我有一个Web应用程序（ASP.NET 4.7.2，C#）。我正在使用HttpClient向WebService发送POST请求。当执行此response = await client.SendAsync(request);时，
它抛出An unhandled exception of type 'System.NullReferenceException' occurred in mscorlib.dll
我把它 Package 在一个try/catch上，但是这个异常没有被捕获。

完全相同的代码在WinForms应用程序和控制台项目上完美工作（！！）.

下面是我得到的例外

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.Threading.Tasks.SynchronizationContextAwaitTaskContinuation.PostAction(Object state)
   at System.Threading.Tasks.AwaitTaskContinuation.RunCallback(ContextCallback callback, Object state, Task& currentTask)
--- End of stack trace from previous location ---
   at System.Threading.Tasks.AwaitTaskContinuation.<>c.<ThrowAsyncIfNecessary>b__18_0(Object s)
   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()</StackTrace><ExceptionString>System.NullReferenceException: Δεν έχει οριστεί αναφορά αντικειμένου at μια παρουσία αντικειμένου.
   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.Threading.Tasks.SynchronizationContextAwaitTaskContinuation.PostAction(Object state)
   at System.Threading.Tasks.AwaitTaskContinuation.RunCallback(ContextCallback callback, Object state, Task&amp;amp; currentTask)
--- End of stack trace from previous location ---
   at System.Threading.Tasks.AwaitTaskContinuation.&amp;lt;&amp;gt;c.&amp;lt;ThrowAsyncIfNecessary&amp;gt;b__18_0(Object s)
   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()</ExceptionString></Exception></TraceRecord>
An unhandled exception of type 'System.NullReferenceException' occurred in mscorlib.dll

这是精简后的代码

public async Task<bool> SendPostRequestAsync()
{
    string endpoint = "endpointname";
    url = endpointUrl;
    var client = new HttpClient();
    var queryString = HttpUtility.ParseQueryString(string.Empty);

    // adding Request headers for authentication
    client.DefaultRequestHeaders.Add(...);
    client.DefaultRequestHeaders.Add(...);
    var uri = url + endpoint + "?" + queryString;

    HttpResponseMessage response = new HttpResponseMessage();

    byte[] byteData = Encoding.UTF8.GetBytes(bodyxml);
    using (var content = new ByteArrayContent(byteData))
    {
        try
        {
            Uri myUri = new Uri(uri, UriKind.Absolute);
            HttpRequestMessage request = new HttpRequestMessage
            {
                Method = HttpMethod.Post,
                RequestUri = myUri,
                Content = content
            };

            content.Headers.ContentType = new MediaTypeHeaderValue("text/plain");
            
            //this is to check if there nothing is null for the action
            bool iAmOk = ((client != null) && (response != null) && (url != null) && (request != null) && (bodyxml != null));
            if (!iAmOk) { return false; }
            
            try
            {
               response = await client.SendAsync(request);
               //here throws the exception
            }
            catch (Exception ex)
            {
                string msg = ex.Message;
                //it never goes here
            }
            var contents = response.Content.ReadAsStringAsync().Result;
            
            if (response.StatusCode == HttpStatusCode.OK)
            {
                // the call is successfull
                return true;
            }
            else
            {
                _log.Error("errordata");
                //the call is not successfull
                return false;
            }
        }
        catch (Exception exx)
        {
            //it doesn't go neither here
            _log.Error("exceptiondata");
            return false;
        }
   }
}

我正在使用

• Microsoft Visual Studio Community 2019版本16.11.29
• ASP.NET和Web Tools 2019 16.11.116.46506
• ASP.NET Web框架和工具2019 16.11.116.46506
• .Net Framework 4.7.2

是应用程序类型的问题吗？或与一些图书馆？？（找到了this post，看起来很相似，但还没有任何答案。任何输入将不胜感激！