Why does PowerShell intermittently not find List[string].Add() method? - powershell

Question

I've got a build script (PowerShell 4 on Windows 2012 R2) that runs NUnit in a background job and returns NUnit's output. This output is collected in a Collections.Generic.List[string].
$nunitJob = Start-Job -ScriptBlock {
param(
[string]
$BinRoot,
[string]
$NUnitConsolePath,
[string[]]
$NUnitParams,
[string]
$Verbose
)
Set-Location -Path $BinRoot
$VerbosePreference = $Verbose
Write-Verbose -Message ('{0} {1}' -f $NUnitConsolePath,($NUnitParams -join ' '))
& $NUnitConsolePath $NUnitParams 2>&1
$LASTEXITCODE
} -ArgumentList $binRoot,$nunitConsolePath,$nunitParams,$VerbosePreference
$nunitJob | Wait-Job -Timeout ($timeoutMinutes * 60) | Out-Null
$jobKilled = $false
if( $nunitJob.JobStateInfo.State -eq [Management.Automation.JobState]::Running )
{
$jobKilled = $true
$errMsg = 'Killing {0} tests: exceeded {1} minute timeout.' -f $assembly.Name,$timeoutMinutes
Write-Error -Message $errMsg
}
$output = New-Object 'Collections.Generic.List[string]'
$nunitJob |
Stop-Job -PassThru |
Receive-Job |
ForEach-Object {
if( -not $_ )
{
[void]$output.Add( '' )
return
}
switch -Regex ( $_ )
{
'^Tests run: (\d+), Errors: (\d+), Failures: (\d+), Inconclusive: (\d+), Time: ([\d\.]+) seconds$'
{
$testsRun = $Matches[1]
$errors = $Matches[2]
$failures = $Matches[3]
$inconclusive = $Matches[4]
$duration = New-Object 'TimeSpan' 0,0,$Matches[5]
break
}
'^ Not run: (\d+), Invalid: (\d+), Ignored: (\d+), Skipped: (\d+)$'
{
$notRun = $Matches[1]
$invalid = $Matches[2]
$ignored = $Matches[3]
$skipped = $Matches[4]
break
}
}
# Error happens here:
[void] $output.Add( $_ )
}
Intermittently, our build will fail with this error:
Cannot find an overload for "Add" and the argument count: "1".
At line:XXXXX char:XXXXX
+ [void] $output.Add( $_ )
+ ~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : NotSpecified: (:) [], MethodException
+ FullyQualifiedErrorId : MethodCountCouldNotFindBest
Any idea why PowerShell would not be able to find List[string]'s Add method?
I've opened a console Window and played around with passing different typed objects to Add without getting an error.
> $o = New-Object 'Collections.Generic.List[string]'
> $o.Add( '' )
> $o.Add( $null )
> $o.Add( 1 )
> $o
1

Answer 1

When you redirect stderr to stdout, you can get System.Management.Automation.ErrorRecords interspersed with strings. Stderr is automatically converted to ErrorRecords while stdout is strings.
So you'll probably want to look to see if the output contains ErrorRecords of interest and if not then filter them out.