Using variables expansion to load a template variables file per environment - azure-devops

Question

I'm attempting to create multiple pipelines in Azure DevOps but I would like to reuse the same pipeline YAML file with the differences per environment being loaded from a separate template variables file.
For that purpose I've created two variable files, which are located in the same folder as the pipeline definition:
# vars.dev.yml
variables:
- name: EnvironmentName
value: Development
# vars.prd.yml
variables:
- name: EnvironmentName
value: Production
And the definition of the pipeline is the following:
trigger: none
pr: none
variables:
- name: EnvironmentCode
value: dev
- name: EnvironmentFileName
value: vars.$('EnvironmentCode').yml
stages:
- stage: LoadVariablesPerEnvironment
displayName: Load Variables Per Environment
variables:
- template: $(EnvironmentFileName)
jobs:
- job: ShowcaseLoadedVariables
steps:
- pwsh: Write-Host "Variables have been loaded for the '$ENV:ENVIRONMENTNAME' environment"
displayName: Output Environment Variables
After importing the pipelines using the Azure DevOps UI, I can go to settings of each and set the Environment Code variable to whatever desired environment code:
However I'm always getting the same error when I try to run the pipeline, regardless of the code I fill in the variable value:
So the question here is: Is this kind of variable expansion not supported or is there a different way that I should use to accomplish this?
Thanks!
EDIT
I was able to expand the variables using another method. The new version of the pipeline is as such:
variables:
- name: EnvironmentCode
value: dev
- name: EnvironmentFileName
value: vars.${{ variables.EnvironmentCode }}.yml
stages:
- stage: LoadVariablesPerEnvironment
displayName: Load Variables Per Environment
variables:
- template: ${{ variables.EnvironmentFileName }}
jobs:
- job: ShowcaseLoadedVariables
steps:
- pwsh: Write-Host "Variables have been loaded for the '$ENV:ENVIRONMENTNAME' environment"
displayName: Output Environment Variables
However there is yet the issue of loading different files. I made different attempts and verified the following:
If you give a different environment code using the UI, when running
the pipeline, the value it assumes is still the one that's on the
pipeline definition;
If you remove from the pipeline definition the
default value or the variable entirely the expression
${{variables.EnvironmentCode}} will return an empty string
assuming the filename to be vars..yml which doesn't exist.

Answer 1

Is this kind of variable expansion not supported or is there a
different way that I should use to accomplish this?
If I am not misunderstand, at first, you want to use $() to get the variable you defined using the UI but failed. But later, ${{ }} can give you the value of the variable EnvironmentCode.
In fact, while you change to use ${{ }}, it just accessing the variable you predefined in the YAML files instead of the one you defined with UI. Just see this doc: Variable templates.
For the variable you defined with UI, it can be get and used with the format $()(Note: ${{ }} is the format of get the variables which defined in YAML file). But also, there some things you need to pay attention is for the variables you defined in UI, it can only be get/accessed after the build begin to run, because the variable which defined with UI exists in environment only after the build compiled begin. In one word, they are the agent-scope variable. That's why the value it used is still the one that's on the pipeline definition instead of on the UI.
If you remove from the pipeline definition the default value or the
variable entirely the expression ${{variables.EnvironmentCode}} will
return an empty string assuming the filename to be vars..yml which
doesn't exist.
As the document defined and I mentioned before, ${{}} is format which used to get the value of variable which defined in YAML files rather than the one which defined using UI.
In the steps of job, the variable that defined in the UI or defined in the YAML file can all be get/accessed with the format $(). Still, for the variable which defined in the YAML file can also get with ${{variables.xxxx}}. But at this time, if the variable name which defined in YAML file is same with the one defined with UI, the server can only get the one defined in YAML file.