Develop Reference

How to pause task running on a worker thread and wait for user input?

If I have a task running on a worker thread and when it finds something wrong, is it possible to pause and wait for the user to intervene before continuing?
For example, suppose I have something like this:
async void btnStartTask_Click(object sender, EventArgs e)
{
await Task.Run(() => LongRunningTask());
}
// CPU-bound
bool LongRunningTask()
{
// Establish some connection here.
// Do some work here.
List<Foo> incorrectValues = GetIncorrectValuesFromAbove();
if (incorrectValues.Count > 0)
{
// Here, I want to present the "incorrect values" to the user (on the UI thread)
// and let them select whether to modify a value, ignore it, or abort.
var confirmedValues = WaitForUserInput(incorrectValues);
}
// Continue processing.
}
Is it possible to substitute WaitForUserInput() with something that runs on the UI thread, waits for the user's intervention, and then acts accordingly? If so, how? I'm not looking for complete code or anything; if someone could point me in the right direction, I would be grateful.

What you're looking for is almost exactly Progress<T>, except you want to have the thing that reports progress get a task back with some information that they can await and inspect the results of. Creating Progress<T> yourself isn't terribly hard., and you can reasonably easily adapt it so that it computes a result.
public interface IPrompt<TResult, TInput>
{
Task<TResult> Prompt(TInput input);
}
public class Prompt<TResult, TInput> : IPrompt<TResult, TInput>
{
private SynchronizationContext context;
private Func<TInput, Task<TResult>> prompt;
public Prompt(Func<TInput, Task<TResult>> prompt)
{
context = SynchronizationContext.Current ?? new SynchronizationContext();
this.prompt += prompt;
}
Task<TResult> IPrompt<TResult, TInput>.Prompt(TInput input)
{
var tcs = new TaskCompletionSource<TResult>();
context.Post(data => prompt((TInput)data)
.ContinueWith(task =>
{
if (task.IsCanceled)
tcs.TrySetCanceled();
if (task.IsFaulted)
tcs.TrySetException(task.Exception.InnerExceptions);
else
tcs.TrySetResult(task.Result);
}), input);
return tcs.Task;
}
}
Now you simply need to have an asynchronous method that accepts the data from the long running process and returns a task with whatever the user interface's response is.

You can use TaskCompletionSource to generate a task that can be awaited within the LongRunningTask.
using System;
using System.Collections.Generic;
using System.Threading.Tasks;
namespace ConsoleApp5
{
class Program
{
private static event Action<string> Input;
public static async Task Main(string[] args)
{
var inputTask = InputTask();
var longRunningTask = Task.Run(() => LongRunningTask());
await Task.WhenAll(inputTask, longRunningTask);
}
private static async Task InputTask()
{
await Task.Yield();
while(true)
{
var input = await Console.In.ReadLineAsync();
Input?.Invoke(input);
}
}
static async Task<bool> LongRunningTask()
{
SomeExpensiveCall();
var incorrectValues = GetIncorrectValuesFromAbove();
if (incorrectValues.Count > 0)
{
var confirmedValues = await WaitForUserInput(incorrectValues).ConfigureAwait(false);
}
// Continue processing.
return true;
}
private static void SomeExpensiveCall()
{
}
private static Task<string> WaitForUserInput(IList<string> incorrectValues)
{
var taskCompletionSource = new TaskCompletionSource<string>();
Console.Write("Input Data: ");
try
{
void EventHandler(string input)
{
Input -= EventHandler;
taskCompletionSource.TrySetResult(input);
}
Input += EventHandler;
}
catch(Exception e)
{
taskCompletionSource.TrySetException(e);
}
return taskCompletionSource.Task;
}
private static IList<string> GetIncorrectValuesFromAbove()
{
return new List<string> { "Test" };
}
}
}
Of course in this example you could have just called await Console.In.ReadLineAsync() directly, but this code is to simulate an environment where you only have an event based API.

