Start-Process / System.Diagnostics.Process ExitCode为$ null，带有-NoNewWindow

Question

The Powershell cmdlet Start-Process is acting weirdly:

When I launch another console process and I specify -NoNewWindow , the ExitCode property (an int !) is null.

Here's a test: Same with something else besides cmd . This test was on a Win10 with PS5, it is also the same with Win7 and PS5:

PS C:\Users\Martin> cmd.exe /Cver

Microsoft Windows [Version 10.0.15063]
PS C:\Users\Martin> $PSVersionTable

Name                           Value
----                           -----
PSVersion                      5.1.15063.296
PSEdition                      Desktop
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}
BuildVersion                   10.0.15063.296
CLRVersion                     4.0.30319.42000
WSManStackVersion              3.0
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1


PS C:\Users\Martin> $pNewWindow = Start-Process -FilePath "cmd.exe" -ArgumentList '/C"exit 42"' -PassThru
PS C:\Users\Martin> $pNewWindow.WaitForExit()
PS C:\Users\Martin> $pNoNewWindow.HasExited
True
PS C:\Users\Martin> $pNewWindow.ExitCode
42
PS C:\Users\Martin> $pNoNewWindow = Start-Process -FilePath "cmd.exe" -ArgumentList '/C"exit 42"' -PassThru -NoNewWindow

PS C:\Users\Martin> $pNoNewWindow.WaitForExit()
PS C:\Users\Martin> $pNoNewWindow.HasExited
True
PS C:\Users\Martin> $pNoNewWindow.ExitCode
PS C:\Users\Martin> $pNoNewWindow.ExitCode -eq $null
True
PS C:\Users\Martin> $pNoNewWindow | Get-Member | ? {$_.Name -imatch "exit"}


   TypeName: System.Diagnostics.Process

Name        MemberType Definition
----        ---------- ----------
Exited      Event      System.EventHandler Exited(System.Object, System.EventArgs)
WaitForExit Method     bool WaitForExit(int milliseconds), void WaitForExit()
ExitCode    Property   int ExitCode {get;}
ExitTime    Property   datetime ExitTime {get;}
HasExited   Property   bool HasExited {get;}


PS C:\Users\Martin>

... So, the Property is there, but it's null , even though it's an int ?

Answer 1

Answer from linked question 's answer's / comment:

had to do was cache the process handle. As soon as I did that, $process.ExitCode worked correctly. If I didn't cache the process handle, $process.ExitCode was null.

and indeed, for a process that doesn't immediately terminate (unlike the cmd.exe in the example), the workaround works:

$proc = Start-Process -NoNewWindow -PassThru ...
$handle = $proc.Handle # cache proc.Handle https://stackoverflow.com/a/23797762/1479211
$proc.WaitForExit()
$proc.ExitCode ... will be set

A user added an explanation in the comments :

This is a quirk of the implementation of the .NET Process object. The implementation of the ExitCode property first checks if the process has exited. For some reason, the code that performs that check not only looks at the HasExited property but also verifies that the proces handle is present in the proces object and throws an exception if it is not . PowerShell intercepts that exception and returns null.

Accessing the Handle property causes the process object to retrieve the process handle and store it internally . Once the handle is stored in the process object, the ExitCode property works as expected.

Since Start-Process cannot launch a process suspended, and the handle can only be obtained as long as the process is running (it appears), the usage is a bit brittle for short-running processes.

Answer 2

We encountered a similar issue in our Environment. While executing the following command:

Start-Process -FilePath C:\Windows\system32\reg.exe -PassThru -Wait -NoNewWindow

or

Start-Process -FilePath C:\Windows\system32\reg.exe -PassThru -Wait -WindowStyle Hidden

As stated by Martin Ba, it seems like the started Process (reg.exe) has stopped before Start-Process was able to get a Handle from the Process.

Using -Wait or .WaitForExit() does not make a difference because both Methods check if the process has exited. If it did Exit, PowerShell won't be able to get the necessary handle for the Process. The Handle is necessary because it holds the ExitCode of the started Process.

The following error is thrown, if the Process has already exited:

System.Management.Automation.CmdletInvocationException: Cannot process request because the process (<ProcessIdHere>) has exited. ---> 
System.InvalidOperationException: Cannot process request because the process (<ProcessIdHere>) has exited.
  at System.Diagnostics.Process.GetProcessHandle(Int32 access, Boolean throwIfExited) 
  at System.Diagnostics.Process.OpenProcessHandle(Int32 access) 
  at System.Diagnostics.Process.get_Handle()

Using -WindowStyle Hidden instead of -NoNewWindow may reduce how often the error occurs. This is because -WindowStyle Hidden is creating a new Shell (--> more Overhead), whereas -NoNewWindow uses the current Shell.

We created a UserVoice entry for this issue. Maybe the PowerShell Team is able to work around this behavior: https://windowsserver.uservoice.com/forums/301869-powershell/suggestions/35737357--bug-start-process-might-not-return-handle-exitco