Gitlab CI and Xamarin build fails - xamarin

Question

I've created a complete new Xamarin Forms App project in Visual Studio for Mac and added it to a GitLab repository. After that I created a .gitlab-ci.yml file for setting up my CI build. But the problem is that I get error messages:
error MSB4019: The imported project "/usr/lib/mono/xbuild/Xamarin/iOS/Xamarin.iOS.CSharp.targets" was not found. Confirm that the expression in the Import declaration "/usr/lib/mono/xbuild/Xamarin/iOS/Xamarin.iOS.CSharp.targets" is correct, and that the file exists on disk.
This error pops up also for Xamarin.Android.Csharp.targets.
My YML file look like this:
image: mono:latest
stages:
- build
build:
stage: build
before_script:
- msbuild -version
- 'echo BUILDING'
- 'echo NuGet Restore'
- nuget restore 'XamarinFormsTestApp.sln'
script:
- 'echo Cleaning'
- MONO_IOMAP=case msbuild 'XamarinFormsTestApp.sln' $BUILD_VERBOSITY /t:Clean /p:Configuration=Debug
Some help would be appreciated ;)

Answer 1

You will need a mac os host to build Xamarin.iOS application and AFAIK it's not there yet in GitLab. You can find the discussion here and private beta here. For now, I would recommend going your own MacOS Host and registered GitLab runner on that host:
https://docs.gitlab.com/runner/
You can set up the host where you want (VM or Physical device) and install the GitLab runner and Xamarin environment there, tag it and use with the GitLab pipelines as with any other shared runner.

Answer 2

From the comments on your question, it looks like Xamarin isn't available in the mono:latest image, but that's ok because you can create your own docker images to use in Gitlab CI. You will need to have access to a registry, but if you use gitlab.com (opposed to a self-hosted instance) the registry is enabled for all users. You can find more information on that in the docs: https://docs.gitlab.com/ee/user/packages/container_registry/
If you are using self-hosted, the registry is still available (even for free versions) but it has to be enabled by an admin (docs here: https://docs.gitlab.com/ee/administration/packages/container_registry.html).
Another option is to use Docker's own registry, Docker Hub. It doesn't matter what registry you use, but you'll have to have access to one of them so your runners can pull down your image. This is especially true if you're using shared runners that you (or your admins) don't have direct control over. If you can directly control your runners, another option is to build the docker image on all of your runners that need it.
I'm not familiar with Xaramin, but here's how you can create a new Docker image based on mono:latest:
# ./mono-xamarin/Dockerfile
FROM mono:latest # this lets us build off of an existing image rather than starting from scratch. Everything in the mono:latest image will be available in this image
RUN ./install_xamarin.sh # Run whatever you need to in order to install xamarin or anything else you need.
RUN apt-get install git # just an example
Once your Dockerfile is written, you can build it like this:
docker build --file path/to/Dockerfile --tag mono-xamarin:latest
If you build the image on your runners, you can use it immediately like:
# .gitlab-ci.yml
image: mono-xamarin:latest
stages:
- build
...
Otherwise you can now push it to whichever registry you want to use.