If you need to implement partial routing in Optimizely CMS 12, this is a good start.

There are of course somebreaking changes as per the documentation:

The interface IPartialRouter changed slightly, and implementations should now be registered in DI container as IPartialRouter, rather than the previous way to register a partial router, which was through the extension method RegisterPartialRoute on RouteCollection.

OK, lets start with adding a partial router class which implements IPartialRouter:

public class MyPagePartialRouter : IPartialRouter<MyPage, MyPage>    {        public MyPagePartialRouter() { }        public object RoutePartial(MyPage content, UrlResolverContext urlResolverContext)        {            urlResolverContext.RouteValues.Add("requestPath", urlResolverContext.RemainingSegments);            urlResolverContext.RemainingSegments = Array.Empty<Char>();            return content;        }        public PartialRouteData GetPartialVirtualPath(MyPage content, UrlGeneratorContext urlGeneratorContext)        {            return null;        }    }

We add our params to theRouteValues collection. Note we useRemainingSegments instead of RemainingPath.

Then we need to register it in a DI container like this:

services.AddSingleton<IPartialRouter, MyPagePartialRouter>();

And that's it!