Debugging Windows Installer Custom Actions

General

1. First, you must know the name of the custom action you want to debug. In this example we’ll call it MyCustomAction.
2. Set the MsiBreak environment variable (user or system) to the name of the custom action. You can do this easily by running setx from the commandline:
   

   C:\Windows\System32>setx MsiBreak MyCustomAction
SUCCESS: Specified value was saved.
C:\Windows\System32>

3. Run your installer

You should get now get a message box prompt like this:

At this point, you can use Visual Studio or another debugger such as WinDBG to attach to the specified process.