Bug 17585 - System.InvalidOperationException: The task has already completed
Summary: System.InvalidOperationException: The task has already completed
Alias: None
Product: iOS
Classification: Xamarin
Component: BCL Class Libraries ()
Version: 7.0.6.x
Hardware: PC Mac OS
: Normal normal
Target Milestone: Untriaged
Assignee: Marek Safar
Depends on:
Reported: 2014-02-04 01:42 UTC by Prashant Cholachagudda
Modified: 2016-07-08 14:43 UTC (History)
7 users (show)

Is this bug a regression?: ---
Last known good build:

SampleTest (9.67 KB, application/zip)
2014-03-03 05:18 UTC, Sadik Ali

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Description Prashant Cholachagudda 2014-02-04 01:42:13 UTC
System.InvalidOperationException when using Async stream operations

System.InvalidOperationException: The task has already completed
at System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1[Elevenworks.Drawing.Core.Figure].SetResult (Elevenworks.Drawing.Core.Figure result) [0x00000] in <filename unknown>:0
at Elevenworks.Drawing.Persistence.StreamingXmlPersistenceInput+<ReadAsync>c__async17.MoveNext () [0x00000] in <filename unknown>:0
at System.Threading.Tasks.SynchronizationContextContinuation.<Execute>m__0 (System.Object l) [0x00000] in <filename unknown>:0
at MonoTouch.UIKit.UIKitSynchronizationContext+<Post>c__AnonStorey89.<>m__A8 () [0x00000] in <filename unknown>:0
at MonoTouch.Foundation.NSAsyncActionDispatcher.Apply () [0x00000] in <filename unknown>:0
at (wrapper managed-to-native) MonoTouch.UIKit.UIApplication:UIApplicationMain (int,string[],intptr,intptr)
at MonoTouch.UIKit.UIApplication.Main (System.String[] args, System.String principalClassName, System.String delegateClassName) [0x00000] in <filename unknown>:0
at Elevenworks.TouchDraw.Application.Main (System.String[] args) [0x00000] in <filename unknown>:0
Comment 2 Marek Safar 2014-02-04 02:16:09 UTC
Without any test case I cannot verify that 7.0.6 TPL hotfix resolves the issue but the changes are quite hight that it does.
Comment 3 Sadik Ali 2014-03-03 05:18:22 UTC
Created attachment 6197 [details]

I have created test project with some Asynchronous methods and checked this issue but unable to reproduce this.

Checked with below environments:

All Mac
XS 4.2.3 (build 59)
Xamarin.iOS : 

I have attached Test project, let me know if I am missing any thing. Also can you please provide me some inputs to verify this issue.
Comment 4 Sebastien Pouliot 2016-05-24 21:14:05 UTC
The information was provided.
Comment 5 Vyacheslav Napadovsky 2016-07-08 14:19:20 UTC
Same exception here. Below is the possible reason:

Unhandled Exception:
System.InvalidOperationException: The task has already completed
  at System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1[System.String].SetException (System.Exception exception) [0x00000] in <filename unknown>:0
  at System.IO.StreamReader+<ReadToEndAsyncCore>c__async2.MoveNext () [0x00000] in <filename unknown>:0
  at (wrapper unbox) System.IO.StreamReader/<ReadToEndAsyncCore>c__async2:MoveNext ()
  at System.Threading.Tasks.AwaiterActionContinuation.Execute () [0x00000] in <filename unknown>:0
  at System.Threading.Tasks.Task.ProcessCompleteDelegates () [0x00000] in <filename unknown>:0
  at System.Threading.Tasks.Task.HandleGenericException (System.AggregateException e) [0x00000] in <filename unknown>:0
  at System.Threading.Tasks.Task.TrySetException (System.AggregateException aggregate, Boolean cancellation, Boolean observed) [0x00000] in <filename unknown>:0
  at System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1[System.Int32].SetException (System.Exception exception) [0x00000] in <filename unknown>:0
  at System.IO.StreamReader+<ReadBufferAsync>c__async3.MoveNext () [0x00000] in <filename unknown>:0
  at (wrapper unbox) System.IO.StreamReader/<ReadBufferAsync>c__async3:MoveNext ()
  at System.Threading.Tasks.AwaiterActionContinuation.Execute () [0x00000] in <filename unknown>:0
  at System.Threading.Tasks.Task.ProcessCompleteDelegates () [0x00000] in <filename unknown>:0
  at System.Threading.Tasks.Task.HandleGenericException (System.AggregateException e) [0x00000] in <filename unknown>:0
  at System.Threading.Tasks.Task.TrySetException (System.AggregateException aggregate, Boolean cancellation, Boolean observed) [0x00000] in <filename unknown>:0
  at System.Threading.Tasks.TaskCompletionSource`1[System.Int32].TrySetException (IEnumerable`1 exceptions) [0x00000] in <filename unknown>:0
  at System.Threading.Tasks.TaskCompletionSource`1[System.Int32].SetException (IEnumerable`1 exceptions) [0x00000] in <filename unknown>:0
  at System.Threading.Tasks.TaskCompletionSource`1[System.Int32].SetException (System.Exception exception) [0x00000] in <filename unknown>:0
  at System.Threading.Tasks.TaskFactory`1[System.Int32].InnerInvoke (System.Threading.Tasks.TaskCompletionSource`1 tcs, System.Func`2 endMethod, IAsyncResult l) [0x00000] in <filename unknown>:0
  at System.Threading.Tasks.TaskFactory`1+<FromAsyncBeginEnd>c__AnonStorey5`3[System.Int32,System.Byte[],System.Int32,System.Int32].<>m__0 (IAsyncResult l) [0x00000] in <filename unknown>:0
  at System.Net.WebAsyncResult.CB (System.Object unused) [0x00000] in <filename unknown>:0

