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?

Answer 1

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);

Answer 2

You want

System.Threading.Interlocked.Add()

Answer 3

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.