Parallel.ForEach doesn't return after loopResult.Stop()

Question

This is what my loop looks like:

var loopResult = Parallel.ForEach(folder.Items.Cast<object>(), (item, loopState) =>
    {
        if (!loadData)
        {
            loopState.Stop();
            return;
        }

        DoSomeWork(item);
    }
);

if (loopResult.IsCompleted)
{
    Debug.WriteLine("done");
}

The problem is the code never gets to if (loopResult.IsCompleted). After executing return; for all the different threads, absolutely nothing happens.

that is actually the right question to ask, it got stuck in that method trying to Invoke a method call on GUI thread, which never returned. — user1151923
– user1151923, Commented Apr 2, 2012 at 15:26

pad · Accepted Answer · 2012-03-31 12:06:58Z

4

The code never gets to the body of if (loopResult.IsCompleted) because the parallel loop has been stopped successfully. You can check it as follows:

/* ForEach loop as above */

if (!loopResult.IsCompleted && 
         !loopResult.LowestBreakIteration.HasValue)
{
   Debug.WriteLine("Loop was stopped");
}

if (loopResult.IsCompleted)
{
    Debug.WriteLine("Loop was done without stopping");
}

You can find useful information how to Stop/Break a parallel loop in this MSDN page, starting from Breaking Out of Loops Early subsection.

edited Mar 31, 2012 at 12:06

answered Mar 31, 2012 at 11:54

pad

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Comments

Tung · Accepted Answer · 2012-03-31 11:58:20Z

2

According to this, IsCompleted will not return true if a loop was ended prematurely:

Gets whether the loop ran to completion, such that all iterations of the loop were executed and the loop didn't receive a request to end prematurely.

Your call to Stop() causes all other loops to exit early, resulting in IsCompleted evaluating to false.

answered Mar 31, 2012 at 11:58

Tung

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