Parallel.For(): Update variable outside of loop

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You can’t do this. sum is being shared across you parallel threads. You need to make sure that the sum variable is only being accessed by one thread at a time:

// DON'T DO THIS!
Parallel.For(0, data.Count, i =>
{
    Interlocked.Add(ref sum, data[i]);
});

BUT… This is an anti-pattern because you’ve effectively serialised the loop because each thread will lock on the Interlocked.Add.

What you need to do is add sub totals and merge them at the end like this:

Parallel.For<int>(0, result.Count, () => 0, (i, loop, subtotal) =>
    {
        subtotal += result[i];
        return subtotal;
    },
    (x) => Interlocked.Add(ref sum, x)
);

You can find further discussion of this on MSDN: http://msdn.microsoft.com/en-us/library/dd460703.aspx

PLUG: You can find more on this in Chapter 2 on A Guide to Parallel Programming

The following is also definitely worth a read…

Patterns for Parallel Programming: Understanding and Applying Parallel Patterns with the .NET Framework 4 – Stephen Toub

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