Evaluate an expression at template expansion time - azure-devops

Question

I have an azure pipeline template that takes parameters, and I'd like to set one of the parameters based on an expression:
- template: templates/mytemplate.yml
parameters:
TestBackCompat: eq('${{ parameters.CIBuildId }}', 'MasterLatestBuildId')
CreateNupkg: ${{ parameters.CreateNupkg }}
As written, it doesn't work, because the expression isn't evaluated until runtime.
Is there a way to evaluate the expression at compile-time? Simple variable replacement (e.g., the usage of CreateNuPkg in the script above) works OK.

Answer 1

From this official document:
https://learn.microsoft.com/en-us/azure/devops/pipelines/process/templates?view=azure-devops#template-expressions
Template expressions can expand template parameters, and also
variables. You can use parameters to influence how a template is
expanded. The object works like the variables object in an expression.
How the variable object works:
https://learn.microsoft.com/en-us/azure/devops/pipelines/process/variables?view=azure-devops&tabs=yaml%2Cbatch#runtime-expression-syntax
So the usage in your situation should be like this:
give_parameters.yml
trigger:
- none
parameters:
- name: CIBuildId
type: string
default: 'MasterLatestBuildId'
extends:
template: using_parameters.yml
parameters:
TestBackCompat: ${{ eq(parameters.CIBuildId, 'MasterLatestBuildId')}}
using_parameters.yml
parameters:
- name: CIBuildId
type: string
default: 'x'
- name: TestBackCompat
type: string
default: 'x'
variables:
- name: test1
value: ${{ parameters.TestBackCompat}}
- name: test2
value: ${{ parameters.CIBuildId}}
steps:
- task: PowerShell#2
inputs:
targetType: 'inline'
script: |
Write-Host $(test1)
Write-Host $(test2)
Write-Host ${{ parameters.TestBackCompat}}
Result: