I didn't plan to start my discussion about the new installation technology in Windows 8, AppX, with this topic. However, a quick scan through this morning's email from wix-users prompted me to start here.

Someone was having a problem getting their registry keys written during a patch. Someone else gave the advice that it's easiest to just use a custom action in the patch to change the values. That proposed solution goes against several best practices (that are probably not written down or not written down well).  Two off the top of my head:

• Avoid custom actions at all cost, they decrease the stability of your installer.
• Do not duplicate functionality provided by the installation engine with a custom action.

In the "reimagined" world of Windows 8 and Metro-style apps, Windows took the hard line and said:

...there are no custom actions during the install.

This basically removes any chance to insert failure from the outside of the AppX installation engine. It also means your application's setup requirements must be fully declarative. We'll talk about that more in the future.

Ultimately, custom actions are still an admission of failure and they don't exist in Window 8's Metro-style applications.