What Happened to `AddJsonOptions` in .NET Core 3.0 and How Do I Configure JSON Serialization Now?

Removal of AddJsonOptions method and JSON serialization configuration method in .NET Core 3.0

After upgrading from .NET Core 2.0 to 3.0, developers found that the IMvcBuilder extension method was missing from the AddJsonOptions interface. This article explains the reasons for removing this method and describes the recommended alternative configuration mechanism.

Changes between Json.NET and ASP.NET Core 3.0

In the past, ASP.NET Core used Json.NET as the JSON serialization library by default. However, in version 3.0, the framework no longer uses Json.NET by default and instead introduces a new, performance-optimized JSON API.

New configuration mechanism

To continue using Json.NET in .NET Core 3.0, developers need to add a reference to the Microsoft.AspNetCore.Mvc.NewtonsoftJson NuGet package. Then, in the Startup class, modify the MVC configuration as follows:

<code class="language-csharp">services.AddControllers()
    .AddNewtonsoftJson();</code>

This configuration will enable the MVC controller to use Json.NET. The AddNewtonsoftJson method provides overloads that allow developers to define custom Json.NET options, similar to the AddJsonOptions method in earlier versions. For example:

<code class="language-csharp">services.AddControllers()
    .AddNewtonsoftJson(options =>
    {
        options.SerializerSettings.ContractResolver = new DefaultContractResolver();
    });</code>

To summarize, although the AddJsonOptions method is no longer available in .NET Core 3.0, developers can easily reintroduce Json.NET by installing the NuGet package and updating the MVC configuration in the Startup class. This ensures compatibility with older projects and access to the full functionality of Json.NET.

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