For Asp.Net Core 3.0, it uses System.Text.Json
for serialization and deserialization.
For using old behavior, you could use Json.NET in an ASP.NET Core 3.0 project by referencing Json.NET support.
Short Answer:
- Install
Microsoft.AspNetCore.Mvc.NewtonsoftJson
which is preview version. - Change to
services.AddControllers().AddNewtonsoftJson();