title: Fastest way to convert List<int> to List<int?>
body: <p>What is the fastest way of taking a list of primitives and converting it to a nullable list of primitives? For example: <code>List&lt;int&gt;</code> to <code>List&lt;int?&gt;</code>.</p>
  
  <p>The easy solution, creating a new list and adding every item with a <code>foreach</code> loop, takes too much time.</p>