January 15th, 2020

Why can’t I use SHSetKnownFolderPath to change the location of FOLDERID_LocalAppData?

Raymond Chen
Raymond Chen

A customer observed that their unit test started failing in Windows 10 version 1803. Specifically, it failed here:

SHSetKnownFolderPath(FOLDERID_LocalAppData, 0, NULL, temporaryDirectory);

with the error E_INVALID­ARG. What happened in Windows 10 version 1803 to make this call invalid?

The change to block relocation of the Local­App­Data folder, as well as its close personal friend Local­App­Data­Low, was an acknowledgement of reality: Earlier versions of Windows allowed you to relocate those directories, but the result was a corrupted user profile, forcing you to delete the user and start over.

The reason is that there are some system files in those directories which are perpetually locked and consequently cannot be moved to their new home. The next time the user logs on, the system tries to reopen the files and can’t, and things rapidly go downhill from there.

Relocation of these directories is blocked to prevent you from shooting yourself in the foot. It’s an acknowledgement of the reality that it not only doesn’t work, but leaves the user profile corrupted.

 

Category
Old New Thing
Topics
Code

Author

Raymond Chen

Raymond has been involved in the evolution of Windows for more than 30 years. In 2003, he began a Web site known as The Old New Thing which has grown in popularity far beyond his wildest imagination, a development which still gives him the heebie-jeebies. The Web site spawned a book, coincidentally also titled The Old New Thing (Addison Wesley 2007). He occasionally appears on the Windows Dev Docs Twitter account to tell stories which convey no useful information.

12 comments

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  • Joshua Hudson

    If you *really* want this, you should have carried out the procedure instead to encrypt your entire user profile.

    1) Log in as another user with admin
    2) Delete your entire profile
    3) Delete the path to your profile from the system registry
    4) Change new profile directory to something in the encrypted drive
    5) Log in as yourself with runas; this will create your new profile.
    6) Change new profile directory back *immediately* -- don't...

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  • Steven Don

    I must wonder why that particular action was in a unit test. I can understand wanting to use a temporary folder to store your own app’s data when doing tests, but this seems like addressing a local problem by modifying global state. What legitimate purpose could that have served?

    • Alexis Ryan

      Yeah what are they trying to test, were they just testing against all FOLDERIDs, a random subset? even just including that function in a unit test seems strange too

  • Simon Clarkstone

    I remember fighting with this when trying to move all my Known Folders into an encrypted partition. Moving AppData/Local was a right pain due to magic files, and once moved, some things like the start menu and Edge wouldn't launch. I eventually gave up and kept AppData/Local unencrypted. (I wanted to encrypt the whole disk but the tool I was using failed to do that without clear explanation, so I switched to having an encrypted...

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    • William Godfrey

      I had a Windows tablet with a small C: on flash memory and a D: on an SD card which was 16 times larger and almost empty. The C: was running low on space and I tried configuring it to use D: for temp files but it just wasn’t having it.

  • Alex Martin

    I know it's far too late to change any of this behavior, and I don't want to start a flame war, but I will still say that this is an example of how Windows mishandles file locking.

    Could you make this work with a pending rename? It would mean a reboot, but it would work. Obviously not through the same interface, but if you had a new one that's documented as moving the folder at the...

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    • Stefan Kanthak

      The pending rename feature can of course rename folders, not just files.
      But how do you tell a user that it only works on the same volume?

      • Alex Martin

        I’m just trying to think of ways to fix the programmatic interface. UI design is not my strong suit…

      • Dave Bacher

        Just require the user to log out.

        When the user is logged out, you can move the directory. User Profile Service can keep a token, if it needs it, while it moves the directory. Then it can mount and update the registry, or just mklink /d and be done with it (which is all around safer). You can create a shadow at the time the move is requested and start copying files from...

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      • cheong00

        @Antonio: Regarding OneDrive mixing files from multiple machines, I think that's not my fault. It's Windows Security Settings claims I have problem not enabling it. And when I enable it it's don't provide much settings for me to set. As soon as it tells me sync complete, I found all the files have moved to another location, and my "Visual Studio Projects" folder contains multiple copies of files from different machine at different time.

        And if...

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      • Antonio Rodríguez

        @cheong00: TRWTF is storing application settings in the My Documents folder, and hard-coding the path to it. That's exactly what the article's LocalAppData is for. You don't need for OneDrive to move it. The user can do as they wishes with it (hey, it's their "My Documents" folder!), and you wouldn't even get to complain about it.

        Also, if you are having problems because OneDrive mixed files from two machines, I guess you either don't have...

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      • cheong00

        I hate to beat the dead horse, but can't help but keep wondering who TH approve OneDrive's action to move the Document folder on backup? Lots of my cross-platform software go broken because they store settings in Document folder with hard-coded path.

        For me, it hurts me more because the backup is not "per-machine". My Visual Studio Projects from different time got mixed into a mess. (I switched back to my desktop for development when SP3...

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