windows - What is a compressed GUID and why is it used?

Question

I know that in Windows, the GUID is used by the Windows Installer to check for already installed products under the registry key HKEY_LOCAL_MACHINESOFTWAREMicrosoftWindowsCurrentVersionUninstall and other product-relevant information is stored using the compressed GUID in HKEY_LOCAL_MACHINESOFTWAREMicrosoftWindowsCurrentVersionInstallerUserData.

Yet a google search to figure out what the compressed GUID actually is and why both forms are being used for saving product-specific data in the registry database only revealed algorithms for converting from one form to another.

Apparently, the compressed GUID is calculated from merely changing the order of the characters in a specific way, giving me more confusion about why this is the compressed form and why it is used this way.

Additionally, some sources appear to refer to the GUID as the product code of the software product, and others specifically use the term product code as a synonym for the compressed GUID.

I do not have much knowledge of the inner workings of Windows and its installers, but hope that someone can enlighten me with the information I was unable to find. I apologize in advance for my mediocre English.

Answer 1

The packed guid is reportedly used to save space in the registry. It is used for component Ids, so once you start getting a million components on the system I assume they were worried about the extra space usage. It's also possible it was used to prevent people trawling the registry to look for Windows Installer items instead of using the APIs. It's an undocumented implementation detail, and if too many people got into habit of using the registry instead of using the APIs there would be issues if that implementation changed. So treat this as interesting information, not something to use in an application.

Darwin Descriptors are different - they are just an encoding of product code, component id, and feature name. They occur in various places where that combination of data is useful for loading COM servers, starting programs etc. Again, it's undocumented, but if you wanted to untangle them you could LoadLibrary on msi.dll and find and call MsiDecomposeDescriptorW with the right parameters.