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Task based behavior is blocking the parent thread [duplicate]

This question already has an answer here:
Async Await Running Synchronously
(1 answer)
Closed 2 years ago.
Assuming that I need to have some concurrent processes and I would like to forget about the old Thread class and stick to Task with an async pattern. Given the following codes:
// Scenario 1
public async Task Accept() {
while(true) {
Connection con = await connection.AcceptAsync();
await HandleConnection(con);
}
}
public Task HandleConnection(Connection con) {
// Long Running Logic
return Task.CompletedTask;
}
// Scenario 2
public async Task Accept() {
while(true) {
Connection con = await connection.AcceptAsync();
HandleConnection(con); // Produces a warning
}
}
public Task HandleConnection(Connection con) {
// Long Running Logic
return Task.CompletedTask;
}
Both these approaches fail and I cannot handle the connection concurrently. For example when the first connection is accepted then I cannot accept the second connection until the HandleConnection method finishes its job. To solve this problem I can do the following:
public async Task Accept() {
while(true) {
Connection con = await connection.AcceptAsync();
HandleConnection(con); // Produces a warning
}
}
public Task HandleConnection(Connection con) {
return Task.Run(()=> {
//Long Running Logic
});
}
Now I am able to handle multiple connections but this behavior raises a few questions:
1- I heard that the await keyword in contrast with wait is non-blocking and in fact the whole async pattern in non-blocking as well. But in this situation it is actually blocking the parent thread.
2- When a method is async, a Task resembling that method must be generated as a result so that it can be awaited, is that true? if that is true then why is it blocking? else how does the await mechanism work?
3- If I don't await the Task, I get a compile time warning which says I have to await the task, but if I use it then the parent thread is blocked.
To ditch the warning I can do HandleConnection(con).Wait(0) but isn't that a bad practice?

I played with this issue and tested that thoroughly. The conclusion is that in a method that returns a Task, if you do not actually return a Task or instead await or return Task.CompletedTask, that method will still run on the Parent thread and will surely block it. You can only benefit from multithreading if you actually raise a Task as I represented in my question.
To prove this, the following is blocking:
public static void Main(String[] args) {
test();
Console.WriteLine("Hello World");
}
private static Task test() {
while(true) {}
return Task.CompletedTask;
}
In this code test will block the main thread.
If you make the test async, it will still block the main thread.
public static void Main(String[] args) {
test();
Console.WriteLine("Hello World");
}
private static async Task test() {
while(true) {}
}
This is while the following does not block the main thread:
public static async Task test() {
await Task.Run(()=> {
while(true) {}
});
}
However, if you don't want to await the Task you should raise the Task in void method:
public void Method() {
Task.Run(()=> ());
}
This is much better than Task.Wait(0).

Async method deadlocks with TestScheduler in ReactiveUI

I'm trying to use the reactiveui test scheduler with an async method in a test.
The test hangs when the async call is awaited.
The root cause seems to be a command that's awaited in the async method.
[Fact]
public async Task Test()
=> await new TestScheduler().With(async scheduler =>
{
await SomeAsyncMethod();
// *** execution never gets here
Debugger.Break();
});
private async Task SomeAsyncMethod()
{
var command = ReactiveCommand.CreateFromTask(async () =>
{
await Task.Delay(100);
});
// *** this hangs
await command.Execute();
}
How can I do an async call in combination with the test scheduler that does not deadlock?
I'm using reactiveui 9.4.1
EDIT:
I've tried the WithAsync() method as suggested in Funks answer, but the behaviour is the same.

