Figure 2. Build Your Solution Carefully. You typically have three projects loaded in your VS.NET solution when developing a plug-in: the ExtensibilityLib DLL, the main application, and the plug-in DLL under development. The last two projects have a reference to the ExtensibilityLib DLL. You can simplify the test phase by changing the output path of the plug-in project so that the DLL is created right in the main app’s Bin directory. Optionally, you can add a fourth DLL project that gathers all the forms used by the main application.