Upgrade to 2019.2.13 causes Namespace name `EventSystems' could not be found

Question

I have upgraded from 2018.3.14f to 2019.2.13 and I started having really strange errors.

I am now having those two errors:

The type or namespace name EventSystems' does not exist in the namespaceUnityEngine'. Are you missing an assembly reference?

and

The type or namespace name UI' does not exist in the namespaceUnityEngine'. Are you missing an assembly reference?

This is happening only to the project which I upgraded from 2018.3.14 to 2019.2.13.

If I create a new blank project with 2019 there is no issue. The namespaces are found plus when I go to New2019Project/Library/ScriptAssemblies/ I can see the following files:

Then If I go to my upgraded old project in this dir OldUpgradedTo2019Project/Library/ScriptAssemblies/ the folder is having only those files:

As you can see I don't have those files in my updated project. If I try to copy the ones from the new working one into my old upgraded, at the time I start the Unity for the upgraded old project it deletes them and the problem stays.

Is there any way I can fix this annoying problem?

Answer 1

I update my Visual Studio 2017 to Visual Studio 2019, then everything works.

Hope this works for you.

Answer 2

What fixed this issue for me was to update to the latest 'Visual Studio Code Editor' in the Unity Package Manager. To do this, go to window -> package manager -> set 'All Packages' to 'In Project', select the Visual Studio Code Editor and update.

Answer 3

I was experiencing the same issue, but my problem was that I had multiple Google.VersionHandler.dll files in separate folders within Assets. As a result, the .asmdef files were not loading. Here's how I fixed it:

1. Close Unity and Visual Studio
2. Remove the extra copy of Google.VersionHandler.dll
3. Delete all .csproj and .sln files
4. Delete Library, obj, and Temp directories
5. Reopen Unity
6. Click Assets > Open C# Project to regenerate .csproj and .sln files

Hope this works for you!