等待使用httpClient.GetAsync（url）并获取错误System.NullReferenceException未处理 [英] await with httpClient.GetAsync(url) and getting error System.NullReferenceException was unhandled

问题描述

我正在使用VS 2012并将目标框架设置为4.0。

I am using VS 2012 and set the target framework as 4.0.

public async Task< HttpResponseMessage> GetResponse（Uri url）            


{                


this.httpClient.DefaultRequestHeaders.Accept.Clear（）;              &NBSP;&NBSP;&NBSP;


的 HttpResponseMessage响应=等待this.httpClient.GetAsync（URL）;&NBSP; &NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP; &NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;


response.EnsureSuccessStatusCode（）;            


}

public async Task<HttpResponseMessage> GetResponse(Uri url)            
{                
this.httpClient.DefaultRequestHeaders.Accept.Clear();                
HttpResponseMessage response = await this.httpClient.GetAsync(url);                
response.EnsureSuccessStatusCode();            
}

在上面一行（标记为粗体），我收到以下错误：

On the above line (marked bold), i am getting following error:

-------------------------------------------------- ----------------

错误：
$
System.NullReferenceException未处理

HResult = - 2147467261

消息=对象引用未设置为对象的实例。

Source = Microsoft.CompilerServices.AsyncTargetingPack.Net4

StackTrace：

服务器堆栈跟踪：System.Web.ThreadContext.AssociateWithCurrentThread（Boolean  setImpersonationContext）at System.Web.HttpApplication.OnThreadEnterPrivate（Boolean setImpersonationContext）at System.Web.LegacyAspNetSynchronizationContext.CallCallbackPossiblyUnderLock（SendOrPostCallback
）在System.Runtime.CompilerS回调，对象状态）在System.Web.LegacyAspNetSynchronizationContext.CallCallback（SendOrPostCallback回调，对象状态）在System.Web.LegacyAspNetSynchronizationContext.Post（SendOrPostCallback回调，对象状态） ervices.TaskAwaiter<> c__DisplayClassa< OnCompletedInternal> b__0（任务
参数0）

&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;在异常重新抛出[0]：在System.Runtime.CompilerServices.AsyncMethodBuilderCore< ThrowAsync> b__1（对象状态）

&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP ;&NBSP; at System.Threading.QueueUserWorkItemCallback.WaitCallback_Context（Object state）

          在System.Threading.ExecutionContext.RunInternal（的ExecutionContext的ExecutionContext，ContextCallback回调，对象的状态，布尔preserveSyncCtx）

&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;在System.Threading.ExecutionContext.Run（的ExecutionContext的ExecutionContext，ContextCallback回调，对象的状态，布尔preserveSyncCtx）

&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP; at System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem（）

           at System.Threading.ThreadPoolWorkQueue.Dispatch（）

          在System.Threading._ThreadPoolWaitCallback.PerformWaitCallback（）

      InnerException： 

-------------------------------------- ------------------------

------------------------------------------------------------------
Error:
System.NullReferenceException was unhandled
HResult=-2147467261
Message=Object reference not set to an instance of an object.
Source=Microsoft.CompilerServices.AsyncTargetingPack.Net4
StackTrace:
Server stack trace: at System.Web.ThreadContext.AssociateWithCurrentThread(Boolean  setImpersonationContext) at System.Web.HttpApplication.OnThreadEnterPrivate(Boolean setImpersonationContext) at System.Web.LegacyAspNetSynchronizationContext.CallCallbackPossiblyUnderLock(SendOrPostCallback callback, Object state) at System.Web.LegacyAspNetSynchronizationContext.CallCallback(SendOrPostCallback callback, Object state) at System.Web.LegacyAspNetSynchronizationContext.Post(SendOrPostCallback callback, Object state) at System.Runtime.CompilerServices.TaskAwaiter.<>c__DisplayClassa.<OnCompletedInternal>b__0(Task param0)
        Exception rethrown at [0]: at System.Runtime.CompilerServices.AsyncMethodBuilderCore.<ThrowAsync>b__1(Object state)
           at System.Threading.QueueUserWorkItemCallback.WaitCallback_Context(Object state)
           at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
           at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
           at System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem()
           at System.Threading.ThreadPoolWorkQueue.Dispatch()
           at System.Threading._ThreadPoolWaitCallback.PerformWaitCallback()
      InnerException: 
--------------------------------------------------------------

虽然成功通话是为 await中传递的URL做出的this.httpClient.GetAsync（url）
因为我检查了fiddler中的调用详细信息并找到了成功的响应，但后来得到了上述错误。


$
 我已根据
https://nuget.org/packages/Microsoft.Bcl.Async 但仍然得到相同的错误。此外，我已为Microsoft.CompilerServices.AsyncTargetingPack.Net4.dll安装了Microsoft.CompilerServices.AsyncTargetingPack.1.0.0。

Though the successfully call is making for the URL passed in await this.httpClient.GetAsync(url) as i checked the call detail in fiddler and found the successful response but then got the above mentioned error.

 I have installed "Microsoft.Bcl.Async" as per the https://nuget.org/packages/Microsoft.Bcl.Async but still getting the same error. Also i have installed the Microsoft.CompilerServices.AsyncTargetingPack.1.0.0 for Microsoft.CompilerServices.AsyncTargetingPack.Net4.dll.

此错误的任何建议和解决方案。

Any suggessions and solutions for this error.

谢谢，

- JP Sharma

Thanks,
--JP Sharma

JP Sharma

JP Sharma

推荐答案

我看不到哪里  this.httpClient。被建造。  你确定
它不是null，并且在调用完成之前没有别的东西将它设置为null。  我会在函数中创建httpClient并在函数内使用

I do not see where this.httpClient. is created.  Are you sure it is not null and there is nothing else setting it to null before the call is complete.  I would create the httpClient in the function and use it inside the function

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