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

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

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

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

// 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();
        }