HttpClient - A task was cancelled? - c#

It works fine when have one or two tasks however throws an error "A task was cancelled" when we have more than one task listed.
List<Task> allTasks = new List<Task>();
allTasks.Add(....);
allTasks.Add(....);
Task.WaitAll(allTasks.ToArray(), configuration.CancellationToken);
private static Task<T> HttpClientSendAsync<T>(string url, object data, HttpMethod method, string contentType, CancellationToken token)
{
HttpRequestMessage httpRequestMessage = new HttpRequestMessage(method, url);
HttpClient httpClient = new HttpClient();
httpClient.Timeout = new TimeSpan(Constants.TimeOut);
if (data != null)
{
byte[] byteArray = Encoding.ASCII.GetBytes(Helper.ToJSON(data));
MemoryStream memoryStream = new MemoryStream(byteArray);
httpRequestMessage.Content = new StringContent(new StreamReader(memoryStream).ReadToEnd(), Encoding.UTF8, contentType);
}
return httpClient.SendAsync(httpRequestMessage).ContinueWith(task =>
{
var response = task.Result;
return response.Content.ReadAsStringAsync().ContinueWith(stringTask =>
{
var json = stringTask.Result;
return Helper.FromJSON<T>(json);
});
}).Unwrap();
}

There's 2 likely reasons that a TaskCanceledException would be thrown:
Something called Cancel() on the CancellationTokenSource associated with the cancellation token before the task completed.
The request timed out, i.e. didn't complete within the timespan you specified on HttpClient.Timeout.
My guess is it was a timeout. (If it was an explicit cancellation, you probably would have figured that out.) You can be more certain by inspecting the exception:
try
{
var response = task.Result;
}
catch (TaskCanceledException ex)
{
// Check ex.CancellationToken.IsCancellationRequested here.
// If false, it's pretty safe to assume it was a timeout.
}

I ran into this issue because my Main() method wasn't waiting for the task to complete before returning, so the Task<HttpResponseMessage> was being cancelled when my console program exited.
C# ≥ 7.1
You can make the main method asynchronous and await the task.
public static async Task Main(){
Task<HttpResponseMessage> myTask = sendRequest(); // however you create the Task
HttpResponseMessage response = await myTask;
// process the response
}
C# < 7.1
The solution was to call myTask.GetAwaiter().GetResult() in Main() (from this answer).

var clientHttp = new HttpClient();
clientHttp.Timeout = TimeSpan.FromMinutes(30);
The above is the best approach for waiting on a large request.
You are confused about 30 minutes; it's random time and you can give any time that you want.
In other words, request will not wait for 30 minutes if they get results before 30 minutes.
30 min means request processing time is 30 min.
When we occurred error "Task was cancelled", or large data request requirements.

Another possibility is that the result is not awaited on the client side. This can happen if any one method on the call stack does not use the await keyword to wait for the call to be completed.

Promoting #JobaDiniz's comment to an answer:
Do not do the obvious thing and dispose the HttpClient instance, even though the code "looks right":
async Task<HttpResponseMessage> Method() {
using (var client = new HttpClient())
return client.GetAsync(request);
}
Disposing the HttpClient instance can cause following HTTP requests started by other instances of HttpClient to be cancelled!
The same happens with C#'s new RIAA syntax; slightly less obvious:
async Task<HttpResponseMessage> Method() {
using var client = new HttpClient();
return client.GetAsync(request);
}
Instead, the correct approach is to cache a static instance of HttpClient for your app or library, and reuse it:
static HttpClient client = new HttpClient();
async Task<HttpResponseMessage> Method() {
return client.GetAsync(request);
}
(The Async() request methods are all thread safe.)

in my .net core 3.1 applications I am getting two problem where inner cause was timeout exception.
1, one is i am getting aggregate exception and in it's inner exception was timeout exception
2, other case was Task canceled exception
My solution is
catch (Exception ex)
{
if (ex.InnerException is TimeoutException)
{
ex = ex.InnerException;
}
else if (ex is TaskCanceledException)
{
if ((ex as TaskCanceledException).CancellationToken == null || (ex as TaskCanceledException).CancellationToken.IsCancellationRequested == false)
{
ex = new TimeoutException("Timeout occurred");
}
}
Logger.Fatal(string.Format("Exception at calling {0} :{1}", url, ex.Message), ex);
}

In my situation, the controller method was not made as async and the method called inside the controller method was async.
So I guess its important to use async/await all the way to top level to avoid issues like these.

