After considering this interesting answer HttpClient.GetAsync(...) never returns..., I still have a situation where my HttpClient is not returning when I use await (sample code below). In addition. I use this helper routine from both asp.net MVC5 Controllers (UI-driven) and the WebApi. What can I do to:
Use await instead of the (dreaded) .Result and still have this function return?
Reuse this same routine from both MVC Controllers and WebApi?
Apparently, I should replace the .Result with .ConfigureAwait(false) but this seems to contradict with the fact that "Task4" in the post cited above works fine with an await httpClient.GetAsync. Or do I need separate routines for Controller and WebApi cases?
public static async Task<IEnumerable<TcMarketUserFullV1>> TcSearchMultiUsersAsync(string elasticQuery)
{
if (string.IsNullOrEmpty(elasticQuery)) return null;
IEnumerable<TcMarketUserFullV1> res = null;
using (var hclient = new HttpClient())
{
hclient.BaseAddress = new Uri("https://addr.servicex.com");
hclient.DefaultRequestHeaders.Accept.Clear();
hclient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
hclient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer",
CloudConfigurationManager.GetSetting("jwt-bearer-token"));
// Why does this never return when await is used?
HttpResponseMessage response = hclient.GetAsync("api/v2/users?q=" + elasticQuery + "&search_engine=v2").Result;
if (response.IsSuccessStatusCode)
{
var content = await response.Content.ReadAsStringAsync();
res = JsonConvert.DeserializeObject<TcMarketUserFullV1[]>(content).AsEnumerable();
}
else{log.Warn("...");}
}
return res;
}
UPDATE: My call chain, which starts with a Telerik Kendo Mvc.Grid DataBinding call is as follows:
[HttpPost]
public async Task<ActionResult> TopLicenseGrid_Read([DataSourceRequest]DataSourceRequest request)
{
var res = await GetLicenseInfo();
return Json(res.ToDataSourceResult(request)); // Kendo.Mvc.Extensions.DataSourceRequest
}
Then:
private async Task<IEnumerable<CsoPortalLicenseInfoModel>> GetLicenseInfo()
{
...
// Never returns
var qry = #"app_metadata.tc_app_user.country:""DE""";
return await TcSearchMultiUsersAsync(qry);
}
Then, the routine shown in full above but now WITHOUT the .Result:
public static async Task<IEnumerable<TcMarketUserFullV1>> TcSearchMultiUsersAsync(string elasticQuery)
{
if (string.IsNullOrEmpty(elasticQuery)) return null;
IEnumerable<TcMarketUserFullV1> res = null;
using (var hclient = new HttpClient())
{
hclient.BaseAddress = new Uri("https://addr.servicex.com");
hclient.DefaultRequestHeaders.Accept.Clear();
hclient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
hclient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer",
CloudConfigurationManager.GetSetting("jwt-bearer-token"));
// This now returns fine
HttpResponseMessage response = hclient.GetAsync("api/v2/users?search_engine=v2&q=" + elasticQuery");
if (response.IsSuccessStatusCode)
{
// This returns my results fine too
var content = await response.Content.ReadAsStringAsync();
// The following line never returns results. When debugging, everything flows perfectly until I reach the following line, which never
// returns and the debugger returns me immediately to the top level HttpPost with a result of null.
res = JsonConvert.DeserializeObject<TcMarketUserFullV1[]>(content).AsEnumerable();
}
else{log.Warn("...");}
}
return res;
}
You need to await everything. It's not enough that you await on one of them, you need to await on all of them:
This:
HttpResponseMessage response = hclient.GetAsync(
"api/v2/users?q=" + elasticQuery + "&search_engine=v2").Result;
Should be:
HttpResponseMessage response = await hclient.GetAsync(
"api/v2/users?q=" + elasticQuery + "&search_engine=v2");
It is enough to have one blocking call to .Result in order to deadlock. You need "async all the way".
Related
Hello I dont get the difference between the following two asnyc functions, could someone explain it to me? Both of them doesnt return IRestResponse, so I cant access StatusCode field. Do I have to cast here?
public async Task<IRestResponse> Post<T>(string Ressource, T ObjTOPost) where T : new()
{
return await Task.Factory.StartNew(() =>
{
var client = new RestClient("test.com");
var request = new RestRequest(Ressource, Method.POST);
var response = client.Execute(request);
return response;
});
}
And this:
public async Task<IRestResponse> Post<T>(string Ressource, T ObjTOPost) where T : new()
{
var client = new RestClient("test.com");
var request = new RestRequest(Ressource, Method.POST);
var response = await client.ExecuteTaskAsync<T>(request);
return response;
}
Both of them doesnt return IRestResponse, so I cant access StatusCode field.