There are several ways to solve this problem, with the Control.Invoke being probably the most familiar. Here is a more TPL-ish approach. You start by declaring a UI related scheduler as a class field:
private TaskScheduler _uiScheduler;
Then initialize it:
public MyForm()
{
InitializeComponent();
_uiScheduler = TaskScheduler.FromCurrentSynchronizationContext();
}
Then you convert your synchronous LongRunning method to an asynchronous method. This means that it must return Task<bool> instead of bool. It must also have the async modifier, and by convention be named with the Async suffix:
async Task<bool> LongRunningAsync()
Finally you use the await operator in order to wait for the user's input, which will be a Task configured to run on the captured UI scheduler:
async Task<bool> LongRunningAsync()
{
// Establish some connection here.
// Do some work here.
List<Foo> incorrectValues = GetIncorrectValuesFromAbove();
if (incorrectValues.Count > 0)
{
// Here, I want to present the "incorrect values" to the user (on the UI thread)
// and let them select whether to modify a value, ignore it, or abort.
var confirmedValues = await Task.Factory.StartNew(() =>
{
return WaitForUserInput(incorrectValues);
}, default, TaskCreationOptions.None, _uiScheduler);
}
// Continue processing.
}
Starting the long running task is the same as before. The Task.Run understands async delegates, so you don't have to do something special after making the method async.
var longRunningTask = Task.Run(() => LongRunningAsync());
This should be enough, provided that you just intend to show a dialog box to the user. The Form.ShowDialog is a blocking method, so the WaitForUserInput method needs not to be asynchronous. If you had to allow the user to interact freely with the main form, the problem would be much more difficult to solve.

Another example using Invoke() and a ManualResetEvent. Let me know if you need help with the form code; setting up a constructor, using DialogResult, or creating a property to hold the "confirmedValues":
bool LongRunningTask()
{
// Establish some connection here.
// Do some work here.
List<Foo> incorrectValues = GetIncorrectValuesFromAbove();
var confirmedValues;
if (incorrectValues.Count > 0)
{
DialogResult result;
ManualResetEvent mre = new ManualResetEvent(false);
this.Invoke((MethodInvoker)delegate
{
// pass in incorrectValues to the form
// you'll have to build a constructor in it to accept them
frmSomeForm frm = new frmSomeForm(incorrectValues);
result = frm.ShowDialog();
if (result == DialogResult.OK)
{
confirmedValues = frm.confirmedValues; // get the confirmed values somehow
}
mre.Set(); // release the block below
});
mre.WaitOne(); // blocks until "mre" is set
}
// Continue processing.
}

Correct way of returning a task in C#

I have a scenario wherein I need to return a method that can be awaited from the caller.
I have here my implementations, and I just want to know which of which is the correct one.
Method 1.
public class ClassA
{
public Task MyTask { get; set; }
public ClassA()
{
MyTask = MyAsyncMethod();
}
private async void MyAsyncMethod()
{
await LongProcessHere();
}
}
public class MyCaller()
{
private async void ExecuteAsync()
{
ClassA ca = new ClassA();
await ca.MyTask;
}
}
Method 2.
public class ClassA
{
public Task MyAsyncMethod()
{
return Task.Run(async()=>
{
await LongProcessHere();
});
}
}
public class MyCaller()
{
private async void ExecuteAsync()
{
ClassA ca = new ClassA();
await ca.MyAsyncMethod();
}
}

If you're asking should I expose a property or method, then that entirely depends on what the Task represents.
If the task is something done once per instance of the class, then having a Task property is appropriate. Usually in this case, the property represents something about the instance, such as "my initialization is complete" or "I am done processing".
If the task is something that you need to do multiple times, then having a Task-returning method is appropriate.
Task-returning methods are vastly more common than Task properties.
On a side note, avoid async void and don't use Task.Run unnecessarily.