I was using a simple call instead of async. As soon I added await and made method async it started working fine.
public async Task<T> ExecuteScalarAsync<T>(string query, object parameter = null, CommandType commandType = CommandType.Text) where T : IConvertible
{
using (IDbConnection db = new SqlConnection(_con))
{
return await db.ExecuteScalarAsync<T>(query, parameter, null, null, commandType);
}
}

Another reason can be that if you are running the service (API) and put a breakpoint in the service (and your code is stuck at some breakpoint (e.g Visual Studio solution is showing Debugging instead of Running)). and then hitting the API from the client code. So if the service code a paused on some breakpoint, you just hit F5 in VS.

Related

Exception while posting to web API, HTTPClient already disposed

I'm posting a bytearray from an Android App in Xamarin.Forms to an .NET Core 2.0 WebAPI. However, I'm getting an exception saying that the NetworkStream already is disposed;
Code making the request;
public async Task PostImageAsync(ImageDTO image)
{
var content = new MultipartFormDataContent();
var byteArrayContent = new ByteArrayContent(image.Content);
content.Add(byteArrayContent, image.FileTile, image.FileName);
try
{
using (var httpClient = GetNewHttpClient())
{
SetBearerToken(httpClient);
var response = await httpClient.PostAsync($"{_apiUrl}/api/images/upload", content);
if (response.IsSuccessStatusCode)
{
}
else
{
}
}
}
catch (Exception e)
{
//Exception occurs here
var msg = e.GetBaseException().Message;
throw;
}
}
Code to get the HttpClient
private HttpClient GetNewHttpClient()
{
//HttpClientHandler is a global variable
var httpClient = new HttpClient(HttpClientHandler, false) {BaseAddress = new Uri(_apiUrl)};
return httpClient;
}
API Endpoint
[HttpPost]
public async Task<IActionResult> Upload(IFormFile file)
{
if (file == null || file.Length == 0) return BadRequest();
return Ok();
}
EDIT - SetBearerToken Method
private static void SetBearerToken(HttpClient client)
{
client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", App.StoredToken);
}
The Exception:
cannot access a disposed object. Object name: 'System.Net.Sockets.NetworkStream'.
It feels like a really obvious mistake I'm making here, but I can't get my head around it. Anybody has any ideas?
Don't dispose objects inside async functions
A using statement in an async method is "odd" in that the Dispose
call may execute in a different thread to the one which acquired the
resource (depending on synchronization context etc) but it will still
happen... assuming the thing you're waiting for ever shows up or
fail, of course. (Just like you won't end up calling Dispose in
non-async code if your using statement contains a call to a method
which never returns.)
#jon-skeet https://stackoverflow.com/a/16566605/2228916
Don’t dispose of the HttpClient:
https://aspnetmonsters.com/2016/08/2016-08-27-httpclientwrong/
Also noticed that you set _apiUrl as the BaseAddress and prefix the url in the post. Pick one or the other.

UWP 10 C# HttpResponseMessage task freezes

I couldn't find from search anyone having similar issues so:
I'm trying to get XML from server with HttpClient, but my UI freezes weirdly at line "task.Wait()". This is the code:
public void register() {
String data = "register=" + (accountName) + "&email0=" +
(email) + "&email1=" + (otherEmail);
var task = MakeRequest(data);
task.Wait(); //freezes here
var response = task.Result;
String resultstring = response.Content.ReadAsStringAsync().Result;
}
private static async Task<System.Net.Http.HttpResponseMessage> MakeRequest(String data)
{
var content = new StringContent(data, Encoding.UTF8, "application/x-www-form-urlencoded");
var httpClient = new System.Net.Http.HttpClient();
System.Net.Http.HttpResponseMessage responseMessage=null;
try
{
responseMessage = await httpClient.PostAsync(server, content);
}
catch(Exception ex)
{
responseMessage.ReasonPhrase = string.Format("RestHttpClient.SendRequest failed: {0}", ex);
}
return responseMessage;
}
Any help is greatly appreciated!
It's not freezing weirdly at all - it's freezing entirely reasonably.
You're calling task.Wait(), which stops your UI thread from doing any more work until that task has completed. However, that task itself needs to get back to the UI thread when the task returned by httpClient.PostAsync completes, in order to continue with the rest of your async method.
Basically, you shouldn't use Task.Wait() or Task.Result unless you know for sure that the task itself won't be blocked waiting to continue on the thread you're currently executing on.
Ideally, your register method (which should be Register to follow .NET naming conventions) should be asynchronous as well, so you can await the task returned by MakeRequest.
Additionally, you should probably change the way MakeRequest awaits the task - as the rest of that method doesn't really need to run on the UI thread, you can use:
responseMessage = await httpClient.PostAsync(server, content).ConfigureAwait(false);
Finally, this line:
responseMessage.ReasonPhrase = string.Format("RestHttpClient.SendRequest failed: {0}", ex);
... will throw a NullReferenceException if it ever executes. If an exception is thrown, responseMessage will still be null.
Answer of Jon Skeet solved my problem, and here is the working code, if some other beginner is having same problems.
public async void Register{
String data = "register=" + (accountName) + "&email0=" +
(email) + "&email1=" + (otherEmail);
var task = await MakeRequest(data);
String resultstring = taskContent.ReadAsStringAsync().Result;
}
private static async Task<System.Net.Http.HttpResponseMessage> MakeRequest(String data)
{
var content = new StringContent(data, Encoding.UTF8, "application/x-www-form-urlencoded");
var httpClient = new System.Net.Http.HttpClient();
return await httpClient.PostAsync(server, content).ConfigureAwait(false);
}
Thank you very much!

