I have created a winforms app and a setup and deployment project for that app (VS 2010).
All I need is this:
When a user runs the msi, right after he selects the installation folder I want to check if the main executable of the application already exists in that folder. In that case I want to break the installation and prompt the user to either uninstall the existing application or choose a different folder.
I would like, if possible, to not use any custom installer action. At first, a launch condition (with a file search) seemed to be the right way, but it seems launch conditions (since they are 'launch') run at the beginning of the msi execution and not after folder picking.
Visual Studio Setup and Deployment Projects don't support this type of authoring. It's one of the many reasons that Microsoft removed that project type from Visual Studio 2012.
The only way to do it using this tool would be to build an MSI and then use ORCA to create a transform authoring the validation custom action and scheduling it into the UI as a gating control event. You could then write a postbuild script to apply the transform to the MSI every time it gets built.
Very advanced stuff and frankly not worth the effort. It would be far more beneficial to switch to a tool that supports doing this such as Windows Installer XML (FOSS) or InstallShield 2012 Professional. ($$)