title: Check for distinct value of a property in list except nulls
body: <p>I have a class with few properties like this.</p>
  
  <pre><code>public class CheckList
  {
      public int ACTION_ID { get; set; }
      public string ACTION_NAME { get; set; }
      public string ACTION_DESCRIPTION { get; set; }
      public bool? ACTIVE { get; set; }
  }
  </code></pre>
  
  <p>and List of this class in my controller.</p>
  
  <pre><code>List&lt;CheckList&gt; validChecklists = _ChecklistRepo.GetAll();
  var ifActionsAreSame = validChecklists .Select(t =&gt; t.ACTION_NAME).Distinct().Count();
  if (ifActionsAreSame &lt; validChecklists .Count)
  {
      return Ok(new {ActionsAreDuplicated= true });
  }
  </code></pre>
  
  <p>Sometimes the ACTION_NAME of multiple items Can be null in the list. This code treats the null value as duplicate. What changes I need to do to exclude null fields in the ACTION_NAME field.</p>