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Changes in the Code of a ScriptedImporter alter .meta files of Reimported files even though the file didn't change the Structure

Asset Importers


Changes to the Import Code of a ScriptedImporter which alters the structure of the Imported Object, even when there are no changes to the Original Source, result in changes to the associated .meta file. This results in .meta files changing for Users when they get new code even though they didn't change any of their content, which
confuses and concerns content creators. The contents of a .meta file are obtuse enough that most content creators (or even most engineers) are not able to determine if the changes are intentional or not. However, content creators can't just ignore .meta files, because they sometimes contain importer settings that are important for them (e.g. .fbx importers).

To reproduce:

1. Download attached project and open in Unity
2. Copy the file Assets / Data / outside of the Unity Assets directory
3. In the Editor, click the Repro > Alternate menu item (it won't change appearance)
4. In the Project window, select Assets / Data / Test, right-click, and select Reimport
5. Check Assets / Data / differences with the copy You made in the prior step

Expected results: Both .meta files are identical
Actual results: Both .meta files are different

- This issue appears on both Windows and macOS
- On Unity 2019.* the process around Custom Import Script has changed but it wasn’t a bugfix, rather a “New Feature”
- On Unity 2018.1.0b7 and lower .meta files had another structure, so there was no Repro
- Project and .meta files differences are in the Comments section

Reproduced on Unity 2018.1.0b8, 2018.1.9f2, 2018.2.21f1 and 2018.3.14f1

  1. Response avatar

    Resolution Note:

    FileIDs are stored in the .meta file for versions of Unity up to and including 2018.3. Starting with Unity 2019.1, fileIDs are generated deterministically by hashing the sub-asset name and are no longer stored in the .meta file.

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