This might have a very simple answer, already posted here somewhere, but there is also a lot of old misleading information about threads.
My question is this: How do I prevent my Console from terminating before the background work is complete? Since the Task is created far down in the business layers of my program, I do not have access to it when the Console terminates (not like in this simple example). Is there some way of awaiting all Tasks that have been created within the context of my application?
public class Program
{
private static void Main(string[] args)
{
DoBackgroundWork();
System.Console.WriteLine("Doing something else during background work");
// Wait for all created Tasks to complete before terminating
???
}
private static void DoBackgroundWork()
{
var task = Task.Run(() =>
{
System.Console.WriteLine("Starting background work");
Thread.Sleep(10000);
System.Console.WriteLine("Background work finished!");
});
}
}
Return the task so that you can wait on it.
private static void Main(string[] args)
{
var task = DoBackgroundWork();
System.Console.WriteLine("Doing something else during background work");
// Wait for all created Tasks to complete before terminating
task.Wait();
}
private static Task DoBackgroundWork()
{
var task = Task.Run(() =>
{
System.Console.WriteLine("Starting background work");
Thread.Sleep(1000);
System.Console.WriteLine("Background work finished!");
});
return task;
}
Related
I have a WPF app running on .net 6 and an external device connected to it.
Initializing the device sometimes fails and I don't want to hold the UI thread trying to initialize it.
I want to run the following method (_device.Init()) in an async fashion and when it returns true, run Start() method.
edit: run it until it returns true from the _device.Init() method, not true for finishing the task
Is there a built-in functionality to do it with tasks? or any other "best practice" way?
Thank you :)
SomeDevice _device = new();
public async void Init()
{
// some other code
while (Task.Run(() => _device.Init()).Result == false)
{
}
Start();
}
public void Start()
{
// some other code
Application.Current.Dispatcher.BeginInvoke(new Action(() =>
{
_device.Start();
}));
}
Instead of getting the Result of the Task (which may block the UI thread) you should await the Task:
public async void Init()
{
// some other code
while (!await Task.Run(() => _device.Init()))
{
}
Start();
}
The method should also be awaitable and be awaited when called, e.g. in an async Loaded event handler:
public async Task Init()
{
// some other code
while (!await Task.Run(() => _device.Init()))
{
}
Start();
}
...
await Init();
public async void Init()
{
var task = _device.Init();
//do work here
await task;
Start();
}
Should do the trick, it'll do the work and then wait for the task to complete before going to Start();
If you want to simply wait for init to finish and then run start it's even simpler with
await _device.Init().ContinueWith((x) => { Start();})
I'm trying to asynchronously show a progress form that says the application is running while the actual application is running.
As following this question, I have the following:
Main Form:
public partial class MainForm : Form
{
public MainForm()
{
InitializeComponent();
}
async Task<int> LoadDataAsync()
{
await Task.Delay(2000);
return 42;
}
private async void Run_Click(object sender, EventArgs e)
{
var runningForm = new RunningForm();
runningForm.ShowRunning();
var progressFormTask = runningForm.ShowDialogAsync();
var data = await LoadDataAsync();
runningForm.Close();
await progressFormTask;
MessageBox.Show(data.ToString());
}
}
Progress Form
public partial class RunningForm : Form
{
private readonly SynchronizationContext synchronizationContext;
public RunningForm()
{
InitializeComponent();
synchronizationContext = SynchronizationContext.Current;
}
public async void ShowRunning()
{
this.RunningLabel.Text = "Running";
int dots = 0;
await Task.Run(() =>
{
while (true)
{
UpadateUi($"Running{new string('.', dots)}");
Thread.Sleep(300);
dots = (dots == 3) ? 0 : dots + 1;
}
});
}
public void UpadateUi(string text)
{
synchronizationContext.Post(
new SendOrPostCallback(o =>
{
this.RunningLabel.Text = text;
}),
text);
}
public void CloseThread()
{
synchronizationContext.Post(
new SendOrPostCallback(o =>
{
this.Close();
}),
null);
}
}
internal static class DialogExt
{
public static async Task<DialogResult> ShowDialogAsync(this Form form)
{
await Task.Yield();
if (form.IsDisposed)
{
return DialogResult.OK;
}
return form.ShowDialog();
}
}
The above works fine, but it doesn't work when I'm calling from outside of another from. This is my console app:
class Program
{
static void Main(string[] args)
{
new Test().Run();
Console.ReadLine();
}
}
class Test
{
private RunningForm runningForm;
public async void Run()
{
var runningForm = new RunningForm();
runningForm.ShowRunning();
var progressFormTask = runningForm.ShowDialogAsync();
var data = await LoadDataAsync();
runningForm.CloseThread();
await progressFormTask;
MessageBox.Show(data.ToString());
}
async Task<int> LoadDataAsync()
{
await Task.Delay(2000);
return 42;
}
}
Watching what happens with the debugger, the process gets to await Task.Yield() and never progresses to return form.ShowDialog() and thus you never see the RunningForm. The process then goes to LoadDataAsync() and hangs forever on await Task.Delay(2000).
