异步方法不返回

Question

我有以下Action方法,它使用Scanner类使用一些webservice来获取一些数据.当我在GetSuggestions方法中使用断点时,我可以看到结果.但是,此数据永远不会返回到我的Action方法.相反,当我检查model内部的值时Index(),它是,

Id = 1, Status = System.Threading.Tasks.TaskStatus.WaitingForActivation, Method = "{null}", Result = "{Not yet computed}"

我检查了这个问题,但它没有帮助我.

控制器动作方法:

[HttpGet]
public ActionResult Index()
{
    var plane = new Scanner();
    var model = plane.GetSuggestions("ISTANBUL");

    return View(model);
}

GetSuggestions方法:

public async Task<List<PlaceDto>> GetSuggestions(string key)
{
    string url = String.Format("URL GOES HERE", key, API_KEY);

    string data = await RequestProvider.Get(url);

    return JObject.Parse(data).SelectToken("Places").ToObject<List<PlaceDto>>();
}

RequestProvider方法:

public static async Task<string> Get(string url)
{
    using (var client = new HttpClient())
    {
        return await client.GetStringAsync(url);
    }
}

---

编辑1

我还尝试使用task包装Action方法并等待GetSuggestions方法,但是我收到了异常client.GetStringAsync(url)

当我使用Task on Action Method时出现异常

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.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 where exception was thrown ---
       at System.Threading.Tasks.AwaitTaskContinuation.<ThrowAsyncIfNecessary>b__1(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()
  InnerException: 

---

编辑2

我删除了await关键字await client.GetStringAsync(url);和代码工作.但是,我认为这将同步运行而不是异步.以下是更新的Get方法,

public static async Task<string> Get(string url)
{
    using (var client = new HttpClient())
    {
        return client.GetStringAsync(url).Result;
    }  
}

Answer 1

你的问题分为两部分.

首先是僵局.您需要替换任何Task.Wait或Task<T>.Result呼叫await.我在博客和您链接的答案中更充分地解释了这种死锁情况.

第二个是例外.您看到此异常是因为您在ASP.NET 4.0上使用async/ await,这是不受支持的.您需要确保定位.NET 4.5并且已<httpRuntime targetFramework="4.5" />在web.config中设置.