To develope Revit add-in applications, the capability to debug the code is very important to find problems, solve bugs etc.

In visual studio, select the project you want to debug, right click and select the lowest “Property” item, in the property page, choose “Debug” in the left panel, and in “Start Action”, choose “Start external program:” and then select the Revit.exe in your installation folder like following picture, then you can add break points in your project and debug your application as general executable projects.

And there is another way to debug your program: run revit first, then in visual studio, click the “Attach to Process” under “Debug” menu:

In the pop-up dialog, select Revit in the “Available Processes”, then click “Attach”, the other is same as previous method: