you can find some info about this class. As I can see, the class was introduced in the .Net 3.5, so this stuff may not be supported by Mono. Just for you to know: not every feature of .NET is supported by Mono. Json.NET is also may not be supported by Mono properly. So I'd suggest to check your Mono version compatibility with appropriate versions of .NET framework. Also, there are several builds of Json.NET for different .NET versions. Try to play with them. Also, Qt framework
is a much better choice for writing crossplatform applications. It's not too hard to learn.