How can I do an async call in combination with the test scheduler?
In short
command.Execute() is a cold observable. You need to subscribe to it, instead of using await.
Given your interest in TestScheduler, I take it you want to test something involving time. However, from the When should I care about scheduling section:
threads created via "new Thread()" or "Task.Run" can't be controlled in a unit test.
So, if you want to check, for example, if your Task completes within 100ms, you're going to have to wait until the async method completes. To be sure, that's not the kind of test TestScheduler is meant for.
The somewhat longer version
The purpose of TestScheduler is to verify workflows by putting things in motion and verifying state at certain points in time. As we can only manipulate time on a TestScheduler, you'd typically prefer not to wait on real async code to complete, given there's no way to fast forward actual computations or I/O. Remember, it's about verifying workflows: vm.A has new value at 20ms, so vm.B should have new val at 120ms,...
So how can you test the SUT?
1\ You could mock the async method using scheduler.CreateColdObservable
public class ViewModelTests
{
[Fact]
public void Test()
{
string observed = "";
new TestScheduler().With(scheduler =>
{
var observable = scheduler.CreateColdObservable(
scheduler.OnNextAt(100, "Done"));
observable.Subscribe(value => observed = value);
Assert.Equal("", observed);
scheduler.AdvanceByMs(99);
Assert.Equal("", observed);
scheduler.AdvanceByMs(1);
Assert.Equal("Done", observed);
});
}
}
Here we basically replaced command.Execute() with var observable created on scheduler.
It's clear the example above is rather simple, but with several observables notifying each other this kind of test can provide valuable insights, as well as a safety net while refactoring.
Ref:
Answer by Paul Betts
Control Time with the TestScheduler
2\ You could reference the IScheduler explicitly
a) Using the schedulers provided by RxApp
public class MyViewModel : ReactiveObject
{
public string Observed { get; set; }
public MyViewModel()
{
Observed = "";
this.MyCommand = ReactiveCommand
.CreateFromTask(SomeAsyncMethod);
}
public ReactiveCommand<Unit, Unit> MyCommand { get; }
private async Task SomeAsyncMethod()
{
await RxApp.TaskpoolScheduler.Sleep(TimeSpan.FromMilliseconds(100));
Observed = "Done";
}
}
public class ViewModelTests
{
[Fact]
public void Test()
{
new TestScheduler().With(scheduler =>
{
var vm = new MyViewModel();
vm.MyCommand.Execute().Subscribe();
Assert.Equal("", vm.Observed);
scheduler.AdvanceByMs(99);
Assert.Equal("", vm.Observed);
scheduler.AdvanceByMs(1);
Assert.Equal("Done", vm.Observed);
});
}
}
Note
CreateFromTask creates a ReactiveCommand with asynchronous execution logic. There's no need to define the Test method as async or await the TestScheduler.
Within the With extension method's scope RxApp.TaskpoolScheduler = RxApp.MainThreadScheduler = the new TestScheduler().
b) Managing your own schedulers through constructor injection
public class MyViewModel : ReactiveObject
{
private readonly IScheduler _taskpoolScheduler;
public string Observed { get; set; }
public MyViewModel(IScheduler scheduler)
{
_taskpoolScheduler = scheduler;
Observed = "";
this.MyCommand = ReactiveCommand
.CreateFromTask(SomeAsyncMethod);
}
public ReactiveCommand<Unit, Unit> MyCommand { get; }
private async Task SomeAsyncMethod()
{
await _taskpoolScheduler.Sleep(TimeSpan.FromMilliseconds(100));
Observed = "Done";
}
}
public class ViewModelTests
{
[Fact]
public void Test()
{
new TestScheduler().With(scheduler =>
{
var vm = new MyViewModel(scheduler); ;
vm.MyCommand.Execute().Subscribe();
Assert.Equal("", vm.Observed);
scheduler.AdvanceByMs(99);
Assert.Equal("", vm.Observed);
scheduler.AdvanceByMs(0);
Assert.Equal("Done", vm.Observed);
});
}
}
Ref:
Kent Boogaert's Answer
Testing Rx code - ISchedulerProvider
Let's close ranks with another quote from Haacked:
Unfortunately, and this next point is important, the TestScheduler doesn’t extend into real life, so your shenanigans are limited to your asynchronous Reactive code. Thus, if you call Thread.Sleep(1000) in your test, that thread will really be blocked for a second. But as far as the test scheduler is concerned, no time has passed.

