DeadLock on task.Wait() with Task which edit UI

问题

I'm trying to find some solutions to my problem here, but with no result (or I just do not get them right) so if anyone could help / explain i will be really gratefull.

I'm just developing a tool for system administrators using Win Form and now I need to create a continuous ping on the selected machine which is running on the background. There is an indicator for Online status on UI which I need to edit with background ping. So right now I'm in this state:

Class A (Win form):

ClassB activeRelation = new ClassB();

public void UpdateOnline(Relation pingedRelation)
{
    //There is many Relations at one time, but form shows Info only for one...
    if (activeRelation == pingedRelation)
    {
        if (p_Online.InvokeRequired)
        {
            p_Online.Invoke(new Action(() =>
                p_Online.BackgroundImage = (pingedRelation.Online) ? Properties.Resources.Success : Properties.Resources.Failure
            ));
        }
        else
        {
            p_Online.BackgroundImage = (pingedRelation.Online) ? Properties.Resources.Success : Properties.Resources.Failure;
        }
    }
}

//Button for tunring On/Off the background ping for current machine
private void Btn_PingOnOff_Click(object sender, EventArgs e)
{
    Button btn = (sender is Button) ? sender as Button : null;

    if (btn != null)
    {
        if (activeRelation.PingRunning)
        {
            activeRelation.StopPing();
            btn.Image = Properties.Resources.Switch_Off;
        }
        else
        {
            activeRelation.StartPing(UpdateOnline);
            btn.Image = Properties.Resources.Switch_On;
        }
    }
}

Class B (class thats represent relation to some machine)

private ClassC pinger;    

public void StartPing(Action<Relation> action)
{
    pinger = new ClassC(this);
    pinger.PingStatusUpdate += action;
    pinger.Start();
}

public void StopPing()
{
    if (pinger != null)
    {
        pinger.Stop();
        pinger = null;
    }
}

Class C (background ping class)

private bool running = false;
private ClassB classb;
private Task ping;
private CancellationTokenSource tokenSource;
public event Action<ClassB> PingStatusUpdate;

public ClassC(ClassB classB)
{
    this.classB = classB;
}

public void Start()
{
    tokenSource = new CancellationTokenSource();
    CancellationToken token = tokenSource.Token;

    ping = PingAction(token);
    running = true;
}

public void Stop()
{
    if (running)
    {
        tokenSource.Cancel();
        ping.Wait(); //And there is a problem -> DeadLock
        ping.Dispose();
        tokenSource.Dispose();
    }

    running = false;
}

private async Task PingAction(CancellationToken ct)
{
    bool previousResult = RemoteTasks.Ping(classB.Name);
    PingStatusUpdate?.Invoke(classB);

    while (!ct.IsCancellationRequested)
    {
        await Task.Delay(pingInterval);

        bool newResult = RemoteTasks.Ping(classB.Name);

        if (newResult != previousResult)
        {
            previousResult = newResult;
            PingStatusUpdate?.Invoke(classB);
        }
    }
}

So the problem is in deadlock when I cancel token and Wait() for task to complete -> it's still running, but While(...) in task is finished right.

回答1:

You have a deadlock because ping.Wait(); blocks UI thread.

You should wait for task asynchronously using await.

So, if Stop() is event handler then change it to:

public async void Stop() // async added here
{
    if (running)
    {
        tokenSource.Cancel();
        await ping; // await here
        ping.Dispose();
        tokenSource.Dispose();
    }

    running = false;
}

If it is not:

public async Task Stop() // async added here, void changed to Task
{
    if (running)
    {
        tokenSource.Cancel();
        await ping; // await here
        ping.Dispose();
        tokenSource.Dispose();
    }

    running = false;
}

As mentioned by @JohnB async methods should have Async suffix so, the method should be named as StopAsync().

Similar problem and solution are explained here - Do Not Block On Async Code

You should avoid synchronous waiting on tasks, so you should always use await with tasks instead of Wait() or Result. Also, as pointed by @Fildor you should use async-await all the way to avoid such situations.

来源：https://stackoverflow.com/questions/53221727/deadlock-on-task-wait-with-task-which-edit-ui

标签

winforms

asynchronous

task