title: IEnumerable<type> does not contain a definition of 'Contains'
body: <p>I have 2 classes and 2 IEnumerable lists of those classes:</p>
  <pre><code>public class Values
  {
      public int Value { get; set; }
      public DateTime SomeDate { get; set; }
  }
  
  public class Holidays
  {
      public DateTime holiday { get; set; }
  }
  
  IEnumerable&lt;Values&gt; values;
  IEnumerable&lt;Holidays&gt; holidays;
  </code></pre>
  <p>Next, I am trying to select those 'values' where 'someDate' is not in 'holidays'</p>
  <pre><code>var query = values.Where(x=&gt; !holidays.Contains(x.someDate));
  </code></pre>
  <p>However, this gives me the error of <code>IEnumerable&lt;Holidays&gt; does not contain a definition of 'Contains'</code>.</p>
  <p>System.Linq is already added in the <code>using</code>s.</p>
  <p>I believe this has to do something with the collections, but am not able to figure what.</p>