Have you tried to use ConfigureAwait(false) when calling nested method?
[Fact]
public async Task Test()
=> await new TestScheduler().With(async scheduler =>
{
// this hangs
await SomeAsyncMethod().ConfigureAwait(false);
// ***** execution will never get to here
Debugger.Break();
}

Please try using .ConfigureAwait(false) on all your async methods.
This will provide you non-blocking behavior.
[Fact]
public async Task Test()
=> await new TestScheduler().With(async scheduler =>
{
await SomeAsyncMethod().ConfigureAwait(false);
// *** execution never gets here
Debugger.Break();
}).ConfigureAwait(false);
private async Task SomeAsyncMethod()
{
var command = ReactiveCommand.CreateFromTask(async () =>
{
await Task.Delay(100).ConfigureAwait(false);
}).ConfigureAwait(false);
// *** this hangs
await command.Execute();
}
Another way to test whether the problem is related with ConfigureAwait is to port your project to Asp.Net Core and test it there.
Asp.net core does not need to use ConfigureAwait to prevent this blocking issue.
Check this for Reference

How can I get the result of my work in a Task

I need to do a work in a Task (infinite loop for monitoring) but how can I get the result of this work?
My logic to do this stuff i wrong? This is a scope problem I think.
There is an example simplified:
The variable is "first" and I want "edit"
namespace my{
public class Program{
public static void Main(string[] args){
Logic p = new Logic();
Task t = new Task(p.process);
t.Start();
Console.WriteLine(p.getVar());// result="first"
}
}
public class Logic{
public string test = "first";
public void process(){
while(true){
//If condition here
this.test = "edit";
}
}
public String getVar(){
return this.test;
}
}
}

It can be done using custom event. In your case it can be something like:
public event Action<string> OnValueChanged;
Then attach to it
p.OnValueChanged += (newValue) => Console.WriteLine(newValue);
And do not forget to fire it
this.test = "edit";
OnValueChanged?.Invoke(this.test);