Why is this happening? Does it have something to do with how Tasks are prioritized (ie: Task.Yield())?
Watching what happens with the debugger, the process gets to await
Task.Yield() and never progresses to return form.ShowDialog() and thus
you never see the RunningForm. The process then goes to
LoadDataAsync() and hangs forever on await Task.Delay(2000).
Why is this happening?
What happens here is that when you do var runningForm = new RunningForm() on a console thread without any synchronization context (System.Threading.SynchronizationContext.Current is null), it implicitly creates an instance of WindowsFormsSynchronizationContext and installs it on the current thread, more on this here.
Then, when you hit await Task.Yield(), the ShowDialogAsync method returns to the caller and the await continuation is posted to that new synchronization context. However, the continuation never gets a chance to be invoked, because the current thread doesn't run a message loop and the posted messages don't get pumped. There isn't a deadlock, but the code after await Task.Yield() is never executed, so the dialog doesn't even get shown. The same is true about await Task.Delay(2000).
I'm more interested in learning why it works for WinForms and not for
Console Applications.
You need a UI thread with a message loop in your console app. Try refactoring your console app like this:
public void Run()
{
var runningForm = new RunningForm();
runningForm.Loaded += async delegate
{
runningForm.ShowRunning();
var progressFormTask = runningForm.ShowDialogAsync();
var data = await LoadDataAsync();
runningForm.Close();
await progressFormTask;
MessageBox.Show(data.ToString());
};
System.Windows.Forms.Application.Run(runningForm);
}
Here, the job of Application.Run is to start a modal message loop (and install WindowsFormsSynchronizationContext on the current thread) then show the form. The runningForm.Loaded async event handler is invoked on that synchronization context, so the logic inside it should work just as expected.
That however makes Test.Run a synchronous method, i. e., it only returns when the form is closed and the message loop has ended. If this is not what you want, you'd have to create a separate thread to run your message loop, something like I do with MessageLoopApartment here.
That said, in a typical WinForms or WPF application you should almost never need a secondary UI thread.
I have the following code in a WinForms application with one button and one label:
using System;
using System.IO;
using System.Threading.Tasks;
using System.Windows.Forms;
namespace WindowsFormsApplication1
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
private async void button1_Click(object sender, EventArgs e)
{
await Run();
}
private async Task Run()
{
await Task.Run(async () => {
await File.AppendText("temp.dat").WriteAsync("a");
label1.Text = "test";
});
}
}
}
This is a simplified version of the real application I'm working on. I was under the impression that by using async/await in my Task.Run I could set the label1.Text property. However, when running this code I get the error that I'm not on the UI thread and I can't access the control.
Why can't I access the label control?
When you use Task.Run(), you're saing that you don't want the code to run on the current context, so that's exactly what happens.
But there is no need to use Task.Run() in your code. Correctly written async methods won't block the current thread, so you can use them from the UI thread directly. If you do that, await will make sure the method resumes back on the UI thread.
This means that if you write your code like this, it will work:
private async void button1_Click(object sender, EventArgs e)
{
await Run();
}
private async Task Run()
{
await File.AppendText("temp.dat").WriteAsync("a");
label1.Text = "test";
}
Try this:
replace
label1.Text = "test";
with
SetLabel1Text("test");
and add the following to your class:
private void SetLabel1Text(string text)
{
if (InvokeRequired)
{
Invoke((Action<string>)SetLabel1Text, text);
return;
}
label1.Text = text;
}
The InvokeRequired returns true if you are NOT on the UI thread. The Invoke() method takes the delegate and parameters, switches to the UI thread and then calls the method recursively. You return after the Invoke() call because the method has already been called recursively prior to the Invoke() returning. If you happen to be on the UI thread when the method is called, the InvokeRequired is false and the assignment is performed directly.
