How to make decision to run a step in extended template based on value of System.PullRequest.TargetBranch & System.PullRequest.SourceBranch

Question

I have three level nested Azure DevOps yaml templates. In my second level nested template I want to conditionally call the third level template based on the target branch for pull request.

For e.g. level3@templates should only be called if pull request target branch is Main.

I know System.PullRequestTargetBranch is not available in templates. I am also familiar with

https://learn.microsoft.com/en-us/azure/devops/pipelines/process/variables?view=azure-devops&tabs=yaml%2Cbatch#runtime-expression-syntax

and limitations pre-defined variables have with nested templates

https://learn.microsoft.com/en-us/azure/devops/pipelines/build/variables?view=azure-devops&tabs=yaml#system-variables-devops-services

I am hoping someone will be able to suggest some workaround for getting around this problem.

Will appreciate all the help.

My yaml is structured as below

main.yaml

resources:
  repositories:
    - repository: templates
      type: git
      name: MyProj/templates
      ref: refs/heads/main

extends:  
  template: extends/level1.yaml@templates
  parameters:     
    MY_STAGES:
      BUILD:
        displayName: "Build"
      L1Config:
          config1: "config1"

Level1.yaml

parameters:
  - name: MY_STAGES
    type: object
    default: {}
  - name: L1Config
    type: object
    default: {}
  
    
variables:  
  - name: var1
    value: "value1"
  - name: var2    
    value: "val2"

stages:
  - ${{ each MY_STAGE in parameters.MY_STAGES }}:
      - stage: ${{ MY_STAGE.key }}
        displayName: "Build"        
        jobs:
          - job: build_dev
            displayName: Build
            steps:
              - template: /jobs/level2@templates
                parameters:
                  L2Config: ${{ MY_STAGE.value.L1Config }}        

level2.yaml

parameters:
  - name: L2Config
    type: object
    default: {}
        
steps:
  - pwsh: Write-Host "Hello World!"   
  
  # This is the template I want to call conditionally, based on the PullRequest Target
  - ${{ if eq(variable['System.PullRequest.TargetBranch'], 'refs/heads/main') }}: 
    # I know above syntax does not work, I have just written it down as an example of what I wish had worked. 
    - template: /steps/level3@templates
      parameters:
        param1: value1

Answer 1

The System.PullRequestBranch information can be used in Task Conditions. So in effort to keep things simple, I have moved the check for PR Target branch to the task condition of task which needed this information.