PostAsync方法的HttpClient错误

Question

使用HttpClient对第三方API进行PostAsync调用时.当我做client.PostAsync时,我正好看到这个错误.知道是什么导致了这个吗？

码:

public class JobController : AsyncController
{
    public ActionResult ViewPage()
    {
        return View("~/Views/Pages/Submit.cshtml");
    }

    private const string ServiceUrl = "https://api.3points.io/v1/applications/";

    [HttpPost]
    public async Task<ActionResult> Submit()
    {
        var client = new HttpClient();
        var formData = new MultipartFormDataContent();
        var encoded = Convert.ToBase64String(Encoding.UTF8.GetBytes("abc123"));
        client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic", encoded);

        foreach (var key in Request.Form.AllKeys)
            formData.Add(new StringContent(Request.Form[key]), String.Format("\"{0}\"", key));

        foreach (string key in Request.Files.AllKeys)
        {
            var file = Request.Files[key];
            if (file == null || file.ContentLength <= 0) continue;

            HttpContent fileStream = new StreamContent(file.InputStream);
            formData.Add(fileStream, key, file.FileName);
        }

        var response = await client.PostAsync(ServiceUrl, formData);

        var success = "True";
        if (!response.IsSuccessStatusCode) success = "False";

        return new JsonResult { Data = success };
    }
}

错误:

System.NullReferenceException was unhandled 
HResult=-2147467261
Message=Object reference not set to an instance of an object.
Source=System.Web
StackTrace:
   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.Threading.Tasks.AwaitTaskContinuation.RunCallback(ContextCallback callback, Object state, Task& currentTask)
--- End of stack trace from previous location where exception was thrown ---
   at System.Threading.Tasks.AwaitTaskContinuation.<ThrowAsyncIfNecessary>b__1(Object s)
   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()

Answer 1

将这两行添加到web.config文件中:

// First tag might exist already
<httpRuntime targetFramework="4.5" />
<appSettings>
    <add key="aspnet:UseTaskFriendlySynchronizationContext" value="true" />

我引用此链接:

启用4.5中引入的新的await-friendly异步管道.早期版本的ASP.NET中的许多同步原语都有不良行为,例如对公共对象进行锁定或违反API协定.实际上,ASP.NET 4的SynchronizationContext.Post实现是一个阻塞同步调用!新的异步管道努力提高效率,同时遵循API的预期合同.新管道还代表开发人员执行少量错误检查,例如检测对异步void方法的意外调用.

WebSockets等某些功能需要设置此开关.重要的是,除非已设置此开关,否则在ASP.NET中未定义async/await的行为.(记住:设置也足够了.)