Why does MSI require the original .msi file to proceed with an uninstall ?

Question

As most of you probably noticed, when uninstalling an MSI package Windows will ask for the original .msi file...

Why is that ?

I can only see disadvantages to that:

• not resilient to network changes.
• not resilient to local disk changes.
• unexpected by users.
• typically requires users to leave their desk and start a crusade to get the correct CD.
• kind of proves installations are not self-contained.
• promotes the use of unsafe tools such as msizap.
• which in turn promotes the "next time I'll just use a zip file" mentality.

Could someone shed some light on this ?

Answer 1

There are a few reasons for keeping the original msi:

• The uninstaller uses it to know what files and registry keys were installed and make sure they are all cleaned up.
• The msi may contain code for special uninstall actions that need to be performed.
• It allows you do to a 'repair' operation from the Add/Remove Programs menu, regardless of whether or not you saved the install file yourself.

The normal way of things is for Windows to keep the file cached for you, so you don't have to think about it. See your %WINDIR%\Installer\ folder. The only reason it would ask your for the original msi is if something is wrong with the saved file. This addresses most of your concerns, though it does raise a new one (disk-space).

Answer 2

The original MSI is not needed for uninstall unless the MSI itself is badly designed. As stated above all MSI files are cached in %WINDIR%\Installer*.* using a random name. This MSI is stripped of all cabs, and hence contains the installer structure only, and no files.

This "stripped" MSI file is used for any maintenance and uninstall operations - and it is sufficient for uninstall in the vast majority of cases. The original source is only needed if files need to be copied to disk (for a maintenance install), or the MSI does an explicit request to resolve the original source via the standard action ResolveSource or via a custom action (which shouldn't be done in a properly authored package).

I hear rumours that Microsoft may be changing this in newer versions of MSI and instead cache the entire MSI file instead of a stripped file.