Parallel.For not handling lock properly

Question

I've done the following test:

private static object threadLocker = new object();

private static long threadStaticVar;
public static long ThreadStaticVar
{
    get
    {
        lock (threadLocker)
        {
            return threadStaticVar;
        }
    }
    set
    {
        lock (threadLocker)
        {
            threadStaticVar = value;
        }
    }
}

Parallel.For(0, 20000, (x) =>
{
    //lock (threadLocker) // works with this lock
    //{
        ThreadStaticVar++;
    //}
});

This Parallel.For invokes the method passing the values from 0 to 19999. So it would execute 20k times.

If I don't wrap ThreadStaticVar++; with a lock, even though it has a lock on its get and set, the result will not be 20000. If I remove the comment bars and lock it inside the .For it gets the right value.

My question is: How does it work? Why the lock on the get and set doesn't work? Why it works only inside my For?

Answer 1

The ++ operator isn't an atomic increment. There will be a call to get followed by a call to set, and those calls can be interleaved among different threads since the lock is only on each individual operation. Think of it like this:

lock {tmp = var}
lock {var = tmp+1}

Those locks don't look so effective now, do they?