ASP.NET Core 3.1: Use Newtonsoft JSON only in some controllers

Question

I have an ASP.NET Core 3.1 application where I use

services.AddControllers().AddNewtonsoftJson();

to use the Newtonsoft JSON serializer. Unfortunately I cannot switch completely to System.Text.Json in one step.

Is there a way to put an attribute like

[NewtonsoftJson]
public class MyController : Controller
{
   // ...

on those controllers that I could not yet migrate -- and only use Newtonsoft for these controllers?

Answer 1

According to your description, I suggest you could consider using the ActionFilterAttribute to achieve your requirement.

Like below :

public class NewtonsoftJsonFormatterAttribute : ActionFilterAttribute
{
   public override void OnActionExecuted(ActionExecutedContext context)
   {
       if (context.Result is ObjectResult objectResult)
       {
           var jsonOptions = context.HttpContext.RequestServices.GetService<IOptions<MvcNewtonsoftJsonOptions>>();
 
            objectResult.Formatters.RemoveType<SystemTextJsonOutputFormatter>();
            objectResult.Formatters.Add(new NewtonsoftJsonOutputFormatter(
                jsonOptions.Value.SerializerSettings,
                context.HttpContext.RequestServices.GetRequiredService<ArrayPool<char>>(),
                context.HttpContext.RequestServices.GetRequiredService<IOptions<MvcOptions>>().Value));
        }
        else
        {
             base.OnActionExecuted(context);
        }
    }
}

More details, you could refer to this article.