 <p>First of all, <code>Where(x =&gt; x is Video)</code> can be replaced by <code>OfType&lt;Video&gt;()</code>.<br> Second, for fluent syntax it's better to use <code>ParallelEnumerable.ForAll</code> extension method:</p> <pre><code>media.OfType&lt;Video&gt;() .AsParallel() .ForAll(this.Update) </code></pre>