The app creates a lot of connections during it's run. The issue happens after 7 hours of running (about 80000 connections was created-closed already).
The fault is happening while executing following code:

async Task<string> RequestGet(Uri uri) {
    var request = WebRequest.Create(uri) as HttpWebRequest;
    request.CookieContainer = _cookies;
    request.UserAgent = _userAgent;
    using (var response = await request.GetResponseTimeout())
    using (var stream = response.GetResponseStream())
    using (var reader = new StreamReader(stream, Encoding.UTF8))
        return await reader.ReadToEndAsync();

// GetRequestTimeout method:
public static Task<WebResponse> GetResponseTimeout(this WebRequest request, int timeout = 30000) {
    request.Timeout = timeout;
    return Task.FromResult(request.GetResponse());

Note, there are no more than one instance of RequestGet running at the same time (I await on all of the its calls), and _cookies container got accessed only via this method (+ its construction).
Comment 6 Vyacheslav Napadovsky 2016-07-08 14:22:59 UTC
Forgot to mention, 
$ mono --version
Mono JIT compiler version 3.2.8 (Debian 3.2.8+dfsg-10)
Copyright (C) 2002-2014 Novell, Inc, Xamarin Inc and Contributors. www.mono-project.com
        TLS:           __thread
        SIGSEGV:       normal
        Notifications: epoll
        Architecture:  armel,vfp+hard
        Disabled:      none
        Misc:          softdebug
        LLVM:          supported, not enabled.
        GC:            sgen
Comment 7 Vyacheslav Napadovsky 2016-07-08 14:43:59 UTC
Additionally, periodically this message below appears on console (0x8 is just a sample there are other numbers):

_wapi_connect: error looking up socket handle 0x8

This message appears usually after connection timeout reached.
For example: among 30998 connections there were 10262 timed out. And only 25 such messages appear on console. The code that creates so much connections is below:

async Task<bool> CheckUrl(string url, CancellationToken ct) {
    bool ret = true;
    try {
        var request = (HttpWebRequest)WebRequest.Create(url);
        using (var response = await request.GetResponseTimeout(12000))
        using (var reader = new BinaryReader(response.GetResponseStream())) {
            const int kilobyte = 1024;
            byte[] buffer = new byte[40 * kilobyte];
            int idx = 0;
            while (ret && (idx > 10 * kilobyte)) {
                int read = reader.Read(buffer, idx, buffer.Length - idx);
                idx += read;
                ret = read > 0;
    catch (OperationCanceledException) {
    catch (Exception) {
        ret = false;
    return ret;

Exceptions in this method does not get logged unfortunately; and I'm gonna add logging now.
CheckUrl method runs in heavy parallel (45 instances max), but sequentially with the RequestGet method mentioned above:
RequestGet -> ... -> RequestGet -> {CheckAvailability: 45 paralled instances of CheckUrl created -> all instances finished} -> loop for RequestGet -> ...
The issue happens on 8th of this loop iteration.
CheckAvailability function code below:

public async Task CheckAvailability(CancellationToken ct) {
    lock (_logger) {
        _logger.WriteLine("CheckAvailability: started", DateTime.Now);
    Dictionary<string, List<Item>> split;   // channel url host -> list of channels
    CheckProgress cp;
    lock (_listLock) {
        split = _list
            .GroupBy(k => (new Uri(k.Value.Url)).Host, e => e.Value)
            .ToDictionary(k => k.Key, e => e.ToList());
        cp = new CheckProgress {
            Processed = 0,
            FailsCount = 0,
            Count = _list.Count,
    const int ConcurrentChecksMax = 45;
    var tasks = new List<Task>();
    foreach (var slice in split.Values) {
        tasks.Add(CheckSequence(slice, cp, ct));
        if (tasks.Count >= ConcurrentChecksMax)
            tasks.Remove(await Task.WhenAny(tasks));
    await Task.WhenAll(tasks);
    lock (_listLock)
        _list = split.SelectMany(d => d.Value).ToDictionary(c => c.Url);
    lock (_logger) {
        _logger.WriteLine("CheckAvailability: finished");
        _logger.WriteLine("CheckAvailability: {0} fails out of {1} channels", cp.FailsCount, cp.Count);

async Task CheckSequence(ICollection<Item> sequence, CheckProgress progress, CancellationToken ct) {
    var random = new Random();
    foreach (var item in new List<Item>(sequence)) {
        bool success = await CheckUrl(item.Url, ct);
        if (!success) {
            await Task.Delay(random.Next(1000, 2000), ct);
            success = await CheckUrl(item.Url, ct);
        lock (Console.Out) {
            int fails = success ? progress.FailsCount : Interlocked.Increment(ref progress.FailsCount);
            int processed = Interlocked.Increment(ref progress.Processed);
            var status = string.Format(" {3:###}% completed ({0}/{1}, {2} fails)",
                processed, progress.Count, fails, processed * 100 / progress.Count);
        if (!success)
        await Task.Delay(random.Next(4000, 10000), ct);

class ConsoleLogger {
    public void WriteLine(string format = "", params object[] args) {
        lock (Console.Out)
            Console.Out.WriteLine("[{0:G}] {1}", DateTime.Now, string.Format(format, args));

class Item {
    public string Header { get; set; }
    public string Title { get; set; }
    public string Url { get; set; }
    public int Index { get; set; }

class CheckProgress {
    public int Processed;
    public int FailsCount;
    public int Count;

all other code I can send via mail request (just don't want to make it fully public).