Optional job templates in YAML Pipelines - azure-devops

Question

Is it possible to optionally include templates based on some kind of template expression? Specifically, I want my top-level definition in azure-pipelines.yml to call out which build job templates to use in an included stage template:
azure-pipelines.yml :
stages:
- template: generic-build-stage.yml # Template reference
parameters:
# Example of optional build templates to use
buildTypes: [SpecificBuildJobs1, SpecificBuildJobs3, SpecificBuildJobs4]
generic-build-stage.yml :
parameters:
buildTypes: ???
stages:
- stage: generic_build
jobs:
${{ }} # ???? What goes here to include the appropriate templates
- template: ???
The template expression above would ideally expand to this:
jobs:
- template: specific-build-jobs1.yml
- template: specific-build-jobs3.yml
- template: specific-build-jobs4.yml
Edit: The "Iterative insertion" example in the docs seems to suggest that some form of dynamic, parse-time insertion is possible.

Answer 1

The following method worked to allow a top-level pipeline definition to consume a variable number of job sets at a lower level.
azure-pipelines.yml :
stages:
- template: generic-build-stage.yml # Template reference
parameters:
# Example of optional build templates to use
buildTypes: [SpecificBuildJobs1, SpecificBuildJobs3, SpecificBuildJobs4]
generic-build-stage.yml :
parameters:
buildTypes: [MissingBuildType] # Use this if buildTypes is not provided
stages:
- stage: build_stage
jobs:
# Note: VS Code extension for Pipelines (1.1574.4) will
# say this is an "Unexpected property", but this works in ADO
- ${{ if containsValue(parameters.buildTypes, 'MissingBuildType') }}:
- template: build-stage-null.yml
- ${{ if containsValue(parameters.buildTypes, 'SpecificBuildJobs1') }}:
- template: specific-build-jobs1.yml
- ${{ if containsValue(parameters.buildTypes, 'SpecificBuildJobs2') }}:
- template: specific-build-jobs2.yml
- ${{ if containsValue(parameters.buildTypes, 'SpecificBuildJobs3') }}:
- template: specific-build-jobs3.yml
- ${{ if containsValue(parameters.buildTypes, 'SpecificBuildJobs4') }}:
- template: specific-build-jobs4.yml

Answer 2

It seems to be impossible, for the template reference is resolved at parse time.
You may have to set multiple templates for main pipeline, and set the value for buildTypes as the specific template name for job templates, and in generic-build-stage.yml use - template:${{parameters.buildTypes}}.yml to call corresponding job template;
Azure-pipelines.yml:
stages:
- template: generic-build-stage.yml
parameters:
buildTypes:specific-build-jobs1
- template: generic-build-stage.yml
parameters:
buildTypes:specific-build-jobs3
generic-build-stage.yml
parameters:
buildTypes: ""
stages:
- stage: generic_build
jobs:
- template: ${{parameters.buildTypes}}.yml