How to create an Observable sequence that will resend HTTP requests after a timeout?

I'm new to Rx and I'm trying to create an Observable sequence that will allow me to do the following:
Send a HTTP POST request to a URI using
System.Net.Http.HttpClient.SendAsync(request, cancelToken)
Wait for a configurable time period for a response to be returned or for the request to timeout.
If the request times out, then repeat the request.
Continue repeating the request until either a response is received (doesn't have to be 200 OK) or a max. number of retries is reached.
If the max. number of retries is reached then I have to know about it so I can log an error.
I've been playing around with:
HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Post, "Some URI");
...
CancellationTokenSource tokenSource = new CancellationTokenSource();
CancellationToken token = tokenSource.Token;
...
try
{
var res = Observable.FromAsync(() => _client.SendAsync(request, token))
.Timeout(TimeSpan.FromSeconds(5));
try
{
HttpResponseMessage response = res.Wait();
// Process response
...
}
catch (TaskCancelledException)
{
....
}
}
catch (TimeoutException)
{
....
}
but I'm not sure of the best way to kick off the request again after a timeout occurs, and also how I should check that I have reached the max. number of retries.
Also, I'm not sure whether I should be putting a timeout policy on the observable sequence or whether I should be setting the Timeout property on the HttpClient object itself.
[Edited on 11 Dec 2014]
Based on comments below, I have updated the code from #TheZenCoder so that it uses an await
var attempts = 0;
HttpResponseMessage response = null;
try
{
response = await Observable
.FromAsync((ct) =>
{
attempts++;
return SendRequest(token, ct);
})
.Retry(5)
.LastAsync();
}
catch (Exception ex)
{
Console.WriteLine(ex.Message);
}
I've only done limited testing so far but it seems to work ok.
For sending the request, setting a request timeout and using a cancellation token you can wrap that into a method like:
private static Task<HttpResponseMessage> SendRequest(CancellationToken token)
{
var client = new HttpClient { Timeout = TimeSpan.FromSeconds(5) };
var request = new HttpRequestMessage(HttpMethod.Get, new Uri("http://www.google.ba"));
return client.SendAsync(request, token);
}
I think setting the timeout on the HTTP client is a cleaner option instead of using the Observable timeout.
Then you can use the IObservable Retry method to retry this operation multiple times. If you are not afraid of opensource there are also some more flexible retry methods in the RXX Extensions as noted by #Dave Sexton in the comments.
var attempts = 0;
try
{
var response = Observable
.FromAsync(() =>
{
attempts++;
return SendRequest(token);
})
.Retry(5)
.Wait();
}
catch (Exception ex)
{
Console.WriteLine(ex.Message);
}
If an HTTP timeout or another error occurs in the SendRequest method the retries will continue. An exception will be thrown after the last retry containing information about the error. The number of retries is set to the attempts variable.
Have in mind that using the Wait method effectively blocks the calling thread execution until a result is available, and that is not something you ever want to do when using Async code. Maybe you have some specific scenario on mind, and that is why i left it there in my example.

The request message was already sent. Cannot send the same request message multiple times

Is there anything wrong with my code here? I keep getting this error:
System.InvalidOperationException: The request message was already sent. Cannot send the same request message multiple times.
My HttpRequestMessage is inside a Func so I figured I get a brand new request each time I pass in func().
public async Task<HttpResponseMessage> GetAsync(HttpRequestMessage request)
{
return await RequestAsync(() => request);
}
public async Task<HttpResponseMessage> RequestAsync(Func<HttpRequestMessage> func)
{
var response = await ProcessRequestAsync(func);
if (response.StatusCode == HttpStatusCode.Unauthorized)
{
WaitForSomeTime();
response = await ProcessRequestAsync(func);
}
return response;
}
private async Task<HttpResponseMessage> ProcessRequestAsync(Func<HttpRequestMessage> func)
{
var client = new HttpClient();
var response = await client.SendAsync(func()).ConfigureAwait(false);
return response;
}
You are calling the same func parameter twice:
var response = await ProcessRequestAsync(func);
//...
response = await ProcessRequestAsync(func);
In this case func returns the same request every single time. It doesn't generate a new one every time you call it. If you truly need a different request each time then func needs to return a new message each call:
var response = await GetAsync(() => new HttpRequestMessage()); // Create a real request.
public async Task<HttpResponseMessage> GetAsync(Func<HttpRequestMessage> requestGenerator)
{
return await RequestAsync(() => requestGenerator());
}
I had the same issue, but no repetition in my code. Turned out I had added a watch on an asynchronous process. That watch called the process while I stepped through the code, so that when I got to the line I was trying to debug it crashed with this error message.
Removing all watches solved the problem.
Leaving this here for other people who might have the same problem.

