I'm using the new Unity 2019, and the update for colyseus that use Net 4.x , if you have some issues regarding this class, neither if you have the System.Web, well, you can download another one than don't required the entire package, you can test with this one: HttpUtility.cs ( https://gist.github.com/bjorn-ali-goransson/b04a7c44808bb2de8cca3fc9a3762f9c) you just need to added in the Colyseus Plugin Folder, then It wil work. Regards!