How to create a comment in Azure DevOps PR in case of build failure?

I have a custom build step that fails under certain conditions during my Pull Request build, within Azure DevOps.

I would like to extend it further by raising a PR comment, similar to this sort of thing in GitHub: https://developer.github.com/v3/issues/comments/#create-a-comment

I don't have code samples to add here as I could not find useful examples to build upon. I use PowerShell for my custom build step - how do I achieve this when running a PR build of my branch?

I can help with an example. There is a bunch of value in posting custom messages\status to PRs from your pipelines.

First things first, make sure your build service has permissions to contribute to pull requests in your repository. (Go to Project Settings -> Repositories -> Security. The user is named <projectname> Build service)

Then you want to add a conditional PowerShell step. This one is just based on it being a PR build, but you might want to add a depends on failure for the previous step, based on your workflow.

- task: PowerShell@2
  condition: eq(variables['Build.Reason'], 'PullRequest')
  displayName: Post Message to PR
  env:
    SYSTEM_ACCESSTOKEN: $(System.AccessToken)  
  inputs:
      targetType: filePath
      filePath: PostToPR.ps1

So the basic workflow is:

• Build the Markdown Message
• Build the JSON Body
• Post the Message to the PR

PostToPR.ps1

#Going to create the comment in an Active state, assuming it needs to be resolved
#See https://learn.microsoft.com/en-us/dotnet/api/microsoft.teamfoundation.sourcecontrol.webapi.commentthreadstatus?view=azure-devops-dotnet
$StatusCode = 1 

$Stuff = $env:Build_Repository_Name
$Things = "Other things you might want in the message"

#Build Up a Markdown Message to 
$Markdown = @"
## Markdown Message here
|Column0 |Column1|
|--------|---------|
|$Stuff|$Things|  
"@

#Build the JSON body up
$body = @"
{
    "comments": [
      {
        "parentCommentId": 0,
        "content": "$Markdown",
        "commentType": 1
      }
    ],
    "status": $StatusCode 
  }
"@

Write-Debug $Body
#Post the message to the Pull Request
#https://learn.microsoft.com/en-us/rest/api/azure/devops/git/pull%20request%20threads?view=azure-devops-rest-5.1
try {
    $url = "$($env:SYSTEM_TEAMFOUNDATIONCOLLECTIONURI)$env:SYSTEM_TEAMPROJECTID/_apis/git/repositories/$($env:Build_Repository_Name)/pullRequests/$($env:System_PullRequest_PullRequestId)/threads?api-version=5.1"
    Write-Host "URL: $url"
    $response = Invoke-RestMethod -Uri $url -Method POST -Headers @{Authorization = "Bearer $env:SYSTEM_ACCESSTOKEN"} -Body $Body -ContentType application/json
  if ($response -ne $Null) {
    Write-Host "*******************Bingo*********************************"
  }
}
catch {
  Write-Error $_
  Write-Error $_.Exception.Message
}

And you end up with a nice markdown table with custom status information in your PR!