Break out of Parallel.Foreach in June CTP

Question

How do you break out of a Parallel.Foreach loop in the June CTP? I see that in the .NET 4.0 extensions they have added the ability to do so, but how do you do it in the previous set of extensions?

Answer 1

The June CTP supported this by using an overload of Parallel.ForEach that used a delegate which accepted a ParallelState parameter.  That ParallelState exposed a Stop method, which could be used to exit a loop early.  Throwing an exception would also cause early exit from a loop.

Answer 2

But the problem is that the ParallelState enumeration is marked as internal. AFAICT, there is no way to create an instance of it.

Answer 3

ParallelState is public; its constructor is internal.  But you don't need to construct one.  Rather, the loop constructs one and passes that instance in to your delegate, e.g.

using System;
using System.Threading;

class Program
{
    static void Main(string[] args)
    {
        int[] data = new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
        Parallel.ForEach(data, (i, state) =>
        {
            if (i == 4) state.Stop();
            Thread.Sleep(i * 1000);
            Console.WriteLine(i);
        });
    }
}

Answer 4

Ah. Using a undeclared variable called "state" seemingly out of thin-air is definitely not intuitive but with multi-threaded development that seems to be par for the course ;->. Thanks!

Answer 5

Happy to help.

I'm using the most condensed lambda syntax.  If it helps from a clarity perspective, you could expand it to the full delegate syntax:

Parallel.ForEach(data, delegate(int i, ParallelState state)
{
     ...
});

or even separating out the delegate:

Action<int, ParallelState> body = delegate(int i, ParallelState state)
{
    ...
});
Parallel.ForEach(data, body);

Finally, of course, you can separate the body out into its own method.