Tasks aren't threads, they don't need a .Start call to start them. All examples and tutorials show the use of Task.Run or Task.StartNew for a reason - tasks are a promise that a function will execute at some point in the future and produce a result. They will run on threads pulled from a ThreadPool when a Task Scheduler decides they should. Creating cold tasks and calling .Start doesn't guarantee they will start, it simply makes the code a lot more difficult to read.
In the simplest case, polling eg a remote HTTP endpoint could be as simple as :
public static async Task Main()
{
var client=new HttpClient(serverUrl);
while(true)
{
var response=await client.GetAsync(relativeServiceUrl);
if(!response.IsSuccessStatusCode)
{
//That was an error, do something with it
}
await Task.Delay(1000);
}
}
There's no need to start a new Task because GetAsync is asynchronous. WCF and ADO.NET also provide asynchronous execution methods.
If there's no asynchronous method to call, or if we need to perform some heavey work before the async call, we can use Task.Run to start a method in parallel and await for it to finish:
public bool CheckThatService(string serviceUrl)
{
....
}
public static async Task Main()
{
var url="...";
//...
while(true)
{
var ok=Task.Run(()=>CheckThatService(url));
if(!ok)
{
//That was an error, do something with it
}
await Task.Delay(1000);
}
}
What if we want to test multiple systems in parallel? We can start multiple tasks in parallel, await all of them to complete and check their results:
public static async Task Main()
{
var urls=new[]{"...","..."};
//...
while(true)
{
var tasks=urls.Select(url=>Task.Run(()=>CheckThatService(url));
var responses=await Task.WhenAll(tasks);
foreach(var response in responses)
{
///Check the value, due something
}
await Task.Delay(1000);
}
}
Task.WhenAll returns an array with the results in the order the tasks were created. This allows checking the index to find the original URL. A better idea would be to return the result and url together, eg using tuples :
public static (bool ok,string url) CheckThatService(string serviceUrl)
{
....
return (true,url);
}
The code wouldn't change a lot:
var tasks=urls.Select(url=>Task.Run(()=>CheckThatService(url));
var responses=await Task.WhenAll(tasks);
foreach(var response in responses.Where(resp=>!resp.ok))
{
///Check the value, due something
}
What if we wanted to store the results from all the calls? We can't use a List or Queue because they aren't thread safe. We can use a ConcurrentQueue instead:
ConcurrentQueue<string> _results=new ConcurrentQueue<string>();
public static (bool ok,string url) CheckThatService(string serviceUrl)
{
....
_results.Enqueue(someresult);
return (true,url);
}
If we want to report progress regularly we can use IProgress<T> as shown in Enabling Progress and Cancellation in Async APIs.
We could put all the monitoring code in a separate method/class that accepts an IProgress< T> parameter with a progress object that can report success, error messages and the URL that caused them, eg :
class MonitorDTO
{
public string Url{get;set;}
public bool Success{get;set;}
public string Message{get;set;}
public MonitorDTO(string ulr,bool success,string msg)
{
//...
}
}
class MyMonitor
{
string[] _urls=url;
public MyMonitor(string[] urls)
{
_urls=url;
}
public Task Run(IProgress<MonitorDTO> progress)
{
while(true)
{
var ok=Task.Run(()=>CheckThatService(url));
if(!ok)
{
_progress.Report(new MonitorDTO(ok,url,"some message");