Try this
private async Task Run()
{
await Task.Run(async () => {
await File.AppendText("temp.dat").WriteAsync("a");
});
label1.Text = "test";
}
Or
private async Task Run()
{
await File.AppendText("temp.dat").WriteAsync("a");
label1.Text = "test";
}
Or
private async Task Run()
{
var task = Task.Run(async () => {
await File.AppendText("temp.dat").WriteAsync("a");
});
var continuation = task.ContinueWith(antecedent=> label1.Text = "test",TaskScheduler.FromCurrentSynchronizationContext());
await task;//I think await here is redundant
}
async/await doesn't guarantee that it will run in UI thread. await will capture the current SynchronizationContext and continues execution with the captured context once the task completed.
So in your case you have a nested await which is inside Task.Run hence second await will capture the context which is not going to be UiSynchronizationContext because it is being executed by WorkerThread from ThreadPool.
Does this answers your question?
Why do you use Task.Run? that start a new worker thread (cpu bound), and it causes your problem.
you should probably just do that:
private async Task Run()
{
await File.AppendText("temp.dat").WriteAsync("a");
label1.Text = "test";
}
await ensure you will continue on the same context except if you use .ConfigureAwait(false);
Because it's on a different thread and cross-thread calls aren't allowed.
You will need to pass on the "context" to the thread you are starting. See an example here: http://reedcopsey.com/2009/11/17/synchronizing-net-4-tasks-with-the-ui-thread/
I am going to give you my latest answer that I have given for async understanding.
The solution is as you know that when you are calling async method you need to run as a task.
Here is a quick console app code that you can use for your reference, it will make it easy for you to understand the concept.
using System;
using System.Threading;
using System.Threading.Tasks;
public class Program
{
public static void Main()
{
Console.WriteLine("Starting Send Mail Async Task");
Task task = new Task(SendMessage);
task.Start();
Console.WriteLine("Update Database");
UpdateDatabase();
while (true)
{
// dummy wait for background send mail.
if (task.Status == TaskStatus.RanToCompletion)
{
break;
}
}
}
public static async void SendMessage()
{
// Calls to TaskOfTResult_MethodAsync
Task<bool> returnedTaskTResult = MailSenderAsync();
bool result = await returnedTaskTResult;
if (result)
{
UpdateDatabase();
}
Console.WriteLine("Mail Sent!");
}
private static void UpdateDatabase()
{
for (var i = 1; i < 1000; i++) ;
Console.WriteLine("Database Updated!");
}
private static async Task<bool> MailSenderAsync()
{
Console.WriteLine("Send Mail Start.");
for (var i = 1; i < 1000000000; i++) ;
return true;
}
}
Here I am trying to initiate task called send mail. Interim I want to update database, while the background is performing send mail task.
Once the database update has happened, it is waiting for the send mail task to be completed. However, with this approach it is quite clear that I can run task at the background and still proceed with original (main) thread.
I have a service running some different tasks in a loop until the service is stopped.
However one of these tasks i calling a web service and this call can take several minutes to complete. I want to be able to stop the service instantly, 'cancelling' the web service call without calling Thread.Abort because that causes some strange behavior even if the only thing the thread is doing is calling this web service method.
How can i cancel or break from a synchronous method call (if it's even possible)?
Or should I try a different approach?
I have tried to use the AutoResetEvent and then calling Thread.Abort which is working fine in the below code sample, but when implementing this solution in the actual service I get some unexpected behavior probably because of what's going on in the external libraries I'm using.
AutoResetEvent and Thread.Abort:
class Program
{
static void Main(string[] args)
{
MainProgram p = new MainProgram();
p.Start();
var key = Console.ReadKey();
if (key.Key == ConsoleKey.Q)
p.Stop();
}
}
class MainProgram
{
private Thread workerThread;
private Thread webServiceCallerThread;
private volatile bool doWork;
public void Start()
{
workerThread = new Thread(() => DoWork());
doWork = true;
workerThread.Start();
}
public void Stop()
{
doWork = false;
webServiceCallerThread.Abort();
}
private void DoWork()
{
try
{
while (doWork)
{
AutoResetEvent are = new AutoResetEvent(false);
WebServiceCaller caller = new WebServiceCaller(are);
webServiceCallerThread = new Thread(() => caller.TimeConsumingMethod());
webServiceCallerThread.Start();
// Wait for the WebServiceCaller.TimeConsumingMethod to finish
WaitHandle.WaitAll(new[] { are });
// If doWork has been signalled to stop
if (!doWork)
break;
// All good - continue
Console.WriteLine(caller.Result);
}
}
catch (Exception e)
{
Console.Write(e);
}
}
}
class WebServiceCaller
{
private AutoResetEvent ev;
private int result;
public int Result
{
get { return result; }
}
public WebServiceCaller(AutoResetEvent ev)
{
this.ev = ev;
}
public void TimeConsumingMethod()
{
try
{
// Simulates a method running for 1 minute
Thread.Sleep(60000);
result = 1;
ev.Set();
}
catch (ThreadAbortException e)
{
ev.Set();
result = -1;
Console.WriteLine(e);
}
}
}
Can someone suggest a solution to this issue?
