JsonIgnore attribute keeps serializing properties in ASP.NET Core 3

Question

I've recently updated my API project to ASP.NET Core 3. Since then, [JsonIgnore] attributes are not working:

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; }
}

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 Newtonsoft.Json to System.Text.Json. This package is not available in .NET Standard 2.1 (it only works in .NET Core) so to use [JsonIgnore] in Models inside .NET Standard projects I am using Newtonsoft.Json .

Answer 1

[JsonIgnore] is an JSON.NET attribute and won't be used by the new System.Text.Json serializer.

Since your application is an ASP.NET Core 3.0 System.Text.Json will be used by default. If you want to continue to consume the JSON.NET annotations, you have to use JSON.NET in ASP.NET Core 3

It's as easy as adding .AddNewtonsoftJson() to your MVC or WebApi Builder

services.AddMvc()
    .AddNewtonsoftJson();

or

services.AddControllers()
    .AddNewtonsoftJson();

for WebAPI-esque applications.