There is a windows project in visual studio, you need to transfer it to the ubuntu distribution of the gnu / linux operating system.
As I understand it, the first step is to get rid of windows forms and make a console interface. I will do it.
The problem is that the project uses a third-party dll library (for connecting and working with an external mysql database) Can it be transferred in any way or should I look for an alternative under gnu / linux?
As I understand it, the first step is to get rid of windows forms
Mono seems to have its own implementation of Winforms, which is at least 99 percent compatible. They say so.
As for WYSIWYG, the standard IDE does not have it for Winforms, only for GTK#, but you can download and install MWF Designer.
The problem is that the project uses a third-party dll library
And again, as with Winforms: why are you sure that it will not go to Mono?
The library for working with the database could theoretically even work on an iron or a microwave if there is enough RAM and ROM. Nothing platform-dependent there should be. (The exception is ADO.NET and others like it, but this is not quite a library for .NET, it is tied to native Windows ADO)
Try writing a MySQL helloworld in Mono – this seems to be the place to start in your case.