[SOLVED] Where did IMvcBuilder AddJsonOptions go in .Net Core 3.0?


I’ve just upgraded my ASP web API project from .Net core 2.0 to 3.0. I was using

             .AddJsonOptions(options =>options.SerializerSettings.ContractResolver 
                                       = new DefaultContractResolver());

previously to ensure lower-casing of the serialized JSON.

After the upgrade to 3.0 I get this error:

Error CS1061 ‘IMvcBuilder’ does not contain a definition for
‘AddJsonOptions’ and no accessible extension method ‘AddJsonOptions’
accepting a first argument of type ‘IMvcBuilder’ could be found (are
you missing a using directive or an assembly reference?)

According to AddJsonOptions for MvcJsonOptions in Asp.Net Core 2.2 the AddJsonOptions extension method is/was provided by the Microsoft.AspNetCore.Mvc.Formatters.Json nuget package. I have tried installing/reinstalling this but still can’t resolve the method. Interestingly, intellisense only shows Microsoft.AspNetCore.Mvc.Formatters.Xml when I try to add the using statement even though I added the Json nuget package.

Any ideas what is going on? The documentation for AddJsonOptions only goes up to .Net 2.2 so perhaps the method has been deprecated in 3.0 in favor of some other configuration mechanism?


As part of ASP.NET Core 3.0, the team moved away from including Json.NET by default. You can read more about that in general in the announcement on breaking changes to Microsoft.AspNetCore.App.

Instead of Json.NET, ASP.NET Core 3.0 and .NET Core 3.0 include a different JSON API that focuses a bit more on performance. You can learn about that more in the announcement about “The future of JSON in .NET Core 3.0”.

The new templates for ASP.NET Core will no longer bundle with Json.NET but you can easily reconfigure the project to use it instead of the new JSON library. This is important for both compatibility with older projects and also because the new library is not supposed to be a full replacement, so you won’t see the full feature set there.

In order to reconfigure your ASP.NET Core 3.0 project with Json.NET, you will need to add a NuGet reference to Microsoft.AspNetCore.Mvc.NewtonsoftJson, which is the package that includes all the necessary bits. Then, in the Startup’s ConfigureServices, you will need to configure MVC like this:


This sets up MVC controllers and configures it to use Json.NET instead of that new API. Instead of controllers, you can also use a different MVC overload (e.g. for controllers with views, or Razor pages). That AddNewtonsoftJson method has an overload that allows you to configure the Json.NET options like you were used to with AddJsonOptions in ASP.NET Core 2.x.

    .AddNewtonsoftJson(options =>
        options.SerializerSettings.ContractResolver = new DefaultContractResolver();

Answered By – poke

Answer Checked By – Gilberto Lyons (BugsFixing Admin)

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