}
await Task.Delay(1000);
}
}
}
This class could be used in this way:
public static async Task Maim()
{
var ulrs=new[]{....};
var monitor=new MyMonitor(urls);
var progress=new Progress<MonitorDTO>(pg=>{
Console.WriteLine($"{pg.Success} for {pg.Url}: {pg.Message}");
});
await monitor.Run(progress);
}
Enabling Progress and Cancellation in Async APIs shows how to use the CancellationTokenSource to implement another important part of a monitoring class - cancelling it. The monitoring method could check the status of a cancellation token periodically and stop monitoring when it's raised:
public Task Run(IProgress<MonitorDTO> progress,CancellationToken ct)
{
while(!ct.IsCancellationRequested)
{
//...
}
}
public static async Task Maim()
{
var ulrs=new[]{....};
var monitor=new MyMonitor(urls);
var progress=new Progress<MonitorDTO>(pg=>{
Console.WriteLine($"{pg.Success} for {pg.Url}: {pg.Message}");
});
var cts = new CancellationTokenSource();
//Not awaiting yet!
var monitorTask=monitor.Run(progress,cts.Token);
//Keep running until the first keypress
Console.ReadKey();
//Cancel and wait for the monitoring class to gracefully stop
cts.Cancel();
await monitorTask;
In this case the loop will exit when the CancellationToken is raised. By not awaiting on MyMonitor.Run() we can keep working on the main thread until an event occurs that signals monitoring should stop.

The getVar method is executed before the process method.
Make sure that you wait until your task is finished before you call the getVar method.
Logic p = new Logic();
Task t = new Task(p.process);
t.Start();
t.Wait(); // Add this line!
Console.WriteLine(p.getVar());
If you want to learn more about the Wait method, please check this link.

Why is await exiting?

Given the following code:
public async Task Send() // part of Sender class
{
// sync code
}
// //
private async Task HandleMessage()
{
// await sender.Send(); // exits HandleMessage immediately
sender.Send().Wait(); // works as expected, waiting to complete
DoOtherStuff(); // doesn't get hit with await
return;
}
RunRecurringTask(async () => await HandleMessage(), result);
public void RunRecurringTask(Action action, RecurringTaskRunResult result)
{
action();
result.DoStuff();
}
I thought that await tells the thread to come back when the awaited thing is complete, but it looks like for some reason that's not happening: the remaining code is never hit and everything just... stops. What could be causing this?
This is a console application in an Azure WebJob, for what it's worth. When Wait is used, I get the expected results, however with await, the job just completes.

You should never do async void unless you are writing a event handler. A Action with the async modifier is a async void method. You need to make the argument a Func<Task> and then do await action() in your RunRecurringTask
private async Task HandleMessage()
{
await sender.Send();
DoOtherStuff();
return;
}
RunRecurringTask(async () => await HandleMessage(), result);
//You also could do
//RunRecurringTask(() => HandleMessage(), result);
public async Task RunRecurringTask(Func<Task> action, RecurringTaskRunResult result)
{
await action();
result.DoStuff();
}
If you had other methods that where not marked with async you will need to change all of them up the call stack till you get to the entry point from the SDK. The SDK understands how to handle functions with a async Task return type since the 0.4.0-beta version.