The correct code would look like:
public class ClassA
{
// always return a Task from an async method
public async Task MyAsyncMethod()
{
await LongProcessHere();
}
}
public class MyCaller
{
private async Task ExecuteAsync()
{
ClassA ca = new ClassA();
await ca.MyAsyncMethod();
}
}

You do not need to use Task.Run to call an async method. Also async methods should have Task return type as opposed to void. And vice versa.
public class ClassA
{
public async Task MyAsyncMethod()
{
return await LongProcessHere();
}
}
public class MyCaller()
{
private async Task ExecuteAsync()
{
ClassA ca = new ClassA();
await ca.MyAsyncMethod();
}
}

Waiting for all jobs to be finished with async await

I'm trying to understand the usage of async-await in C#5. If I have 2 jobs started in a method, is there a best way to wait for their completion in C#5+ ? I've done the example below but I fail to see what the async await keywork brings here besides free documentation with async keyword.
I made the following example, I want "FINISHED !" to be printed last. It is not the case however. What did I miss ? How can I make the async method wait until all jobs are finished ? Is there a point using async-await here ? I could just do Task.WaitAll with a non-async method here. I don't really understand what async brings in case you want to wait.
class Program
{
static void Main(string[] args)
{
var fooWorker = new FooWorker();
var barWorker = new BarWorker();
var test = new Class1(fooWorker, barWorker);
test.SomeWork();
Console.ReadLine();
}
}
public class Foo
{
public Foo(string bar) => Bar = bar;
public string Bar { get; }
}
public class Class1
{
private IEnumerable<Foo> _foos;
private readonly FooWorker _fooWorker;
private readonly BarWorker _barWorker;
public Class1(FooWorker fooWorker, BarWorker barWorker)
{
_fooWorker = fooWorker;
_barWorker = barWorker;
}
public void SomeWork()
{
_foos = ProduceManyFoo();
MoreWork();
Console.WriteLine("FINISHED !");
}
private async void MoreWork()
{
if (_foos == null || !_foos.Any()) return;
var fooList = _foos.ToList();
Task fooWorkingTask = _fooWorker.Work(fooList);
Task barWorkingTask = _barWorker.Work(fooList);
await Task.WhenAll(fooWorkingTask, barWorkingTask);
}
private IEnumerable<Foo> ProduceManyFoo()
{
int i = 0;
if (++i < 100) yield return new Foo(DateTime.Now.ToString(CultureInfo.InvariantCulture));
}
}
public abstract class AWorker
{
protected virtual void DoStuff(IEnumerable<Foo> foos)
{
foreach (var foo in foos)
{
Console.WriteLine(foo.Bar);
}
}
public Task Work(IEnumerable<Foo> foos) => Task.Run(() => DoStuff(foos));
}
public class FooWorker : AWorker { }
public class BarWorker : AWorker { }

You are firing off tasks and just forgetting them, while the thread continues running. This fixes it.
Main:
static async Task Main(string[] args)
{
var fooWorker = new FooWorker();
var barWorker = new BarWorker();
var test = new Class1(fooWorker, barWorker);
await test.SomeWork();
Console.ReadLine();
}
SomeWork:
public async Task SomeWork()
{
_foos = ProduceManyFoo();
await MoreWork();
Console.WriteLine("FINISHED !");
}
MoreWork signature change:
private async Task MoreWork()
The obvious code smell which should help make the problem clear is using async void. Unless required this should always be avoided.
When using async and await you'll usually want to chain the await calls to the top-level (in this case Main).
await is non-blocking, so anything that calls an async method should really care about the Task being returned.

Windows Form gets stuck on multiple async tasks

I am trying to execute parallel methods, but Form gets stuck whenever
I call them.
Please correct what I am doing wrong. Here is the Code:
public partial class Form1 : Form
{
private async void button1_Click(object sender, EventArgs e)
{
var itemList = new List<string>() { "Field1", "Field2", "Field3" };
await Task.WhenAll(itemList.Select(item =>
new WorkToDo(item).StartWork()
));
}
}
public class WorkToDo
{
private string id;
public WorkToDo(string id)
{
this.id = id;
}
public async Task<bool> StartWork()
{
Calculate();
Analyze();
SomeToDo();
var result = Save();
await Task.Delay(100);
return result;
}
private bool Calculate()
{
//Some complex and time taking calculation will be here
return true;
}
private bool Analyze()
{
//Some complex and time taking calculation will be here
return true;
}
private bool SomeToDo()
{
//Some complex and time taking calculation will be here
return true;
}
private bool Save()
{
//Some complex and time taking calculation will be here
return true;
}
}

