title: ASP.Net Core 3.1 WEB API - Controller method doesn't return correct JSON
body: <p>I have an ASP.Net Core 2.2 Website that I have upgraded to ASp.Net Core 3.0. Most of the WEB API methods works as expected and returns data just as before the upgrade.</p>
  <p>But this API methods returns an array with empty elements:</p>
  <pre><code>    [Route(&quot;api/TripAnalysis/{id:int}&quot;)]
      public List&lt;TurAnalyse&gt; GetTripAnalysis(int id)
      {
          var x = new List&lt;TurAnalyse&gt;() { new TurAnalyse { ArtId = 1, ArtNavn = &quot;Havørn&quot;, C1 = &quot;X&quot;, Su = false } };
          return x;
      }
  </code></pre>
  <p>The TurAnalyse model class is here:</p>
  <pre><code>public class TurAnalyse
  {
      public int ArtId;
      public bool Su;
      public string ArtNavn;
  
      public string C1;
      public string C2;
      public string C3;
      public string C4;
      public string C5;
      public string C6;
  }
  </code></pre>
  <p>I get this back when calling the controller method:</p>
  <pre><code>[{}]
  </code></pre>
  <p>So it returns an array with 1 element, but no properties or values - and don't understand why?</p>