How to run a Task on a custom TaskScheduler using await?

I have some methods returning Task<T> on which I can await at will. I'd like to have those Tasks executed on a custom TaskScheduler instead of the default one.
var task = GetTaskAsync ();
await task;
I know I can create a new TaskFactory (new CustomScheduler ()) and do a StartNew () from it, but StartNew () takes an action and create the Task, and I already have the Task (returned behind the scenes by a TaskCompletionSource)
How can I specify my own TaskScheduler for await ?

I think what you really want is to do a Task.Run, but with a custom scheduler. StartNew doesn't work intuitively with asynchronous methods; Stephen Toub has a great blog post about the differences between Task.Run and TaskFactory.StartNew.
So, to create your own custom Run, you can do something like this:
private static readonly TaskFactory myTaskFactory = new TaskFactory(
CancellationToken.None, TaskCreationOptions.DenyChildAttach,
TaskContinuationOptions.None, new MyTaskScheduler());
private static Task RunOnMyScheduler(Func<Task> func)
{
return myTaskFactory.StartNew(func).Unwrap();
}
private static Task<T> RunOnMyScheduler<T>(Func<Task<T>> func)
{
return myTaskFactory.StartNew(func).Unwrap();
}
private static Task RunOnMyScheduler(Action func)
{
return myTaskFactory.StartNew(func);
}
private static Task<T> RunOnMyScheduler<T>(Func<T> func)
{
return myTaskFactory.StartNew(func);
}
Then you can execute synchronous or asynchronous methods on your custom scheduler.

The TaskCompletionSource<T>.Task is constructed without any action and the scheduler
is assigned on the first call to ContinueWith(...) (from Asynchronous Programming with the Reactive Framework and the Task Parallel Library — Part 3).
Thankfully you can customize the await behavior slightly by implementing your own class deriving from INotifyCompletion and then using it in a pattern similar to await SomeTask.ConfigureAwait(false) to configure the scheduler that the task should start using in the OnCompleted(Action continuation) method (from await anything;).
Here is the usage:
TaskCompletionSource<object> source = new TaskCompletionSource<object>();
public async Task Foo() {
// Force await to schedule the task on the supplied scheduler
await SomeAsyncTask().ConfigureScheduler(scheduler);
}
public Task SomeAsyncTask() { return source.Task; }
Here is a simple implementation of ConfigureScheduler using a Task extension method with the important part in OnCompleted:
public static class TaskExtension {
public static CustomTaskAwaitable ConfigureScheduler(this Task task, TaskScheduler scheduler) {
return new CustomTaskAwaitable(task, scheduler);
}
}
public struct CustomTaskAwaitable {
CustomTaskAwaiter awaitable;
public CustomTaskAwaitable(Task task, TaskScheduler scheduler) {
awaitable = new CustomTaskAwaiter(task, scheduler);
}
public CustomTaskAwaiter GetAwaiter() { return awaitable; }
public struct CustomTaskAwaiter : INotifyCompletion {
Task task;
TaskScheduler scheduler;
public CustomTaskAwaiter(Task task, TaskScheduler scheduler) {
this.task = task;
this.scheduler = scheduler;
}
public void OnCompleted(Action continuation) {
// ContinueWith sets the scheduler to use for the continuation action
task.ContinueWith(x => continuation(), scheduler);
}
public bool IsCompleted { get { return task.IsCompleted; } }
public void GetResult() { }
}
}
Here's a working sample that will compile as a console application:
using System;
using System.Collections.Generic;
using System.Runtime.CompilerServices;
using System.Threading.Tasks;
namespace Example {
class Program {
static TaskCompletionSource<object> source = new TaskCompletionSource<object>();
static TaskScheduler scheduler = new CustomTaskScheduler();
static void Main(string[] args) {
Console.WriteLine("Main Started");
var task = Foo();
Console.WriteLine("Main Continue ");
// Continue Foo() using CustomTaskScheduler
source.SetResult(null);
Console.WriteLine("Main Finished");
}
public static async Task Foo() {
Console.WriteLine("Foo Started");
// Force await to schedule the task on the supplied scheduler
await SomeAsyncTask().ConfigureScheduler(scheduler);
Console.WriteLine("Foo Finished");
}
public static Task SomeAsyncTask() { return source.Task; }
}
public struct CustomTaskAwaitable {
CustomTaskAwaiter awaitable;
public CustomTaskAwaitable(Task task, TaskScheduler scheduler) {
awaitable = new CustomTaskAwaiter(task, scheduler);
}
public CustomTaskAwaiter GetAwaiter() { return awaitable; }
public struct CustomTaskAwaiter : INotifyCompletion {
Task task;
TaskScheduler scheduler;
public CustomTaskAwaiter(Task task, TaskScheduler scheduler) {
this.task = task;
this.scheduler = scheduler;
}
public void OnCompleted(Action continuation) {
// ContinueWith sets the scheduler to use for the continuation action
task.ContinueWith(x => continuation(), scheduler);
}
public bool IsCompleted { get { return task.IsCompleted; } }
public void GetResult() { }
}
}
public static class TaskExtension {
public static CustomTaskAwaitable ConfigureScheduler(this Task task, TaskScheduler scheduler) {
return new CustomTaskAwaitable(task, scheduler);
}
}
public class CustomTaskScheduler : TaskScheduler {
protected override IEnumerable<Task> GetScheduledTasks() { yield break; }
protected override bool TryExecuteTaskInline(Task task, bool taskWasPreviouslyQueued) { return false; }
protected override void QueueTask(Task task) {
TryExecuteTask(task);
}
}
}

