HttpClient GetAsync never returns when using await async

Question

Edit  This question looks like it might be the same problem  but has no responses     Edit  In test case 5 the task appears to be stuck in WaitingForActivation state   I ve encountered some odd behaviour using the System Net Http HttpClient in  NET 4 5 - where  awaiting  the result of a call to  e g   httpClient GetAsync      will never return   This only occurs in certain circumstances when using the new async await language functionality and Tasks API - the code always seems to work when using only continuations   Here s some code which reproduces the problem - drop this into a new  MVC 4 WebApi project  in Visual Studio 11 to expose the following GET endpoints    api test1  api test2  api test3  api test4  api test5  lt --- never completes  api test6   Each of the endpoints here return the same data  the response headers from stackoverflow com  except for  api test5 which never completes   Have I encountered a bug in the HttpClient class  or am I misusing the API in some way   Code to reproduce   public class BaseApiController   ApiController            lt summary gt          Retrieves data using continuations          lt  summary gt      protected Task lt string gt  Continuations GetSomeDataAsync                 var httpClient   new HttpClient             var t   httpClient GetAsync  http   stackoverflow com   HttpCompletionOption ResponseHeadersRead            return t ContinueWith t1   gt  t1 Result Content Headers ToString                     lt summary gt          Retrieves data using async await          lt  summary gt      protected async Task lt string gt  AsyncAwait GetSomeDataAsync                 var httpClient   new HttpClient             var result   await httpClient GetAsync  http   stackoverflow com   HttpCompletionOption ResponseHeadersRead            return result Content Headers ToString             public class Test1Controller   BaseApiController            lt summary gt          Handles task using Async Await          lt  summary gt      public async Task lt string gt  Get                 var data   await Continuations GetSomeDataAsync             return data           public class Test2Controller   BaseApiController            lt summary gt          Handles task by blocking the thread until the task completes          lt  summary gt      public string Get                 var task   Continuations GetSomeDataAsync             var data   task GetAwaiter   GetResult             return data           public class Test3Controller   BaseApiController            lt summary gt          Passes the task back to the controller host          lt  summary gt      public Task lt string gt  Get                 return Continuations GetSomeDataAsync             public class Test4Controller   BaseApiController            lt summary gt          Handles task using Async Await          lt  summary gt      public async Task lt string gt  Get                 var data   await AsyncAwait GetSomeDataAsync             return data           public class Test5Controller   BaseApiController            lt summary gt          Handles task by blocking the thread until the task completes          lt  summary gt      public string Get                 var task   AsyncAwait GetSomeDataAsync             var data   task GetAwaiter   GetResult             return data           public class Test6Controller   BaseApiController            lt summary gt          Passes the task back to the controller host          lt  summary gt      public Task lt string gt  Get                 return AsyncAwait GetSomeDataAsync

User · Accepted Answer

You are misusing the API   Here s the situation  in ASP NET  only one thread can handle a request at a time  You can do some parallel processing if necessary  borrowing additional threads from the thread pool   but only one thread would have the request context  the additional threads do not have the request context    This is managed by the ASP NET SynchronizationContext   By default  when you await a Task  the method resumes on a captured SynchronizationContext  or a captured TaskScheduler  if there is no SynchronizationContext   Normally  this is just what you want  an asynchronous controller action will await something  and when it resumes  it resumes with the request context   So  here s why test5 fails    Test5Controller Get executes AsyncAwait GetSomeDataAsync  within the ASP NET request context   AsyncAwait GetSomeDataAsync executes HttpClient GetAsync  within the ASP NET request context   The HTTP request is sent out  and HttpClient GetAsync returns an uncompleted Task  AsyncAwait GetSomeDataAsync awaits the Task  since it is not complete  AsyncAwait GetSomeDataAsync returns an uncompleted Task  Test5Controller Get blocks the current thread until that Task completes  The HTTP response comes in  and the Task returned by HttpClient GetAsync is completed  AsyncAwait GetSomeDataAsync attempts to resume within the ASP NET request context  However  there is already a thread in that context  the thread blocked in Test5Controller Get  Deadlock    Here s why the other ones work     test1  test2  and test3   Continuations GetSomeDataAsync schedules the continuation to the thread pool  outside the ASP NET request context  This allows the Task returned by Continuations GetSomeDataAsync to complete without having to re-enter the request context   test4 and test6   Since the Task is awaited  the ASP NET request thread is not blocked  This allows AsyncAwait GetSomeDataAsync to use the ASP NET request context when it is ready to continue    And here s the best practices    In your  library  async methods  use ConfigureAwait false  whenever possible  In your case  this would change AsyncAwait GetSomeDataAsync to be var result   await httpClient GetAsync  http   stackoverflow com   HttpCompletionOption ResponseHeadersRead  ConfigureAwait false   Don t block on Tasks  it s async all the way down  In other words  use await instead of GetResult  Task Result and Task Wait should also be replaced with await     That way  you get both benefits  the continuation  the remainder of the AsyncAwait GetSomeDataAsync method  is run on a basic thread pool thread that doesn t have to enter the ASP NET request context  and the controller itself is async  which doesn t block a request thread    More information    My async await intro post  which includes a brief description of how Task awaiters use SynchronizationContext  The Async Await FAQ  which goes into more detail on the contexts  Also see Await  and UI  and deadlocks  Oh  my  which does apply here even though you re in ASP NET rather than a UI  because the ASP NET SynchronizationContext restricts the request context to just one thread at a time  This MSDN forum post  Stephen Toub demos this deadlock  using a UI   and so does Lucian Wischik    Update 2012-07-13  Incorporated this answer into a blog post

