Trouble with Azure Pipelines in defining stages and branches

Why can’t I do this:

azure-pipeline.yml: extends build-and-deploy.yml@demo

...

parameters:
- name: allowDevDeployment
  displayName: "Allow this build to be deployed to DEV"
  type: boolean
  default: false

- name: selectDeploymentType
  displayName: Select deployment type
  type: string
  default: 0
  values:
    - 0 # (Standard/regular deployment)
    - 1 #(Single branch; master deploys to  DEV -> DEVTEST -> STAGE -> PROD)
    - 2 # (develop to DEV -> Auto PR to master -> DEVTEST -> STAGE -> PROD)


extends:
  template: templates/build-and-deploy.yml@demo
  parameters:
    allowDevDeployment: ${{ parameters.allowDevDeployment }}
    deploymentType: ${{ parameters.selectDeploymentType}}
    ...

In templates/build-and-deploy.yml: Based on what’s selected during pipeline runtime, either 0.yml, 1.yml or 2.yml is called (up till this point it works because I have a pwsh step (before it calls the template) that write-host the parameter that’s selected at runtime, and it outputs the right parameter. The step isn’t included here)

parameters:
  ...
- name: deploymentType
  type: string
  default: none

steps:
  - ${{ if eq(parameters.deploymentType, 0)}}:
    - template: deploymentTypes/0.yml
      parameters:
        allowDevDeployment: ${{ parameters.allowDevDeployment }}
        ...
  - ${{ if eq(parameters.deploymentType, 1)}}:
    - template: deploymentTypes/1.yml
      parameters:
        allowDevDeployment: ${{ parameters.allowDevDeployment }}
        ...
  - ${{ if eq(parameters.deploymentType, 2)}}:
    - template: deploymentTypes/2.yml
      parameters:
        allowDevDeployment: ${{ parameters.allowDevDeployment }}
        ...

and in deploymentTypes/0.yml (or 2.yml or 3.yml): At this point, I get an error that reads "/templates/deploymentTypes/0.yml@demo (Line: 184, Col: 1): Unexpected value ‘stages’"

This file (0.yml or 2.yml or 3.yml) essentially defines what stages must run depending on what branch is built or the pipeline is run from.

parameters:
...
- name: allowDevDeployment
  type: boolean
  default: false
    
stages:
- ${{ if gt(length(parameters.buildJobs), 0) }}:
  - stage: Build
    jobs:
    - ${{ each job in parameters.buildJobs }}:
      - ${{ if startsWith(job.pool.vmImage, 'ubuntu') }}:
        - job: ${{ job.job }}
          pool: ${{ job.pool }}
          ${{ if job.condition }}:
            condition: ${{ job.condition }}
            ...
# Develop, Nightly or Support Branch
- ${{ if or(in(variables['Build.SourceBranchName'], 'develop', 'nightly'), contains(variables['Build.SourceBranch'], '/support/'), eq(parameters.allowDevDeployment, true)) }}:
  - ${{ if gt(length(parameters.devJobs), 0) }}:
    - stage: Dev
      ...

# Develop or Support Branch
- ${{ if or(eq(variables['Build.SourceBranchName'], 'develop'), contains(variables['Build.SourceBranch'], '/support/')) }}:
  - ${{ if gt(length(parameters.devTestJobs), 0) }}:
    - stage: DevTest
      ${{ if ne(parameters.devTestStageLabel, 'none') }}:
        displayName: ${{ parameters.devTestStageLabel }}
        ...

>Solution :

I can reproduce the same error with your yaml sample.

The error is caused by the steps parameter in templates/build-and-deploy.yml, you should change it to stages since it invokes stage level template in 0.yml,1.yml…

templates/build-and-deploy.yml

parameters:
- name: allowDevDeployment
  type: boolean
  default: false

- name: deploymentType
  type: string
  default: none
 
stages:                  # change steps to stages here
  - ${{ if eq(parameters.deploymentType, 0)}}:
    - template: deploymentTypes/0.yml
      parameters:
        allowDevDeployment: ${{ parameters.allowDevDeployment }}

  - ${{ if eq(parameters.deploymentType, 1)}}:
    - template: deploymentTypes/1.yml
      parameters:
        allowDevDeployment: ${{ parameters.allowDevDeployment }}

  - ${{ if eq(parameters.deploymentType, 2)}}:
    - template: deploymentTypes/2.yml
      parameters:
        allowDevDeployment: ${{ parameters.allowDevDeployment }}