JsonResult Serializer Settings in .NET Core3.1

If you are returning a JSON representation of an object from an MVC controller it may be necessary to determine how the JSON is formatted.

This can be done using the JsonSerializerSettings object like so.

public async Task<JsonResult> AuditableResources(DateTime? startDate, DateTime? endDate, string resourceType)
{
	JsonSerializerSettings jsonSerializerSettings = new JsonSerializerSettings()
	{
		ReferenceLoopHandling = ReferenceLoopHandling.Ignore,
		ContractResolver = new CamelCasePropertyNamesContractResolver()
	};

	List<Resource> resources = new List<Resource>();

	// Get resources here...

	return new JsonResult(resources, jsonSerializerSettings);
}

This used to work as is with version 2 of .NET Core but with the addition of the System.Text.Json namespace in v3 this will no longer work and the below error will be recieved.

System.InvalidOperationException: Property 'JsonResult.SerializerSettings' must be an instance of type 'System.Text.Json.JsonSerializerOptions'.

In order to fix this you need to explicitly tell the controller that Newtonsoft should be used to format JSON as stated here.

This can be done by adding the package Microsoft.AspNetCore.Mvc.NewtonsoftJson to your project and adding the following code to the ConfigureServices method of your startup file.

public void ConfigureServices(IServiceCollection services)
{
    services.AddControllers()
        .AddNewtonsoftJson();
}