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Fixed in 2018.3.X
Newly created script is not attached to a prefab if the script was created in the prefab edit mode
How to reproduce:
1. Open the attached Unity project
2. Open "SampleScene"
3. Select Sphere's prefab in the scene and go to its edit mode
4. Press "Add Component"
5. Create and add a new script
Expected result: a newly created script is added to the prefab
Actual result: an error message appears; the script is not added to the prefab (can be seen only after closing the prefab edit mode)
Reproducible with - 2018.3.0a6, 2018.3.0a9
Could not test with - 2017.1.5f1, 2017.2.3p3, 2017.4.9f1, 2018.1.9f1, 2018.2.5f1, 2018.3.0a5 (does not have a prefab edit mode)
"You are trying to replace or create a Prefab from the instance 'Sphere' that contains the script 'test', which does not derive from MonoBehaviour. This is not allowed.
Please change the script to derive from MonoBehaviour or remove it from the GameObject.
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Sep 06, 2022 16:22
This seemed like an incorrect error message. I had a syntax error in a completely different file, which caused this issue. Fixing the error in the unrelated file fixed this issue for me.
Jan 27, 2021 15:48
I am having this issue in 2019.3.14f1 on Linux with a project migrated from 2018.2.21f1.
Jul 08, 2020 10:15
Same problem in Unity 2019.2.17f1. Not resolved.
Jun 20, 2020 12:21
I have the same problem in 2019.3.15f1
Mar 01, 2020 01:48
This continues to happen with the 2019.2.13f1 Unity
Nov 29, 2019 02:30
I have the same problem in 2019.1.0f2
Apr 05, 2019 07:35
i have this problem in unity 2018.3.11f