Introduction

The recent addition to Azure DevOps of the StringList parameter type can be really useful to dynamically create parallel stages or jobs in an Azure DevOps YAML pipeline.

A StringList parameter can be used to present a list of values to the user queuing a pipeline run, thus allowing the selection of one or more values that can be accessed using a YAML expression loop.

This can be combined with YAML templates, where the StringList is passed into a template as an Object. Noting that you can’t use the StringList type for a parameter type in the template definition. However, this is not a problem as there is a dynamic conversion from StringList to Object e.g.

# Azure-Pipeline.yml
parameters:
  - name: environments
    type: stringList
    default: 
      - dev
    values:
      - dev
      - test
      - prod

trigger: none

extends:
  template: template.yml
  parameters:
    environments: ${{parameters.environments }}

# template.yml
parameters:
  - name: environments
    type: object

stages:
  - stage: Deploy
    pool:
      vmImage: ubuntu-latest
    jobs:
      - job: DeployJob
        steps:
          - checkout: none
          - ${{ each value in parameters.environments }}:
            - script: |
                echo "Selected environments: ${{ value }}"

The Problem

Whilst implementing this new parameter type on an existing pipeline we got a very strange set of error messages ‘Unexpected a mapping’, ‘Unexpected at least one-value pair in the mapping’ and ‘Unexpected state while attempting t read the mapping end state’

After much experimentation we found the issue was the indenting of a steps block later in the YAML. When the new loop expression was added only part of the jobs block was correctly indented.

The indenting was

  jobs:
  - ${{ each environment in parameters.Environments }}:
    - job: build_${{ environment }}
    steps:
      ...

When it should have been

  jobs:
  - ${{ each environment in parameters.Environments }}:
    - job: build_${{ environment }}
      steps:
        ...

So as usual when you get an unexpected and unclear error in Azure DevOps YAML, assume the problem is due to whitespace.