title: Function for checking up two lists against each other?
body: <p>I have to List's.</p>
  
  <pre><code>List&lt;User&gt;
  </code></pre>
  
  <p>and</p>
  
  <pre><code>List&lt;Friends&gt;
  </code></pre>
  
  <p>In User are 10 Coloumns, one of them is "UserID", in Friends we havbe 4 Coloumns, one of them is "UserIDActive".</p>
  
  <p>Now i Wanna have a new List in which only all Users are in it, which are also in Friends. Connected through the Coloum UserID == UserIDActive with have the same value.</p>
  
  <p>How to do that?
  With .contain I can only check two Lists with the same objects in it.</p>