When you’re starting a project using a technology which is still in alpha/beta status, sometimes things break.
I experienced this with .NET Core. I created a new project with it in the beginning of 2016, using .NET Core 1.0.0-rc1-update1, which had the original json-based project files (global.json and project.json).

I needed to show localized messages, so I as soon as .NET Core supported it, I created a resource file in my project.
As we know today, there were still lots of moving targets and breaking changes in the first .NET Core releases, and some people felt that RC1/RC2 were more like alpha/beta, even though they were promised as production-ready.

So when I later converted my project to Visual Studio 2017 and .NET Core 1.0 RTM (at least I think it was 1.0 RTM), the json-based project files auto-changed into .csproj files, and the automatic generation of the designer file stopped working.
From then, whenever I changed something in Resource.resx and saved it, Resource.Designer.cs was not updated with the changes.

Only recently, after upgrading to .NET Core 2.0, I investigated this issue by creating a new project, adding a resource file and looking at diffs.

I found out that adding a resource file also adds the following to the project’s .csproj file:

<ItemGroup>
  <Compile Update="Resource.Designer.cs">
    <DesignTime>True</DesignTime>
    <AutoGen>True</AutoGen>
    <DependentUpon>Resource.resx</DependentUpon>
  </Compile>
</ItemGroup>

<ItemGroup>
  <EmbeddedResource Update="Resource.resx">
    <Generator>ResXFileCodeGenerator</Generator>
    <LastGenOutput>Resource.Designer.cs</LastGenOutput>
  </EmbeddedResource>
</ItemGroup>

This was missing in my actual project…so as soon as I changed it, Resource.Designer.cs was re-generated again.

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I was still wondering why it stopped working in the first place, so I (again) looked at the past commits where I updated the project to newer .NET Core versions.

Apparently, the resource files were never mentioned in the old json-based project files. So I suspect that the first .NET Core versions auto-detected the presence of a Resource.resx file and auto-generated the Resource.Designer.cs when building the project.

When I updated my project to .NET Core 1.0, the upgrade wizard automatically replaced the .json files by .csproj files, but it didn’t detect the presence of the resource files. IMO, this should have happened and the code above should have been added to the .csproj automatically.