They return a Task<IRestResponse>. You can get the interface by awaiting the task, e.g.
var task = Post(resource, objectToPost);
IRestResponse response = await task;
Or in one line (more common):
var response = await Post(resource, objectToPost);
Difference between these two async functions
The second example is far more straightforward. The first example spins up an additional task and passes its awaitable back to the caller, whereas the second example awaits the RestClient directly. I see no reason to use the structure in the first example.
I have some problem with async method.
public async void MakePost()
{
var cookieArray = GetCookies().Result;
(...)
}
async public Task<string[]> GetCookies()
{
(...)
var response = await httpClient.SendAsync(request);
string cookieTempSession = response.Headers.ToString();
(...)
return cookieArray;
}
Nothing happening after var response = await httpClient.SendAsync(request); I put breakpoint in next line string cookieTempSession = response.Headers.ToString(); but it never reach it. I tried to "try catch" but also nothing happend. When I merge this two methods into one it works perfect but it's not so pretty. I just wondering what happened there.
Since the first method is async, you should use await instead of Result:
var cookieArray = await GetCookies();
If you are not programming front end, add ConfigureAwait(false) (why?) to the call, like this:
var cookieArray = await GetCookies().ConfigureAwait(false);
...
var response = await httpClient.SendAsync(request).ConfigureAwait(false);
When testing my web API with Postman my API get executes fine!
When it comes to running the code with HttpClient in my client application the code executes without error but without the expected result on the server.
What could be happening?
From my client application:
private string GetResponseFromURI(Uri u)
{
var response = "";
HttpResponseMessage result;
using (var client = new HttpClient())
{
Task task = Task.Run(async () =>
{
result = await client.GetAsync(u);
if (result.IsSuccessStatusCode)
{
response = await result.Content.ReadAsStringAsync();
}
});
task.Wait();
}
return response;
}
Here is the API controller:
[Route("api/[controller]")]
public class CartsController : Controller
{
private readonly ICartRepository _cartRepo;
public CartsController(ICartRepository cartRepo)
{
_cartRepo = cartRepo;
}
[HttpGet]
public string GetTodays()
{
return _cartRepo.GetTodaysCarts();
}
[HttpGet]
[Route("Add")]
public string GetIncrement()
{
var cart = new CountedCarts();
_cartRepo.Add(cart);
return _cartRepo.GetTodaysCarts();
}
[HttpGet]
[Route("Remove")]
public string GetDecrement()
{
_cartRepo.RemoveLast();
return _cartRepo.GetTodaysCarts();
}
}
Note these API calls work as expected when called from Postman.
You shouldn't use await with client.GetAsync, It's managed by .Net platform, because you can only send one request at the time.
just use it like this
var response = client.GetAsync("URL").Result; // Blocking call!
if (response.IsSuccessStatusCode)
{
// Parse the response body. Blocking!
var dataObjects = response.Content.ReadAsAsync<object>().Result;
}
else
{
var result = $"{(int)response.StatusCode} ({response.ReasonPhrase})";
// logger.WriteEntry(result, EventLogEntryType.Error, 40);
}
You are doing fire-and-forget approach. In your case, you need to wait for the result.
For example,
static async Task<string> GetResponseFromURI(Uri u)
{
var response = "";
using (var client = new HttpClient())
{
HttpResponseMessage result = await client.GetAsync(u);
if (result.IsSuccessStatusCode)
{
response = await result.Content.ReadAsStringAsync();
}
}
return response;
}
static void Main(string[] args)
{
var t = Task.Run(() => GetResponseFromURI(new Uri("http://www.google.com")));
t.Wait();
Console.WriteLine(t.Result);
Console.ReadLine();
}
Simple sample used to get page data.
public string GetPage(string url)
{
HttpResponseMessage response = client.GetAsync(url).Result;
if (response.IsSuccessStatusCode)
{
string page = response.Content.ReadAsStringAsync().Result;
return "Successfully load page";
}
else
{
return "Invalid Page url requested";
}
}
I've had a problem with chace control when using httpclient.
HttpBaseProtocalFilter^ filter = ref new HttpBaseProtocolFilter();
filter->CacheControl->ReadBehavior = Windows::Web::Http::Filters::HttpCacheReadBehavior::MostRecent;
HttpClient^ httpClient = ref new HttpClient(filter);
I'm not really sure what the expected results are or what results your getting at all so this is really just a guessing game right now.