Try this
public void Start()
{
workerThread = new Thread(() => DoWork());
doWork = true;
workerThread.IsBackground = true;
workerThread.Start();
}
A thread is either a background thread or a foreground thread.
Background threads are identical to foreground threads, except that
background threads do not prevent a process from terminating. Once all
foreground threads belonging to a process have terminated, the common
language runtime ends the process. Any remaining background threads
are stopped and do not complete.
For more details see http://msdn.microsoft.com/en-us/library/system.threading.thread.isbackground.aspx
The solution is really this simple: Don't make calls that block for several minutes unless you want to block for several minutes. If there is no way to do a particular thing without blocking, potentially for several minutes, complain loudly to whoever wrote the code that imposes that painful requirement (or fix it yourself, if possible).
Once you've made the call, it's too late. You're committed. If the function you are calling doesn't provide a safe way to abort it, then there's no safe way.
As all you want to do is make one an asynchonrous web service call at a time and on each response make another call you can dispense with the worker thread and simply make an aynchronous call, register a callback and make another async call from the callback:
class Program
{
private static WebServiceCaller.TCMDelegate _wscDelegate;
private static readonly WebServiceCaller _wsCaller = new WebServiceCaller();
static void Main(string[] args)
{
_wscDelegate = _wsCaller.TimeConsumingMethod;
MakeWSCallAsync();
Console.WriteLine("Enter Q to quit");
while (Console.ReadLine().ToUpper().Trim()!="Q"){}
}
public static void MakeWSCallAsync()
{
_wscDelegate.BeginInvoke(OnWSCallComplete, null);
}
public static void OnWSCallComplete(IAsyncResult ar)
{
Console.WriteLine("Result {0}", _wscDelegate.EndInvoke(ar));
MakeWSCallAsync();
}
}
class WebServiceCaller
{
public delegate int TCMDelegate();
public int TimeConsumingMethod()
{
try
{
// Simulates a method running for 1 minute
Thread.Sleep(1000);
return 1;
}
catch (ThreadAbortException e)
{
return -1;
}
}
}
No blocking (well, the console thread is blocking on ReadLine()) and no windows kernal mode sync objects (AutoResetEvent) which are expensive.
I am making an application in C# which uses a winform as the GUI and a separate thread which is running in the background automatically changing things. Ex:
public void Run()
{
while(true)
{
printMessageOnGui("Hey");
Thread.Sleep(2000);
// Do more work
}
}
How would I make it pause anywhere in the loop, because one iteration of the loop takes around 30 seconds. So I wouldn't want to pause it after its done one loop, I want to pause it on time.
var mrse = new ManualResetEvent(false);
public void Run()
{
while (true)
{
mrse.WaitOne();
printMessageOnGui("Hey");
Thread.Sleep(2000); . .
}
}
public void Resume() => mrse.Set();
public void Pause() => mrse.Reset();
You should do this via a ManualResetEvent.
ManualResetEvent mre = new ManualResetEvent();
mre.WaitOne(); // This will wait
On another thread, obviously you'll need a reference to the mre
mre.Set(); // Tells the other thread to go again
A full example which will print some text, wait for another thread to do something and then resume:
class Program
{
private static ManualResetEvent mre = new ManualResetEvent(false);
static void Main(string[] args)
{
Thread t = new Thread(new ThreadStart(SleepAndSet));
t.Start();
Console.WriteLine("Waiting");
mre.WaitOne();
Console.WriteLine("Resuming");
}
public static void SleepAndSet()
{
Thread.Sleep(2000);
mre.Set();
}
}
You can pause a thread by calling thread.Suspend but that is deprecated. I would take a look at autoresetevent for performing your synchronization.