title: Do list except with two list of different classes instances
body: <p>I want to compare two list with objects of two different classes. These are the classes:</p>
  
  <pre><code>namespace AngularWebApplication.Models
  {
      public class AggregationLevelConfigurationPresentation
      {
          public byte AggregationLevelConfigurationId { get; set; }
          public int ProductionOrderId { get; set; }
          public string Name { get; set; }
          [ ... ]
      }
  }
  
  public class AggregationLevelConfiguration : IEquatable&lt;AggregationLevelConfiguration&gt;
  {
      public byte AggregationLevelConfigurationId { get; set; }
      public int ProductionOrderId { get; set; }
      public string Name { get; set; }
      [ ... ]
  }
  </code></pre>
  
  <p>I want to get the elements in <code>presentations</code> list that are not in <code>currentLevels</code>:</p>
  
  <p>List presentations;
  List currentLevels;</p>
  
  <pre><code>List&lt;Models.AggregationLevelConfigurationPresentation&gt; newLevels =
      presentations
          .Select(l =&gt; new { l.AggregationLevelConfigurationId, l.ProductionOrderId })
          .Except(currentLevels.Select(l =&gt; new { l.AggregationLevelConfigurationId, l.ProductionOrderId }))
          .ToList();
  </code></pre>
  
  <p>But I get the following error when I do the <code>Except</code>:</p>
  
  <pre><code>Error   CS0029  Cannot implicitly convert type 
  'System.Collections.Generic.List&lt;&lt;anonymous type: byte AggregationLevelConfigurationId, int ProductionOrderId&gt;&gt;' to 
  'System.Collections.Generic.List&lt;AngularWebApplication.Models.AggregationLevelConfigurationPresentation&gt;'
  </code></pre>
  
  <p>I think the problem is in the <code>new { l.AggregationLevelConfigurationId, l.ProductionOrderId }</code> but I don't know how to do the <code>Except</code> with list of objects from different classes.</p>
  
  <p><strong>I need the objects in <code>presentations</code> list that they aren't in <code>currentLevels</code> using <code>AggregationLevelConfigurationId</code> and <code>ProductionOrderId</code> as primary key.</strong></p>