How to stop a for loop using a button?

Question

I have a loop that I would like to stop using a button. Edited for better understanding:

I do realize that you cannot stop a button while a loop was running since it will not work as long as that current UI is running. What I'm really asking for is the most efficient way of creating a thread or using BGWorker to stop this. I have seen some methods, but most of them are for Java and not C#.

What I would like to do is:

private void start_Click(object sender, EventArgs e)
{

    for(int i = 0; i < nums; i++)
    {
        doSomething();
    }
}

private void stop_Click(object sender, EventArgs e)
{

    stops start_Click()
}

Answer 1

You can't do that. For starters, the for loop is running synchronously on the UI thread, which means you won't even be able to click the "Stop" button.

Hence, you need to move the operations of the for loop onto another thread, which means you likely won't be using a for loop at all. You need to think about how the code inside actually needs to be executed, then based on how you are doing the processing, you can implement the "Stop" button.

A very simple way to do this would be to just:

new Thread(() =>
{
   int i = 0;
   while (!stop && i < num)
   {
      doSomething();
      i++;
   }
 }).Start();

And set stop to stop the processing loop. In a more realistic scenario, you could queue up functions that you want to process, then stop dequeuing via a similar method. Unfortunately, its hard to reccommend a setup without knowing more details.

Any solution based on your code will also have the problem of the current doSomething() completing execution (which could take a while). Again, without more info, its hard to say what the best approach to fixing that is.

Answer 2

Try using an async/await approach. It's quite easy!

public partial class MyForm : Form
{
    public MyForm()
    {
        InitializeComponent();
    }

    private CancellationTokenSource _tokenSource;

    private async void start_Click(object sender, EventArgs e)
    {
        if (_tokenSource != null)
            return;

        _tokenSource = new CancellationTokenSource();
        var ct = _tokenSource.Token;

        await Task.Factory.StartNew(() =>
        {
            for (; ; )
            {
                if (ct.IsCancellationRequested)
                    break;
                doSomething();
            }
        }, ct);

        _tokenSource = null;
    }

    private int _labelCounter;

    private void doSomething()
    {
        // do something
        Invoke((Action)(() =>
        {
            myLabel.Text = (++_labelCounter).ToString();
        }));
    }

    private void stop_Click(object sender, EventArgs e)
    {
        if (_tokenSource == null)
            return;

        _tokenSource.Cancel();
    }
}

Answer 3

IMHO, it's likely the best approach here is to convert your work to an asynchronous operation and then use the async/await idiom for the loop. E.g.:

private bool _stopLoop;

private async void start_Click(object sender, EventArgs e)
{
    _stopLoop = false;

    for(int i = 0; i < nums && !_stopLoop; i++)
    {
        await Task.Run(() => doSomething());
    }
}

private void stop_Click(object sender, EventArgs e)
{
    _stopLoop = true;
}

This allows the loop itself to execute in the UI thread where the _stopLoop variable is being managed, but without actually blocking the UI thread (which among other things would prevent the "Stop" button from being clicked).

Unfortunately, you didn't provide details about how doSomething() works. It's possible there's a good way to convert that whole method to be an async method, but I can't comment on that without the actual code.

Note that this approach will only interrupt the loop at a point in between each operation. If you want to be able to interrupt the doSomthing() operation itself, you'll have to provide a mechanism for that. One likely approach would be to use CancellationSource and CancellationToken, which provides a convenient way to express cancellation semantics.

Answer 4

To keep your UI responsive to be able to cancel the running operation you can use a backgrounworker. The backgroundworker does the work in an other thread while keeping your UI responsive:

    private readonly BackgroundWorker _backgroundWorker;

    public Form1()
    {
        InitializeComponent();

        _backgroundWorker = new BackgroundWorker
        {
            WorkerSupportsCancellation = true
        };
        _backgroundWorker.DoWork += backgroundWorker_DoWork;
        _backgroundWorker.RunWorkerCompleted += backgroundWorker_RunWorkerCompleted;

        Disposed += Form1_Disposed;
    }

    private void Form1_Disposed(object sender, EventArgs e)
    {
        _backgroundWorker.Dispose();
    }

    private void StartLoop()
    {
        if ( !_backgroundWorker.IsBusy )
        {
            _backgroundWorker.RunWorkerAsync();
        }
    }

    private void StopLoop()
    {
        _backgroundWorker.CancelAsync();
    }

    private void backgroundWorker_DoWork( object sender , DoWorkEventArgs e )
    {
        var backgroundWorker = ( BackgroundWorker ) sender;

        for ( var i = 0; i < 100; i++ )
        {
            if ( backgroundWorker.CancellationPending )
            {
                e.Cancel = true;
                return;
            }
            // Do Work
        }
    }

    private void backgroundWorker_RunWorkerCompleted( object sender , RunWorkerCompletedEventArgs e )
    {
        if ( e.Cancelled )
        {
            // handle cancellation
        }
        if ( e.Error != null )
        {
            // handle error
        }

        // completed without cancellation or exception
    }

Answer 5

try this :

bool stop=false;

private void start_Click(object sender, EventArgs e)
{
for(int i = 0; i < nums&& !bool; i++)
{
doSomething();
}
}

and in the click event

set

stop=true;