If your solution contains a setup project and this setup project has been built at least once, you get an Assembly Load error when you run the Inheritance Picker in order to inherit from an existing form in your main project.

This problem occurs because the setup project generates several executable files – such as InstMsiA.exe, InstMsiW.exe, and Setup.exe – that aren’t valid .NET assemblies. However, the Inheritance Picker interprets these files as assemblies and attempts to load them, which results in a failure.

The only solution to this problem is removing the setup project from the solution, and re-add it to the solution only when you’re about to package the solution.