User · Answer

I m going to put this in here more for completeness than direct relevance to the OP   I spent nearly a day debugging an HttpClient request  wondering why I was never getting back a response   Finally found that I had forgotten to await the async call further down the call stack   Feels about as good as missing a semicolon

User · Answer

I was using to many await  so i was not getting response    i converted in to sync call its started working             using  var client   new HttpClient                using  var request   new HttpRequestMessage                                  client DefaultRequestHeaders Accept Add new MediaTypeWithQualityHeaderValue  quot application json quot                     request Method   HttpMethod Get                  request RequestUri   new Uri URL                   var response   client GetAsync URL  Result                  response EnsureSuccessStatusCode                    string responseBody   response Content ReadAsStringAsync   Result

User · Answer

Edit  Generally try to avoid doing the below except as a last ditch effort to avoid deadlocks  Read the first comment from Stephen Cleary   Quick fix from here  Instead of writing   Task tsk   AsyncOperation    tsk Wait      Try   Task Run      gt  AsyncOperation    Wait      Or if you need a result   var result   Task Run      gt  AsyncOperation    Result    From the source  edited to match the above example        AsyncOperation will now be invoked on the ThreadPool  where there   won   t be a SynchronizationContext  and the continuations used inside   of AsyncOperation won   t be forced back to the invoking thread    For me this looks like a useable option since I do not have the option of making it async all the way  which I would prefer    From the source      Ensure that the await in the FooAsync method doesn   t find a context to   marshal back to  The simplest way to do that is to invoke the   asynchronous work from the ThreadPool  such as by wrapping the   invocation in a Task Run  e g       int Sync             return Task Run       Library FooAsync    Result          FooAsync will now be invoked on the ThreadPool  where there won   t be a   SynchronizationContext  and the continuations used inside of FooAsync   won   t be forced back to the thread that   s invoking Sync

User · Answer

Since you are using  Result or  Wait or await this will end up causing a deadlock in your code   you can use ConfigureAwait false  in async methods for preventing deadlock   like this   var result   await httpClient GetAsync  http   stackoverflow com   HttpCompletionOption ResponseHeadersRead                                ConfigureAwait false        you can use ConfigureAwait false  wherever possible for Don t Block Async Code

User · Answer

In my case  await  never finished because of exception while executing the request  e g  server not responding  etc  Surround it with try  catch to identify what happened  it ll also complete your  await  gracefully  public async Task lt Stuff gt  GetStuff string id        string path     quot  api v2 stuff  id  quot       try               HttpResponseMessage response   await client GetAsync path           if  response StatusCode    HttpStatusCode OK                        string json   await response Content ReadAsStringAsync                return JsonUtility FromJson lt Stuff gt  json                     else                       Debug LogError   quot Could not retrieve stuff  id  quot                        catch  Exception exception                Debug LogError   quot Exception when retrieving stuff  exception  quot              return null

User · Answer

These two schools are not really excluding   Here is the scenario where you simply have to use     Task Run      gt  AsyncOperation    Wait       or something like     AsyncContext Run AsyncOperation     I have a MVC action that is under database transaction attribute  The idea was  probably  to roll back everything done in the action if something goes wrong  This does not allow context switching  otherwise transaction rollback or commit is going to fail itself   The library I need is async as it is expected to run async   The only option  Run it as a normal sync call   I am just saying to each its own

[c#] HttpClient.GetAsync(...) never returns when using await/async

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