When I POST something using HttpClient I found adding headers by hand seemed to work more often than using default headers.
auto httpClient = ref new HttpClient();
Windows::Web::Http::Headers::HttpMediaTypeHeaderValue^ type = ref new Windows::Web::http::Headers::HttpMediaTypeHeaderValue("application/json");
content->Headers->ContentType = type;
If I don't do these 2 things I found, for me anyways, that half the time my web requests were either not actually being sent or the headers were all messed up and the other half of the time it worked perfectly.
I just read a comment where you said it would only fire once, that makes me think it is the cachecontrol. I think what happens is something (Windows?) sees 2 requests being sent that are the exact same, so to speed things up it just assumes the same answer and never actually sends the request a 2nd time
I have this async method:
public async Task<RES> PostAsync<RES>(string url, string content) where RES : new()
{
using (var client = new HttpClient())
{
HttpResponseMessage message = await client.PostAsync(url, new StringContent(content, Encoding.UTF8, "application/json"));
var readAsStringAsync = await message.Content.ReadAsStringAsync();
return await readAsStringAsync.FromJsonAsync<RES>(mySerializerSettings);
}
}
Where FromJsonAsync is implemented as an extension method:
public async static Task<T> FromJsonAsync<T>(this string data, JsonSerializerSettings settings) where T : new()
{
return (T)(await JsonConvert.DeserializeObjectAsync<T>(data, settings));
}
Now I want to add a regular synchronous Post method and I thought the implementation would be:
public RES Post<RES>(string url, string content) where RES : new()
{
return PostAsync<RES>(url, content).Result;
}
But this doesn't really work. I see that the request is sent via a Http sniffer and I get a response back, but I get stuck when debugging and can't continue.
BTW, this does work (with Result instead of await):
public RES Post<RES>(string url, string content) where RES : new()
{
using (var client = new HttpClient())
{
HttpResponseMessage message = client.PostAsync(url, new StringContent(content, Encoding.UTF8, "application/json")).Result;
var readAsStringAsync = message.Content.ReadAsStringAsync().Result;
return readAsStringAsync.FromJson<RES>(mySerializerSettings);
}
}
Where FromJson is implemented as an extension method:
public static T FromJson<T>(this string data, JsonSerializerSettings settings) where T : new()
{
return (T)JsonConvert.DeserializeObject<T>(data, settings);
}
The application is a web backend (WebApi).
What am I doing wrong?
You probably have a deadlock on your hands.
Asp.net uses a SynchronizationContext to post continuations back to the request context. If the context is blocked (like it is in your case on PostAsync<RES>(url, content).Result) then the continuation can't be executed and so the async method can't complete and you have a deadlock.
You can avoid it by using ConfigureAwait(false):
public async Task<RES> PostAsync<RES>(string url, string content) where RES : new()
{
using (var client = new HttpClient())
{
HttpResponseMessage message = await client.PostAsync(url, new StringContent(content, Encoding.UTF8, "application/json"));
var readAsStringAsync = await message.Content.ReadAsStringAsync().ConfigureAwait(false);
return await readAsStringAsync.FromJsonAsync<RES>(mySerializerSettings).ConfigureAwait(false);
}
}
But it's better to just avoid blocking synchronously on async code to begin with and having two different versions for sync and async.
Although possible, I wouldn't use the answer provided by #i3arnon. Generally, you shouldn't block on async code. Although ConfigureAwait(false) does work, it can lead to confusion in your code-base where other developers may also end up blocking using .Result, without using ConfigureAwait or understanding the implications of that.
Instead, expose synchronous methods which are really synchronous:
public RES Post<RES>(string url, string content) where RES : new()
{
using (var client = new WebClient())
{
client.Headers[HttpRequestHeader.ContentType] = "application/json";
var result = client.UploadString(url, content);
return JsonConvert.DeserializeObject<RES>(result, jsonSerializerSettings);
}
}
It seems you have a non-async function and you want to start a task that will call PostAsync and wait for this task to finish and return the result of the Task. Is this your problem?
