Break Parallel Foreach Earlier

Breaking out of a Parallel.ForEach isn’t as straightforward as a normal foreach, but .NET gives you built-in control methods for it.

1. Use Break()

Stops starting new iterations after a certain point, but lets already-running ones finish.

Parallel.ForEach(collection, (item, state) =>
{
if (SomeCondition(item))
{
state.Break();
return;
}

// Work here
});

When to use:

• When order matters (mainly useful with indexed loops)
• You want to stop future iterations, not cancel everything immediately

2. Use Stop() (most common for “break early”)

Stops all iterations as soon as possible.

Parallel.ForEach(collection, (item, state) =>
{
if (SomeCondition(item))
{
state.Stop();
return;
}

// Work here
});

Behavior:

• Prevents new iterations from starting
• Already-running iterations may still complete

3. Check IsStopped (optional optimization)

Parallel.ForEach(collection, (item, state) =>
{
if (state.IsStopped)
return;

if (SomeCondition(item))
{
state.Stop();
return;
}

// Work
});

4. Using CancellationToken (best for external control)

Important Notes

• There is no instant hard break like in a normal loop
• Parallel loops are cooperative — running tasks must finish or check flags
• Stop() is usually what people want when they say “break early”

One more example

Use the ParallelLoopState.Break or ParallelLoopState.Stop method to end parallel foreach loop:

public void BreakParallelForeachEarlier()
{
    var list = new List<Int32>();
    for (var i = 0; i < 100; ++i)
        list.Add(i);

    var startTime = DateTime.Now;
    var maxRunTimeInMs = 2500;

    Parallel.ForEach(list, (elem, state) =>
    {
        Console.WriteLine("Running for {0}", elem);
        Thread.Sleep(1000 + elem); // Running exhausting algorithm

        var duration = (DateTime.Now - startTime).TotalMilliseconds;
        if (duration > maxRunTimeInMs)
        {
            Console.WriteLine("Exiting the parallel foreach block...");
            state.Break();
        }
    });
}