See the question and my original answer on StackOverflow

I needed a JSON serializer and deserializer without any 3rd party dependency or nuget, that can support old systems, so you don't have to choose between Newtonsoft.Json, System.Text.Json, DataContractSerializer, JavaScriptSerializer, etc. depending on the target platform.

So I have started this "ZeroDepJson" open source (MIT licensed) project here:

It's just one C# file ZeroDepJson.cs, compatible with .NET Framework 4.x to .NET Core/5/6/7+.

Note absolute performance is not a goal. Although its performance is, I hope, decent, if you're after the best performing JSON serializer / deserializer ever, then don't use this.