Question about weird behavior referencing a YAML pipeline resource using a variable for the pipeline resource name

I am experiencing weird behavior with YAML variables, parameters, and Azure pipeline resource references. The following shows the original implementation that works compared to my new implementation with a single line change that fails.

Working Implementation

Template A (makes a call to template B):

- template: Templates\TemplateB.yml
  serviceBuildResourceName: resourceName

Template B (uses serviceBuildResourceName param to get pipeline run information):

$projectId = '$(resources.pipeline.${{ parameters.serviceBuildResourceName }}.projectID)'
$pipelineId ='$(resources.pipeline.${{ parameters.serviceBuildResourceName }}.PipelineID)'

Template B goes on to use the values in $projectId and $pipelineId (along with other values not listed here since it is irrelevant) to successfully retrieve information about the a pipeline run from the specific pipeline resource, serviceBuildResourceName. Note that all pipeline resources are correctly defined at the beginning yaml file for the pipeline. In this implementation above, everything works perfectly.

Failing Implementation

Template A (makes a call to template B):

- template: Templates\TemplateB.yml
  serviceBuildResourceName: $(ServiceBuildResourceName)

Template B (uses serviceBuildResourceName param to get pipeline run information):

$projectId = '$(resources.pipeline.${{ parameters.serviceBuildResourceName }}.projectID)'
$pipelineId ='$(resources.pipeline.${{ parameters.serviceBuildResourceName }}.PipelineID)'

Note that the only difference is the following: instead of passing the hard-coded string into the serviceBuildResourceName parameter, I pass in a variable, which has the same value as before, resourceName. The variable is defined in an earlier template as such:

- name: ServiceBuildResourceName
  value: resourceName

I feel it should still work the same, but I know get the following error in my pipeline run:

WARNING: 2023-02-12 15:52:29.5071 Response body: {"$id":"1","innerException":null,"message":"The value is not an integer.
$(resources.pipeline.resourceName.PipelineID)

I know that the variable is being correctly populated since the error message above contains "resourceName" in resources.pipeline.resourceName.PipelineID, as it should.

However, for reasons unknown to me, it now throughs an error. It seems like it doesn't recognize the pipeline resource, and instead recognizes it as a string.

Any help or insight here would be greatly appreciated, thanks!

As far as I can tell, this is because of how predefined variables work in YAML. Since resources.pipeline... is a predefined variable, it gets resolved at compile time. Thus, you can't use run-time defined variables like I am doing. Instead of resolving it as a predefined variable, it will get resolved to be a string at runtime.