title: JsonIgnore attribute keeps serializing properties in ASP.NET Core 3
body: <p>I've recently updated my API project to ASP.NET Core 3. Since then, <code>[JsonIgnore]</code> attributes are not working:</p>
  
  <pre><code>public class Diagnostico
  {
      [JsonIgnore]
      public int TipoDiagnostico { get; set; }
  
      [JsonIgnore]
      public int Orden { get; set; }
  
      [JsonIgnore]
      public DateTime? FechaInicio { get; set; }
  
      public string TipoCodificacion { get; set; }
  
      public string Codigo { get; set; }
  
      public string Descripcion { get; set; }
  }
  </code></pre>
  
  <p>All the properties of classes are being serialized. The API endpoints are in .NET Core 3, but all the logic is in .NET Standard 2.1. I have realized that the serializer has changed from <code>Newtonsoft.Json</code> to 
  <code>System.Text.Json</code>. This package is not available in .NET Standard 2.1 (it only works in .NET Core) so to use <code>[JsonIgnore]</code> in Models inside .NET Standard projects I am using <code>Newtonsoft.Json</code> .</p>