You need to remember that normal async / await will still be performed on the UI thread.
So to be sure that a real long action is pushed to a background thread, you need to wrap it in Task.Run... like Task.Run(() => Task.WhenAll(tasks));
To complete the question a bit more ( seeing the other answer available ), Task.Run usage is not something to take lightly. It all depends on what sort of code needs to be wrapped.
There is a good write up series on this up on the blog of Stephen Cleary here http://blog.stephencleary.com/2013/11/taskrun-etiquette-examples-even-in.html so take some time to go through this and see what fits your project.
Or look here for some other details from Stephen https://stackoverflow.com/a/18015586

The problem that you have is that StartWork claims to be asynchronous, and yet it isn't. It does all of it's work synchronously.
Marking a method as async doesn't make it asynchronous. it just allows you to use the await keyword from within that method. If you perform long running synchronous operations from an async method then that method is still going to be doing that work synchronously.
There are really two approaches to take here. If there are some number of the things done in StartWork that really are inherently asynchronous, then you need to wrap whatever synchronous CPU bound work you have in calls to Task.Run so that the synchronous work that you have can be done asynchronously in a thread pool thread.
If there is no inherently asynchronous operations you have to do in StartWork then make the method clearly synchronous. Have it return a boolean, not a Task, and adjust the name to reflect the fact that it is synchronous. Then have the caller that calls it use Task.Run to do that whole operation on a thread pool thread asynchronously.
Having StartWork falsely claim to be asynchronous, and then still using Task.Run to do the purportedly asynchronous work in another thread is going to be extremely confusing to other readers of your code, as there should be no reason to offload an asynchronous method to a non-UI thread.

IMHO, If you are using Async operation you don't need Task.Run() if you have Sync Task and do it asynchronously you need Task.Run()
If you are using normal synchronous process just return Task<T> and use this Task.Run(()) to use background thread to process. See this answer
private async void button1_Click(object sender, EventArgs e)
{
var itemList = new List<string>() { "Field1", "Field2", "Field3" }; // more than 50 items
Task.Run(() => Task.WhenAll(tasks));
}
public class WorkToDo
{
private string id;
public WorkToDo(string id)
{
this.id = id;
}
public async Task<bool> StartWork()
{
var t1 = Calculate();
var t2 = Analyze();
var t3 = SomeToDo();
//Assuming you need to do all this before you save
// so wait for the all.
await Task.WhenAll(t1,t2,t3);
var result = await Save();
return result;
}
private async Task<bool> Calculate()
{
//Some complex and time taking calculation will be here
//Assuming here you have some DoAsync() method
return true;
}
private async Task<bool> Analyze()
{
//Some complex and time taking calculation will be here
return true;
}
private async Task<bool> SomeToDo()
{
//Some complex and time taking calculation will be here
return true;
}
private async Task<bool> Save()
{
//Some complex and time taking calculation will be here
return true;
}
Using WhenAll() has some advantage like propagating all error at once, see this

Dissimilar behavior of async-await

I have stumbled upon a strange behavior of async-await.
Example code:
public class foo
{
public async static Task<myobj> method1()
{
var result = await method2();
return result;
}
private async static Task<myobj> method2()
{
// omitted for brevity.
}
}
public class bar
{
public void caller()
{
var result = foo.method1().Result;
pass(result);
}
}
This freezes the UI. The solution is to implement async-await on caller().
But how about this:
public class foo
{
public static myobj method1()
{
var result = method2().Result;
return result;
}
private async static Task<myobj> method2()
{
// omitted for brevity.
}
}
public class bar
{
public void caller()
{
var result = foo.method1();
pass(result);
}
}
This works freely.
What is different with private call vs. the one made to upstream method from other class?

As I mentioned in the comment, the first case is described in great details by Stephen Cleary in his blog.
The deadlock occurs at await method2(). The await continuation was posted to the UI thread's synchronization context via SynchronizationContext.Post. But the UI thread is already blocked waiting at this line: foo.method1().Result. The message pump is blocked and the continuation callback never gets pumped and invoked, deadlock.
In the second case, I don't see await anywhere. I.e., the code as you shown it doesn't do any asynchrony. I guess that's why it works.