There is no way to embed rich async features into a custom TaskScheduler. This class was not designed with async/await in mind. The standard way to use a custom TaskScheduler is as an argument to the Task.Factory.StartNew method. This method does not understand async delegates. It is possible to provide an async delegate, but it is treated as any other delegate that returns some result. To get the actual awaited result of the async delegate one must call Unwrap() to the task returned.
This is not the problem though. The problem is that the TaskScheduler infrastructure does not treat the async delegate as a single unit of work. Each task is split into multiple mini-tasks (using every await as a separator), and each mini-task is processed individually. This severely restricts the asynchronous functionality that can be implemented on top of this class. As an example here is a custom TaskScheduler that is intended to queue the supplied tasks one at a time (to limit the concurrency in other words):
public class MyTaskScheduler : TaskScheduler
{
private readonly SemaphoreSlim _semaphore = new SemaphoreSlim(1);
protected async override void QueueTask(Task task)
{
await _semaphore.WaitAsync();
try
{
await Task.Run(() => base.TryExecuteTask(task));
await task;
}
finally
{
_semaphore.Release();
}
}
protected override bool TryExecuteTaskInline(Task task,
bool taskWasPreviouslyQueued) => false;
protected override IEnumerable<Task> GetScheduledTasks() { yield break; }
}
The SemaphoreSlim should ensure that only one Task would run at a time. Unfortunately it doesn't work. The semaphore is released prematurely, because the Task passed in the call QueueTask(task) is not the task that represents the whole work of the async delegate, but only the part until the first await. The other parts are passed to the TryExecuteTaskInline method. There is no way to correlate these task-parts, because no identifier or other mechanism is provided. Here is what happens in practice:
var taskScheduler = new MyTaskScheduler();
var tasks = Enumerable.Range(1, 5).Select(n => Task.Factory.StartNew(async () =>
{
Console.WriteLine($"{DateTime.Now:HH:mm:ss.fff} Item {n} Started");
await Task.Delay(1000);
Console.WriteLine($"{DateTime.Now:HH:mm:ss.fff} Item {n} Finished");
}, default, TaskCreationOptions.None, taskScheduler))
.Select(t => t.Unwrap())
.ToArray();
Task.WaitAll(tasks);
Output:
05:29:58.346 Item 1 Started
05:29:58.358 Item 2 Started
05:29:58.358 Item 3 Started
05:29:58.358 Item 4 Started
05:29:58.358 Item 5 Started
05:29:59.358 Item 1 Finished
05:29:59.374 Item 5 Finished
05:29:59.374 Item 4 Finished
05:29:59.374 Item 2 Finished
05:29:59.374 Item 3 Finished
Disaster, all tasks are queued at once.
Conclusion: Customizing the TaskScheduler class is not the way to go when advanced async features are required.
Update: Here is another observation, regarding custom TaskSchedulers in the presence of an ambient SynchronizationContext. The await mechanism by default captures the current SynchronizationContext, or the current TaskScheduler, and invokes the continuation on either the captured context
or the scheduler. If both are present, the current SynchronizationContext is preferred, and the current TaskScheduler is ignored. Below is a demonstration of this behavior, in a WinForms application¹:
private async void Button1_Click(object sender, EventArgs e)
{
await Task.Factory.StartNew(async () =>
{
MessageBox.Show($"{Thread.CurrentThread.ManagedThreadId}, {TaskScheduler.Current}");
await Task.Delay(1000);
MessageBox.Show($"{Thread.CurrentThread.ManagedThreadId}, {TaskScheduler.Current}");
}, default, TaskCreationOptions.None,
TaskScheduler.FromCurrentSynchronizationContext()).Unwrap();
}
Clicking the button causes two messages to popup sequentially, with this information:
1, System.Threading.Tasks.SynchronizationContextTaskScheduler
1, System.Threading.Tasks.ThreadPoolTaskScheduler
This experiment shows that only the first part of the asynchronous delegate, the part before the first await, was scheduled on the non-default scheduler.
This behavior limits even further the practical usefulness of custom TaskSchedulers in an async/await-enabled environment.
¹ Windows Forms applications have a WindowsFormsSynchronizationContext installed automatically, when the Application.Run method is called.

