.Insert(TKey key, TValue value, Boolean add)
   at System.Collections.Generic.Dictionary12.Add(TKey key, TValue value)
   at MobileDatingAPI.Models.LoginTokenManager.GetUserIdFromToken(String token) in d:\FPT University\Capstone 2\TFSRepo\Projects\MobileDatingAPI\MobileDatingAPI\Models\LoginTokenManager.cs:line 48
   at MobileDatingAPI.Models.Utils.GetUserFromTokenID(Controller controller, String token, BaseApiViewModels model) in d:\FPT University\Capstone 2\TFSRepo\Projects\MobileDatingAPI\MobileDatingAPI\Models\Utils\Utils.cs:line 68
   at MobileDatingAPI.Controllers.CommunityController.UpdateActivity(String token, Nullable11 longitude, Nullable11 latitude) in d:\FPTUniversity\Capstone 2\TFSRepo\Projects\MobileDatingAPI\MobileDatingAPI\Controllers\CommunityController.cs:line 84
   at lambda_method(Closure , ControllerBase , Object[] )
   at System.Web.Mvc.ActionMethodDispatcher.Execute(ControllerBase controller, Object[] parameters)
   at System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext controllerContext, IDictionary12 parameters)
   at System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary12 parameters)
   at System.Web.Mvc.Async.AsyncControllerActionInvoker.ActionInvocation.InvokeSynchronousActionMethod()
   at System.Web.Mvc.Async.AsyncControllerActionInvoker.<BeginInvokeSynchronousActionMethod>b__39(IAsyncResult asyncResult, ActionInvocation innerInvokeState)
   at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResult12.CallEndDelegate(IAsyncResult asyncResult)
   at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResultBase`1.End()
   at System.Web.Mvc.Async.AsyncResultWrapper.End[TResult](IAsyncResult asyncResult, Object tag)
   at System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethod(IAsyncResult asyncResult)
   at System.Web.Mvc.Async.AsyncControllerActionInvoker.AsyncInvocationWithFilters.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3d()
   at System.Web.Mvc.Async.AsyncControllerActionInvoker.AsyncInvocationWithFilters.<>c__DisplayClass46.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3f()

Answer 1

我使用多线程

这几乎肯定是问题所在:因为代码中的所有内容null都是使用非值的方式检查和/或创建的null,所以问题可能发生的唯一地方就是实现Dictionary.

当达到字典的容量时,集合将重新分配其内部数据结构.如果一个线程在另一个线程执行的重新分配过程中捕获字典,则桶内的一些变量将是未初始化的,即null默认值,具体取决于类型.由于这不是存储桶的通常状态,因此字典将尝试取消引用它,从而导致空指针异常.

为了避免此问题,请在以下调用周围添加同步Add:

// Add this declaration where you declare TokenCache dictionary
object TokenCacheLock = new object();
...
lock (TokenCacheLock) {
    // Add a lock around your access of TokenCache
    if (this.TokenCache.ContainsKey(token)) ...
}

请注意,由于字典的添加与读取同时发生,因此TokenCache需要同步所有访问.

一个更简单的解决方案是使用ConcurrentDictionary<K,V>.