How do I use expressions when deviging variables in Azure DevOps yaml pipelines

When defining a variable in an Azure DevOps .yaml pipeline:

variables:
  - name: environment
    value: "PROD"

How can I use an expression to dynamically set the value of the expression?

I have tried doing it like this (note parameter name/value is not real):

variables:
  ${{ if eq(parameters['PARAM'], 'VALUE') }}
    - name: environment
      value: "PROD"

Expression usage with variables can be achieved in a few ways. In my example, I will define a parameter named envName, and dynamically create a variable named envCode based on the value of envName.

The important part to note about what is wrong in your example is the usage of the - and the spacing. A simple method to know how to format an expression within .yaml is if the statement directly after the expression contains a - at the start, then your expression should also contain the - at the start.

---

Example one:

parameters:
  - name: envName
    type: string
    values:
      - "Production"
      - "Development"

variables:
  - name: envCode
    ${{ if eq(parameters['envName'], 'Production') }}:
      value: "PROD"
    ${{ elseif eq(parameters['envName'], 'Development') }}:
      value: "DEV"
    ${{ else }}:
      value: "INVALID"   

---

Example two:

parameters:
  - name: envName
    type: string
    values:
      - "Production"
      - "Development"

variables:
  ${{ if eq(parameters['envName'], 'Production') }}:
    envCode: "PROD"
  ${{ elseif eq(parameters['envName'], 'Development') }}:
    envCode: "DEV"
  ${{ else }}:
    envCode: "INVALID"

---

Example three:

parameters:
  - name: envName
    type: string
    values:
      - "Production"
      - "Development"

variables:
  - ${{ if eq(parameters['envName'], 'Production') }}:
    - name: envCode
      value: "PROD"
  - ${{ elseif eq(parameters['envName'], 'Development') }}:
    - name: envCode
      value: "DEV"
  - ${{ else }}:
    - name: envCode
      value: "INVALID"

---

Example four: Variable file

parameters:
  - name: envName
    type: string
    values:
      - "Production"
      - "Development"

variables:
  - template: .\environments-vars.yaml
    parameters:
      envName: ${{ parameters.envName}}

The environments-vars.yaml file:

parameters:
  - name: envName

variables:
  ${{ if eq(parameters['envName'], 'Production') }}:
    envCode: "PROD"
  ${{ elseif eq(parameters['envName'], 'Development') }}:
    envCode: "DEV"
  ${{ else }}:
    envCode: "INVALID"