Question

I want to do something like the following – basically I am calling an async operation which will call a callback in another thread and I want to wait for it to complete “inline”. My worry is that that changes variables shared across threads (bar & event) may not be synchronized due to being stored in registers for example. If they were member variables I could mark them volatile but volatile can’t be used on local variables created on the stack. I could use member variables but I think its cleaner not clutter my class by keeping it all local.

Bar bar = null;
ManualResetEvent event = new ManualResetEvent(false);

foo.AsyncOperation(new Action(()=>{    
    // This delegate will be called in another thread
    bar = ...
    event.Set();
}));

event.WaitOne(timeout);
// use bar

Answer 1

Yes, it will work correctly. Read here

http://www.albahari.com/threading/part4.aspx

The following implicitly generate full fences: Setting and waiting on a signaling construct

and in the signaling constructs the ManualResetEvent is included.

If you want to know what a full fence is, in the same page:

Full fences The simplest kind of memory barrier is a full memory
barrier (full fence) which prevents any kind of instruction reordering
or caching around that fence. Calling Thread.MemoryBarrier generates a
full fence;