Question

If I Invoke a method onto the UI Thread is it searilized by the Windows message queue and subsequently doesn’t need to be re-entrant?

    private void CalledFromWorkerThread()
    {
        //changed from 'InvokeRequired' Anti-Pattern
        this.Invoke((Action) (() => _counter++; /* Is this ok? */ ));
    }

Clarification: It is only the UI thread that will be accessing _counter.

Answer 1

What you have is fine, assuming _counter is only accessed by the UI thread.

If two threads call your CalledFromWorkerThread, then _counter will be properly incremented and thread-safe with what you have.