JsonOutputFormatter in ASP.NET Core 3.0

To revert back to NewtonsoftJson and also configure its output formatter, first add a package reference to Microsoft.AspNetCore.Mvc.NewtonsoftJson and then in ConfigureServices you need to call Configure after calling AddController and AddNewtonsoftJson:

public void ConfigureServices(IServiceCollection services)
{
    services.AddControllers()
            .AddNewtonsoftJson();
    
    services.Configure<MvcOptions>(options =>
        {
            NewtonsoftJsonOutputFormatter jsonOutputFormatter = options.OutputFormatters.OfType<NewtonsoftJsonOutputFormatter>().Single();
        
            // makes changes to the Newtonsoft JSON Output Formatter here.
        });
}

I personally use Json.NET

• Simply add a package reference to Microsoft.AspNetCore.Mvc.NewtonsoftJson.
• Update Startup.ConfigureServices to call AddNewtonsoftJson.

services.AddMvc().AddNewtonsoftJson();

Json.NET settings can be set in the call to AddNewtonsoftJson:

services.AddMvc()
    .AddNewtonsoftJson(options =>
           options.SerializerSettings.ContractResolver =
              new CamelCasePropertyNamesContractResolver());

I am using the default options with compatibility mode

services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_3_0)
            .AddNewtonsoftJson(options => { options.SerializerSettings.ContractResolver =
             new DefaultContractResolver(); });

Reference Migrate from ASP.Net 2.2 to 3.0