You can create one of the forms in a separate class library or as a user control in a separate class library and simply reference it from your application. Each project in your solution can be either VB.Net or C #, so it works in both directions ...
In a simple solution example, you can have the following projects
- MyApp.CSharpControls - Project in C #
- MyApp.VBControls - Project in Vb.Net
- MyApp.UI - # VB.Net,
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winforms??