How to use Newtonsoft.Json as default in Asp.net Core Web Api?

ASP.NET Core already uses JSON.NET as JavaScriptSerializer isn't implemented/ported to .NET Core.

Microsoft.AspNetCore.Mvc depends on Microsoft.AspNetCore.Formatter.Json which depends on Microsoft.AspNetCore.JsonPatch, which depends on Newtonsoft.Json (see source).

Update

This is only true for ASP.NET Core 1.0 to 2.2. ASP.NET Core 3.0 removes the dependency on JSON.NET and uses it's own JSON serializer.

here is a code snippet to adjust the settings for a .net core application

public void ConfigureServices(IServiceCollection services)
{
    services
        .AddMvc()
        .AddJsonOptions(options => {
            // send back a ISO date
            var settings = options.SerializerSettings;
            settings.DateFormatHandling = Newtonsoft.Json.DateFormatHandling.IsoDateFormat;
            // dont mess with case of properties
            var resolver = options.SerializerSettings.ContractResolver as DefaultContractResolver;
            resolver.NamingStrategy = null;
        });
}

In .NET Core 3.0+ include the NuGet package Microsoft.AspNetCore.Mvc.NewtonsoftJson and then replace

services.AddControllers();

in ConfigureServices with

services.AddControllers().AddNewtonsoftJson();

This is a pre-release NuGet package in .NET Core 3.0 but a full release package in .NET Core 3.1.

I came across this myself, but I've found that the same answer with some additional info is in this SO question and answer.

Edit: As a useful update: code with the call to AddNewtonsoftJson() will compile and run even without installing the Microsoft.AspNetCore.Mvc.NewtonsoftJson NuGet package. If you do that, it runs with both converters installed, but defaulting to the System.Text.Json converter which you presumably don't want since you're reading this answer. So you must remember to install the NuGet package for this to work properly (and remember to re-install it if you ever clear down and redo your NuGet dependencies).