You should keep MessageBox out of your nonvisual assemblies - to make them nonvisual. MessageBox in an assembly running in a web API on a server would have nowhere to display and no user to hit [OK] button.

That said, MessageBox is a class in System.Window.Forms.

The C# code converter we provide is to migrate your business logic only into an MVC architecture. It is not going to take care of the UI for you or automatically extract the business logic from your app (if you haven't already encapsulated the logic in NVOs).

And Michael is exactly right that in a REST Web API you should not be doing UI things like Message Box... it is running on the server and is non-visual. In fact, I question if you should be even doing file functions.

If you plan to have the C# on the client side (i.e. not re-architect your app into Web APIs running on the server side), then you will be faced with rewriting your UI by hand. As I mentioned, the solution will only automate migration of business logic.

Actually, many of my non-visual assemblies are part of COMs (now) and consumed by the PB desktop apps. So, they do display message boxes, but have no visual screens.