Can you fit for this method call:
await Task.Factory.StartNew(
() => { /* to do what you need */ },
CancellationToken.None, /* you can change as you need */
TaskCreationOptions.None, /* you can change as you need */
customScheduler);

After the comments it looks like you want to control the scheduler on which the code after the await is run.
The compile creates a continuation from the await that runs on the current SynchronizationContext by default. So your best shot is to set up the SynchronizationContext before calling await.
There are some ways to await a specific context. See Configure Await from Jon Skeet, especially the part about SwitchTo, for more information on how to implement something like this.
EDIT:
The SwitchTo method from TaskEx has been removed, as it was too easy to misuse. See the MSDN Forum for reasons.

Faced with same issue, tried to use LimitedConcurrencyLevelTaskScheduler, but it does not support async tasks. So...
Just wrote my own small simple Scheduler, that allow to run async Tasks based on global ThreadPool (and Task.Run method) with ability to limit current max degree of parallelism. It is enough for my exact purposes, maybe will also help you, guys.
Main demo code (console app, dotnet core 3.1) :
static async Task Main(string[] args)
{
//5 tasks to run per time
int concurrentLimit = 5;
var scheduler = new ThreadPoolConcurrentScheduler(concurrentLimit);
//catch all errors in separate event handler
scheduler.OnError += Scheduler_OnError;
// just monitor "live" state and output to console
RunTaskStateMonitor(scheduler);
// simulate adding new tasks "on the fly"
SimulateAddingTasksInParallel(scheduler);
Console.WriteLine("start adding 50 tasks");
//add 50 tasks
for (var i = 1; i <= 50; i++)
{
scheduler.StartNew(myAsyncTask);
}
Console.WriteLine("50 tasks added to scheduler");
Thread.Sleep(1000000);
}
Supporting code (place it in the same place) :
private static void Scheduler_OnError(Exception ex)
{
Console.WriteLine(ex.ToString());
}
private static int currentTaskFinished = 0;
//your sample of async task
static async Task myAsyncTask()
{
Console.WriteLine("task started ");
using (HttpClient httpClient = new HttpClient())
{
//just make http request to ... wikipedia!
//sorry, Jimmy Wales! assume,guys, you will not DDOS wiki :)
var uri = new Uri("https://wikipedia.org/");
var response = await httpClient.GetAsync(uri);
string result = await response.Content.ReadAsStringAsync();
if (string.IsNullOrEmpty(result))
Console.WriteLine("error, await is not working");
else
Console.WriteLine($"task result : site length is {result.Length}");
}
//or simulate it using by sync sleep
//Thread.Sleep(1000);
//and for tesing exception :
//throw new Exception("my custom error");
Console.WriteLine("task finished ");
//just incrementing total ran tasks to output in console
Interlocked.Increment(ref currentTaskFinished);
}
static void SimulateAddingTasksInParallel(ThreadPoolConcurrentScheduler taskScheduler)
{
int runCount = 0;
Task.Factory.StartNew(() =>
{
while (true)
{
runCount++;
if (runCount > 5)
break;
//every 10 sec 5 times
Thread.Sleep(10000);
//adding new 5 tasks from outer task
Console.WriteLine("start adding new 5 tasks!");
for (var i = 1; i <= 5; i++)
{
taskScheduler.StartNew(myAsyncTask);
}
Console.WriteLine("new 5 tasks added!");
}
}, TaskCreationOptions.LongRunning);
}
static void RunTaskStateMonitor(ThreadPoolConcurrentScheduler taskScheduler)
{
int prev = -1;
int prevQueueSize = -1;
int prevFinished = -1;
Task.Factory.StartNew(() =>
{
while (true)
{
// getting current thread count in working state
var currCount = taskScheduler.GetCurrentWorkingThreadCount();
// getting inner queue state
var queueSize = taskScheduler.GetQueueTaskCount();
//just output overall state if something changed
if (prev != currCount || queueSize != prevQueueSize || prevFinished != currentTaskFinished)
{
Console.WriteLine($"Monitor : running tasks:{currCount}, queueLength:{queueSize}. total Finished tasks : " + currentTaskFinished);
prev = currCount;
prevQueueSize = queueSize;
prevFinished = currentTaskFinished;
}
// check it every 10 ms
Thread.Sleep(10);
}
}
, TaskCreationOptions.LongRunning);
}
Scheduler :
public class ThreadPoolConcurrentScheduler
{
private readonly int _limitParallelThreadsCount;
private int _threadInProgressCount = 0;
public delegate void onErrorDelegate(Exception ex);
public event onErrorDelegate OnError;
private ConcurrentQueue<Func<Task>> _taskQueue;
private readonly object _queueLocker = new object();
public ThreadPoolConcurrentScheduler(int limitParallelThreadsCount)
{
//set maximum parallel tasks to run
_limitParallelThreadsCount = limitParallelThreadsCount;
// thread-safe queue to store tasks
_taskQueue = new ConcurrentQueue<Func<Task>>();
}
//main method to start async task
public void StartNew(Func<Task> task)
{
lock (_queueLocker)
{
// checking limit
if (_threadInProgressCount >= _limitParallelThreadsCount)
{
//waiting new "free" threads in queue
_scheduleTask(task);
}
else
{
_startNewTask(task);
}
}
}
private void _startNewTask(Func<Task> task)
{
Interlocked.Increment(ref _threadInProgressCount);
Task.Run(async () =>
{
try
{
await task();
}
catch (Exception e)
{
//Console.WriteLine(e);
OnError?.Invoke(e);
}
}).ContinueWith(_onTaskEnded);
}
//will be called on task end
private void _onTaskEnded(Task task)
{
lock (_queueLocker)
{
Interlocked.Decrement(ref _threadInProgressCount);
//queue has more priority, so if thread is free - let's check queue first
if (!_taskQueue.IsEmpty)
{
if (_taskQueue.TryDequeue(out var result))
{
_startNewTask(result);
}
}
}
}
private void _scheduleTask(Func<Task> task)
{
_taskQueue.Enqueue(task);
}
//returning in progress task count
public int GetCurrentWorkingThreadCount()
{
return _threadInProgressCount;
}
//return number of tasks waiting to run
public int GetQueueTaskCount()
{
lock (_queueLocker) return _taskQueue.Count;
}
}
Few notes :
First - check comments to it, maybe it is the worst code ever!
Did not test in prod
Did not implement cancellation tokens and any other functionality, that should be there, but i'm too lazy. Sorry