Trigger Azure DevOps pipeline only for "main" AND changes in a certain path

I have an Azure DevOps pipeline that should trigger after a merge to main only if changes occured under a certain path.

trigger:
  branches:
    include:
      - main
  paths:
    include:
      - 'Infrastructure/Some.Service.Api.Client/*'

My problem is that this will also trigger on pull request builds which do not originate in main. This pipeline generates a new version of an API client for a service. I only want to create a new version, if the code on main has changed.

How do I configure the trigger correctly?

If the PR trigger is not expected, you can disable it:

1. If you Git repository is on Azure DevOps, disable or remove the Build validation set on the branch policies of main branch.

2. If the Git repository is not on Azure DevOps, such as on GitHub or Bitbucket, you need to do like as below.

   • In the YAML, set "pr: none".

     pr: none
     trigger:
       branches:
         include:
           - main
       paths:
         include:
           - 'Infrastructure/Some.Service.Api.Client/*'
     

   • Disable or remove the pipeline from the checks set on the branch protect policies of main branch.

If the PR trigger is expected, you can use the predefined variable "Build.SourceBranch" to identify whether the pipeline is run for main branch or a PR.

• If the pipeline is triggered for the main branch, its value is "refs/heads/main".
• If the pipeline is triggered by the PR, its value is "refs/pull/<PR-Id>/merge".

So, you use this variable to set a condition for the step which to generate the new version number.

• If value is "refs/heads/main", execute the step.
• If the value is not "refs/heads/main", skip the step.

trigger:
  branches:
    include:
      - main
  paths:
    include:
      - 'Infrastructure/Some.Service.Api.Client/*'

steps:
- ${{ if eq(variables['Build.SourceBranch'], 'refs/heads/main') }}:
  # The step to generate the new version number.
  - task: xxxx
    displayName: 'Generate Version Number'
    . . .

# Some other steps need to run always.
- task: xxxx
. . .