To start a Task, use Task.Run( () => ...);
To wait for the Task use Task.Wait(...);
To see if the task stopped because of an exception: Task.IsFaulted
The result of the task is in Task.Result
Your code could be:
public async Task<RES> PostAsync<RES>(string url, string content) where RES : new()
{
// start the task that will call PostAsync:
var postTask = Task.Run( () => PostAsync(url, content));
// while this task is running you can do other things
// once you need the result: wait for the task to finish:
postTask.Wait();
// If needed check Task.IsFaulted / Task.IsCanceled etc. to check for errors
// the returned value is in Task.Result:
return postTask.Result;
}
I'm working on a new Windows Phone 8 app. I'm connecting to a webservice which returns valid json data. I'm using longlistselector to display the data. This works fine when i'm using the string json in GetAccountList(); but when receiving data from the DataServices class i'm getting the error "Cannot implicitly convert type 'System.Threading.Tasks.Task'to string". Don't know what goes wrong. Any help is welcome. Thanks!
DataServices.cs
public async static Task<string> GetRequest(string url)
{
HttpClient httpClient = new HttpClient();
await Task.Delay(250);
HttpResponseMessage response = await httpClient.GetAsync(url);
response.EnsureSuccessStatusCode();
string responseBody = await response.Content.ReadAsStringAsync();
Debug.WriteLine(responseBody);
return await Task.Run(() => responseBody);
}
AccountViewModel.cs
public static List<AccountModel> GetAccountList()
{
string json = DataService.GetRequest(url);
//string json = #"{'accounts': [{'id': 1,'created': '2013-10-03T16:17:13+0200','name': 'account1 - test'},{'id': 2,'created': '2013-10-03T16:18:08+0200','name': 'account2'},{'id': 3,'created': '2013-10-04T13:23:23+0200','name': 'account3'}]}";
List<AccountModel> accountList = new List<AccountModel>();
var deserialized = JsonConvert.DeserializeObject<IDictionary<string, JArray>>(json);
JArray recordList = deserialized["accounts"];
foreach (JObject record in recordList)
{
accountList.Add(new AccountModel(record["name"].ToString(), record["id"].ToString()));
}
return accountList;
}
UPDATE: I changed it slightly and works like a charm now. Thanks for your help!
DataServices.cs
//GET REQUEST
public async static Task<string> GetAsync(string url)
{
var httpClient = new HttpClient();
var response = await httpClient.GetAsync(url);
string content = await response.Content.ReadAsStringAsync();
return content;
}
AccountViewModel.cs
public async void LoadData()
{
this.Json = await DataService.GetAsync(url);
this.Accounts = GetAccounts(Json);
this.AccountList = GetAccountList(Accounts);
this.IsDataLoaded = true;
}
public static IList<AccountModel> GetAccounts(string json)
{
dynamic context = JObject.Parse(json);
JArray deserialized = (JArray)JsonConvert.DeserializeObject(context.results.ToString());
IList<AccountModel> accounts = deserialized.ToObject<IList<AccountModel>>();
return accounts;
}
public static List<AlphaKeyGroup<AccountModel>> GetAccountList(IList<AccountModel> Accounts)
{
List<AlphaKeyGroup<AccountModel>> accountList = AlphaKeyGroup<AccountModel>.CreateGroups(Accounts,
System.Threading.Thread.CurrentThread.CurrentUICulture,
(AccountModel s) => { return s.Name; }, true);
return accountList;
}
That line is your problem:
return await Task.Run(() => responseBody);
Did you try that? :
return responseBody;
Try this too:
public async static List<AccountModel> GetAccountList()
{
string json = await DataService.GetRequest(url);
...
}
A few things here. First the error
Cannot implicitly convert type 'System.Threading.Tasks.Task' to string
This error is coming from the call to DataService.GetRequest(url). This method does return a string. Tt returns a Task where T is a string. There are many ways that you can use the result of this method. the first (and best/newest) is to await the call to the method.
string json = await DataService.GetResult(url);
Making this change requires you to add the async keyboard to your method
public async static List<AccountModel> GetAccountList()
This is the new async/await pattern. Adding these words tells the compiler that the method cal is asynchronous. It allows you to make asynchronous calls but write code as if it is synchronous.
The other ways to call the method are to work the Task object directly.
// First is to use the Result property of the Task
// This is not recommended as it makes the call synchronous, and ties up the UI thread
string json = DataService.GetResult(url).Result;
// Next is to continue work after the Task completes.
DataService.GetResult(url).ContinueWith(t =>
{
string json = t.Result;
// other code here.
};
Now for the GetResult method. Using the async/await pattern requires you to return Task from methods. Even though the return type is Task, your code should return T. So as Krekkon mentioned, you should change the return line to
return responseBody;
Here is a great article about returning Task from an async method.