Extract List of object from another List of objects
<p>I have a <code>List&lt;User&gt; users</code>, where class<code>User</code> has one property <code>username</code>. I also have another<code>List&lt;User&gt; activeUsers</code>. Let <code>users  = activeUsers + inactiveUsers</code>. Now I want to extract <code>inactiveUsers</code> from <code>users</code> based on <code>username</code> property. I solved this problem using two for-loops. I think this is not efficient way. so if anyone know how it can be done efficiently please let me know.</p>

<p>e.g. I have activeUsers[1,3] and users[1,2,3,4] and want to build inactiveUsers[2,4].</p>
