Calling C# Classes from JS

I’m using an extension that is written in C#, but I want to access classes from it in a JS script. I found somewhere else that in order to do this, the C# code must be compiled first, and the way to ensure that is to place it in the Plugins, Standard Assets, or Pro Standard Assets folders. The only problem is that once I do that, the editor for the extension stops working. It just says “multi-object editing not supported”. But once I move the code back out into its own folder, the editor works again (but now I can’t access it from a JS script).

Is there a better way to access C# classes from a JS script? Or a better way to ensure that the C# code gets compiled first?