Add delegate to event - thread safety
<p>It is possible to execute the following code from multiple threads simultaneously.</p>

<pre><code>this._sequencer.Completed += OnActivityFinished;
</code></pre>

<p>Is it thread safe to add delegate to an event handler from multiple threads?  </p>

<p>Is it thread safe to remove delegate to from event handler from multiple threads?   </p>

<p>What is the simplest and maintainable way of making this thread safe?</p>
