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Fixed in 5.6.0



Found in


Issue ID




UnityEngine.UI.dll is in timestamps but is not known in guidmapper...

Error message shown on first open of attached project:

Asset 'C:/Program Files (x86)/Unity/Editor/Data/UnityExtensions/Unity/GUISystem/4.6.0/UnityEngine.UI.dll' is in timestamps but is not known in guidmapper...

Comments (85)

  1. Marulian

    Jun 12, 2015 20:43

    Happened today randomly after a pull from git, Unity 5.0.2f1

  2. Jinja

    Jun 11, 2015 17:45

    I'm getting this on windows 5.1.0f3 personal

  3. MarkOxenham

    Jun 03, 2015 07:30

    Hey all, my whole team just encountered this issue, 1 by 1 as we each got latest from source control. We encountered this on both PC (Win8) and Mac. We're all on the same version of Unity (4.6.3)

    While deleting /Library or using Reimport Assets did work when we first encountered the issue, it was quite a brute force approach, and our project is large enough that this process would cost us 30-60 minutes each just to get Unity working again.

    Eventually we found that the solution TYRANNICUS100BC found of moving and replacing the .dlls did work, and vastly sped up the recovery from this error.

    Many of the comments about this have mentioned renaming files/folders and this caught my attention as this was something we had been doing recently - especially the changing of case of an Asset - before we encountered the issue.

  4. Obsidanse

    Jun 02, 2015 16:40

    This happened after attempting to rename a nested folder, containing prefabs, from caps to lowercase.
    TEST Floor Elements, to Test Floor Elements.
    The folder remained in caps and then, along with the dlls in question, was the source of the issues upon reload.

  5. aresundnes

    Jun 01, 2015 13:40

    Happens always when unity crashes in a large project (still in 5.0.2p3), and sometimes when just quitting and restarting unity normally. Only way to fix is to reimport library, or to move out and back the DLL's as the previous comment explains

  6. Rene-Damm

    May 29, 2015 08:51

    We're looking at what's going on. I'll keep you updated.

  7. Tyrannicus100BC

    May 26, 2015 18:37

    Best work around we've found:
    1. Quit Unity
    2. Move UnityEngine.UI.dll and UnityEditor.UI.dll to your desktop
    3. Relaunch Unity
    4. Quit Unity
    5. Move UnityEngine.UI.dll and UnityEditor.UI.dll back to their original location
    6. Relaunch Unity

    It appears that this is a bug inside of Unity keeping track of it's own dll's. It seems to happen most often when there was an unexpected Unity crash. With these steps, you are completely removing the UI system form Unity so it completely forgets about it. You'll get tons of errors on step #3 above, because it won't be able to find any of the UI classes. When you restore the dlls, Unity is above to fix itself back up.


    May 26, 2015 04:49

    occurs when you change an asset's name from lowercase to uppercase or uppercase to lowercase , unity 5.0.2f1

  9. Chiao-Wei Chiu

    May 25, 2015 09:26

    just happen in 4.6.4f1 Mac

  10. BS_Dev

    May 22, 2015 04:31

    this issue is still present in 5.0.2f1 - deleting library folder and reimporting does nothing to help the issue. Renaming scripts or scenes causes them to be disassociated in the editor... will there ever be a fix for the sleight of problems present with Unity 5?

    If Unity 5 was a game on Steam, it would have a metacritic score under 20% and would be reverting to early access...

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