How to stop a Parallel.ForEach loop?

Question

I have this code to start a Parallel ForEach loop:

Parallel.ForEach<ListViewItem>(filesListView.Items.Cast<ListViewItem>(), new ParallelOptions() { MaxDegreeOfParallelism = Environment.ProcessorCount }, item => {
    if (CallToStop == true)
        {  
            //Code here to stop the loop!
        }        
    internalProcessStart(item);
});

I have some code which will check if there is a call to stop the threads, and then I would like to break; the code, but this doesn't work with Parallel.

I found the same question by someone else, but their code is slighly different to mine, and I'm not sure where to put the ParallelLoopState state.

Thanks!

Can you edit in the ForEach loop body as well? – Kevin Le Sep 29 '16 at 18:36 — Kevin Le, Sep 29 '16 at 18:36

score 6 · Accepted Answer · edited Apr 23 '18 at 13:06

6

Rewrite as below

Parallel.ForEach<ListViewItem>(filesListView.Items.Cast<ListViewItem>(), 
            new ParallelOptions() { MaxDegreeOfParallelism = Environment.ProcessorCount }, 
           (item, state) => {
    if (CallToStop == true)
        {  
            state.Break();
        }        
    internalProcessStart(item);
});

Hope this helps.

edited Apr 23 '18 at 13:06

Liam

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answered Sep 29 '16 at 18:51

Cinchoo

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Thanks, this is the code I used and it's working nicely. Thanks – user2924019 Sep 29 '16 at 18:58

score 1 · Answer 2 · answered Sep 29 '16 at 18:46

I think using this overload should work:

    Parallel.ForEach<ListViewItem>(filesListView.Items.Cast<ListViewItem>(), new ParallelOptions() { MaxDegreeOfParallelism = Environment.ProcessorCount }, (item , state)  => 
    {
        if (/*Stop condition here*/)
        { 
            state.Break(); 
        }        
        internalProcessStart(item);
    });

roslav · Answer 3 · 2016-09-29T19:05:02.167

Try using CancellationToken as in this example (MSDN).

Try something like this:

CancellationTokenSource cts = new CancellationTokenSource();

       // Use ParallelOptions instance to store the CancellationToken
        ParallelOptions po = new ParallelOptions();
        po.CancellationToken = cts.Token;
        po.MaxDegreeOfParallelism = Environment.ProcessorCount;

        try
        {
            Parallel.ForEach<ListViewItem>(filesListView.Items.Cast<ListViewItem>(), po, item => {
                // po.CancellationToken.ThrowIfCancellationRequested(); //1
                if (CallToStop == true)
                {  
                    //Code here to stop the loop!
                    cts.Cancel();
                }
                if (po.CancellationToken.IsCancellationRequested == false)
                {        
                    internalProcessStart(item);
                }
            });
        }
        catch (OperationCanceledException e)
        {
            // handle
        }
        finally
        {
            cts.Dispose();
        }

Or instead of setting CallToStop = true call cts.Cancel() directly.

Or you can uncomment //1 and let all threds that did not finished throw OperationCanceledException (only use whenyou are stopping all parallel thread because of CallToStop = true is caused by error).

You can even pass the cts.Token into your internalProcessStart(item,token) and handle what to do if you cancel when internal process is already running.

score 1 · Answer 4 · answered Sep 29 '16 at 18:53

Try something like this. Conceptually, you need to pass the loopState to your lambda function.

Parallel.ForEach<int>(new List<int>(),
            new ParallelOptions() { MaxDegreeOfParallelism = Environment.ProcessorCount },
            (val, loopState) =>
            {
                if (val == 9) //enter your stopcondition here
                {
                    loopState.Stop();
                    return;
                }
            });

How to stop a Parallel.ForEach loop?

4 Answers4

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