Is C# += thread safe?

Question

Just ran into a program where += is used on a shared variable among threads, so is += thread safe, i.e. performs addition and assignment atomically?

No. It's reading, doing the addition, and then writing to the variable. It is not atomic. — Gray, Oct 11 '13 at 20:48
This may help http://stackoverflow.com/questions/4628243/is-the-operator-thread-safe?rq=1. And the answer is Never — Sriram Sakthivel, Oct 11 '13 at 20:49

score 12 · Answer 1 · answered Oct 11 '13 at 20:50

12

No it isn't thread safe since it's equivalent to:

int temp = orig + value;
orig = temp;

You can use Interlocked.Add instead:

Interlocked.Add(ref orig, value);

answered Oct 11 '13 at 20:50

Lee

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score 2 · Answer 2 · answered Oct 11 '13 at 20:50

2

You want

System.Threading.Interlocked.Add()

answered Oct 11 '13 at 20:50

Mike Goodwin

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score 1 · Answer 3 · answered Oct 11 '13 at 20:51

1

string s += "foo";

is

string s = s + "foo";

s is read and then re-assigned. If in between these two actions the value of s is changed by another thread, the result with be different, so it's not thread safe.

answered Oct 11 '13 at 20:51

Sam Leach

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score 0 · Answer 4 · answered Oct 12 '13 at 04:22

Thank you all for the quick replies. Yes, += is not thread safe and to verify this one may run the following simple program.

int count = 0;

        Parallel.For(0, 10000, i =>
        {
            count +=1; // not thread safe
        });
        Console.WriteLine(count);

Is C# += thread safe?

4 Answers4

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