You can open the .NET IDE directly from Integration Studio, with the advantage of synchronizing the elements definition with the extension source code. In fact, before launching the IDE, Integration Studio updates the source code. Learn more about Update Source Code.
How to edit the extension source code
In the File menu or in the toolbar, click Edit Source Code.
When developing an extension you must edit the source code in the .NET IDE. In this situation you are responsible of ensuring the synchronization of the written code, since Integration Studio only guarantees the synchronization of the extension definition.
The IDE you specified in the Options Window is launched with the .NET solution.