System.Management.Automation.PowerShell returns warning messages as error messages

Steps to reproduce

Expected behavior

 PowerShell redirects warning messages to warning streams

Actual behavior

Directly invoke cli: invoke through powershell:

It turns out PowerShell redirects warning messages to error streams

Environment data

Name Value

---

PSVersion 7.0.0 PSEdition Core GitCommitId 7.0.0 OS Microsoft Windows 10.0.19042 Platform Win32NT PSCompatibleVersions {1.0, 2.0, 3.0, 4.0…} PSRemotingProtocolVersion 2.3 SerializationVersion 1.1.0.1 WSManStackVersion 3.0

[jborean93]

That part doesn’t make much sense to me, maybe having errorOutput be null causes issues trying to do += in your ErrorDataRecieved event handler.

Regardless here is a reproducer for all this behaviour by using something I know for sure writes to stdout/stderr:

$proc = [Diagnostics.Process]::new()
$proc.StartInfo.FileName = 'powershell.exe'
$proc.StartInfo.Arguments = "-Command [Console]::Error.WriteLine('stderr'); [Console]::Out.WriteLine('stdout')"
$proc.StartInfo.UseShellExecute = $false
$proc.StartInfo.RedirectStandardError = $true
$proc.StartInfo.RedirectStandardOutput = $true

$proc.Start()
$proc.WaitForExit()

$stdout = $proc.StandardOutput.ReadToEnd()
$stderr = $proc.StandardError.ReadToEnd()

"STDOUT: {0}`nSTDERR: {1}`nRC: {2}" -f $stdout, $stderr, $proc.Exitcode

You can see that running this same command as a PowerShell script writes the stderr to the error stream

$ps = [PowerShell]::Create()
$stdout = $ps.AddScript('powershell.exe -Command "[Console]::Error.WriteLine(''stderr''); [Console]::Out.WriteLine(''stdout'')"').Invoke()[0]

$stderr = $ps.Streams.Error[0]

"OUTPUT: {0}`nERROR: {1}`nHAD_ERRORS: {2}" -f $stdout, $stderr, $ps.HadErrors

I unfortunately don’t have bicep installed or an environment set up to test it out but using these examples show how stderr data is sent to the error stream.

[BethanyZhou]

Thank you all @jborean93 , @mklement0 for explanation!!! Will communicate with bicep team and close this issue.