Why does Write-Host break EndInvoke() result collection from RunspacePool?

Question

If I invoke a simple script, such as 'childScript.ps1':

[CmdletBinding()]
param (
    [Parameter(ValueFromPipeline=$true)]
    $object
)

Write-Host $object

Using BeginInvoke, and ending with EndIvoke(), the write-host output is not available in the result.

E.g:

$childScript = get-content ".\childScript.ps1"
$myObject = "Example string"
$MaxThreads = 5
$RunspacePool = [runspacefactory]::CreateRunspacePool(1, $MaxThreads)
$RunspacePool.Open()
$asyncJob = [PowerShell]::Create().AddScript($childScript).AddParameter('object', $myObject)
$asyncJob.RunspacePool = $HostRunspacePool
$asyncJob.BeginInvoke()
$result = $asyncJob.EndInvoke($job.AsyncHandle) #will be empty

However, if i modify the childScript to be:

[CmdletBinding()]
param (
    [Parameter(ValueFromPipeline=$true)]
    $object
)

Write-Output $object

The result would be "Example string".

If i mix write-host and write-output, the result would always be empty. Is there good documentation on why this occurs?

How can i force only specific output to be captured in the result, even if we use write-xxx in addition to write-output?

Answer 1

Using BeginInvoke(), and ending with EndInvoke(), the Write-Host output is not available in the result.

That is correct, because EndInvoke() only returns the actual output from your runspace:

The output buffer created to hold the results of the asynchronous invoke, or null if the caller provided their own buffer.

---

Side note, by "if the caller provided their own buffer", they mean if you're using one of the *Invoke<TInput, TOutput>(...) overloads, i.e.:

$mybuffer = [System.Management.Automation.PSDataCollection[psobject]]::new()
$ps = [powershell]::Create().AddScript({ 'hello' })
$ps.EndInvoke($ps.BeginInvoke(
    [System.Management.Automation.PSDataCollection[psobject]] $null,
    $mybuffer)) # No output from here

$mybuffer # Output is in this custom buffer

---

If you want the output generated by Write-Host, you need to check your instances Streams.Information property:

$ps = [powershell]::Create().
    AddScript({ Write-Host 'hello world' })
$ps.EndInvoke($ps.BeginInvoke())

$ps.Streams.Information # => `hello world`

Do note that Write-Host sends its output to the Information stream as opposed to Write-Output which sends it to Standard output (stdout), referred as Success stream by PowerShell.