Get response from PostAsJsonAsync

I have this line of code
var response = new HttpClient().PostAsJsonAsync(posturi, model).Result;
The Called WebAPI controller returns a bool to make sure the object was saved, but how do I return that bool response?
Continue to get from content:
var httpClient = new HttpClient();
var response = httpClient.PostAsJsonAsync(posturi, model).Result;
bool returnValue = response.Content.ReadAsAsync<bool>().Result;
But, this is really naive approach for quick way to get result. PostAsJsonAsync and ReadAsAsync is not designed to do like this, they are designed to support async await programming, so your code should be:
var httpClient = new HttpClient();
var response = await httpClient.PostAsJsonAsync(posturi, model);
bool returnValue = await response.Content.ReadAsAsync<bool>();
Also, instead of using a flag to check whether an object is saved or not, you should make use of HTTP codes by returning 200 OK to determine that saving is successfully.
The accepted answer is technically correct but blocks the current thread on calls to .Result. If you are using .NET 4.5 or higher, you should avoid that in almost all situations. Instead, use the equivalent asynchronous (non-blocking) version:
var httpClient = new HttpClient();
var response = await httpClient.PostAsJsonAsync(posturi, model);
bool returnValue = await response.Content.ReadAsAsync<bool>();
Note that the method containing the above code needs to be marked async, and should itself be awaited.
Since its an Async operation don't immediately do .Result as its wrong
Instead you need to do it async by doing this:
var httpClient = new HttpClient()
var task = httpClient.PostAsJsonAsync(posturi, model)
.ContinueWith( x => x.Result.Content.ReadAsAsync<bool>().Result);
// 1. GETTING RESPONSE - NOT ASYNC WAY
task.Wait(); //THIS WILL HOLD THE THREAD AND IT WON'T BE ASYNC ANYMORE!
bool response = task.Result
// 2. GETTING RESPONSE - TASK ASYNC WAY (usually used in < .NET 4.5
task.ContinueWith( x => {
bool response = x.Result
});
// 3. GETTING RESPONSE - TASK ASYNC WAY (usually used in >= .NET 4.5
bool response = await task;
NOTE: I just wrote them in here, so I didnt actually test them but more or less that's what you want.
I hope it helps!
I used HttpStatusCode to check the result.
public HttpStatusCode PostStaffPositions(Foo foo)
{
string uri = myapiuri;
using (HttpClient httpClient = new HttpClient())
{
var response = httpClient.PostAsJsonAsync(uri, foo).Result;
return response.StatusCode;
}
}
And then in Controller check it like this:
HttpStatusCode update = staffrest.PostStaffPositions(foo);
if (update == HttpStatusCode.OK)
{
//Update Succeed
}
else
{
//Update Failed
}
If you call the generic version, it should give you back the bool:
var response = new HttpClient().PostAsJsonAsync<bool>(posturi, model).Result;
At least according to the docs.
It's July 2021 and I'm using .net 5 (namely the .net core 5).
I did not see any generic methods above in System.Net.Http. Now the code looks like this (tested):
using (var client = new HttpClient())
{
client.BaseAddress = new Uri("https://localhost:44330/api/Book/Add");
var response = client.PostAsJsonAsync(client.BaseAddress,
JsonSerializer.Serialize(_teamSummaries));
MessageBox.Show(#"Result is " + JsonSerializer.Serialize(response));
var returnValue = response.Result.Content.ReadAsStringAsync();
MessageBox.Show(#"Return value is " + returnValue.Result);
}
There are also ReadAsStringAsync, ReadAsByteArrayAsync, ReadAsStream, ReadFromJsonAsync, ReadFromJsonAsync<T> (this method returns Task<T>).
But from the text meaning "ReadFromJsonAsync", I think the T is not the bool mentioned above, but a class that contains the bool member. If you want to return something like book, give it a try.
On the other hands, since code on the server looks like this(.net 5):
[HttpPost]
[Route("Add")]
public async Task<ActionResult<IEnumerable<Book>>> Add(string value)
{
var all = await _dbCollection.FindAsync(Builders<Book>.Filter.Empty);
return Ok("Everything is ok.");
}
So, if we want to return true by bool, we should return Ok(...). If we want to return false by bool, we should return something else. There are more than 20 other types of results, which contains much more information rather than just "false".

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