title: how to select an item from generic list by linq
body: <p>I have a LINQ query which contains a method GetInstanceForDatabase()</p>
  
  <pre><code>principlesList.Select(p =&gt; p.GetInstanceForDatabase()).ToList()
  </code></pre>
  
  <p>where</p>
  
  <pre><code>List&lt;PrincipleInstance&gt;() principlesList = ...
  // (contains list of principle like "Manual Trades", "OPM", "Flora")
  </code></pre>
  
  <p><code>GetInstanceForDatabase()</code> is a method  which takes all other info about a principle (like manual trades).</p>
  
  <p>My problem is that I want to sort out only principle like only "Manual Trades".</p>
  
  <p>I want to put a <code>where</code> clause. I tried but it is fails.</p>