这是版本控制问题.您所指的JSON .NET for Unity只是使用(如注释中所述)Newtonsoft.Json 8.0.3.

This is a versioning issue. The JSON .NET for Unity you are referring to as just Json.NET uses (as mentioned in the comments) Newtonsoft.Json 8.0.3.

如果您想使用Newtonsoft.Json的新功能，建议您使用其他解决方案. Newton.Json为Unity制作的当前最新分支在这里: https://github.com/jilleJr/Newtonsoft.Json-for-Unity#readme ，在撰写本文时位于Newtonsoft.Json 12.0.3.

If you want to use fresh features from Newtonsoft.Json I suggest jumping to a different solution. The currently most up-to-date fork of Newtonsoft.Json made for Unity is here: https://github.com/jilleJr/Newtonsoft.Json-for-Unity#readme, being at Newtonsoft.Json 12.0.3 at the time of writing.

jilleJr/Newtonsoft.Json-for-Unity也通过内置的Unity软件包管理器进行部署，因此，使用户保持最新状态也很容易.

jilleJr/Newtonsoft.Json-for-Unity is also deployed via the builtin Unity Package Manager, so staying up-to-date will be easier as a user as well.