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Windows Installer FAQ (the basics)
Custom Actions
Types of Custom Actions
Immediate Custom Actions
Immediate custom actions are the opposite of deferred ones and
can be scheduled at any time.
All immediate custom actions that fall between
"InstallInitialize" and "InstallFinalize" are executed before deferred ones.
As these actions don't have
commit and Rollback support they
shouldn't change machine state.
An immediate custom action can change the MSI database to dynamically
alter its "settings", see "Dynamically Update Dialog Controls"
for an example.