Discussion: Defaults for Thread.ApartmentState

This is to discuss and potentially document current behaviour (that is inconsistent with Windows PowerShell). Maybe some porting work was forgotten since the APIs were initially not available in .Net Core 1.

When creating a new PowerShell instance using [Management.Automation.PowerShell]::Create() (or when creating a new runspace using [runspacefactory]::CreateRunspace()), then PowerShell Core and Windows PowerShell are both in MTA mode. This seems to be reasonable.

When being in the shell of PowerShell Core, the default ApartmentState is also MTA, which seems OK as well at first. However, Windows PowerShell is in STA mode by default and powershell.exe even has an MTA and STA switch, which PowerShell Core does not have.

My questions:

• Why was STA chosen to be the default when being in a shell or ConsoleHost of Windows PowerShell? If there was a reason, we should reconsider it for PowerShell Core. I should add though that this inconsistency could lead to a situation that is difficult to debug (we had a case where execution in the shell was fine but when using our PowerShell runner that uses the PowerShell APIs, we were experiencing threading issues and scratching our heads for some time what the reason could be until we found out about the inconsistency in Windows PowerShell)
• Does pwsh need an STA command line option? In our case we can workaround it by setting it ourselves by setting it in the InitialSessionState that we then pass to PowerShell.Create()

[BrucePay]

And now that XAML for .NET Core 3 has been announced, we should make sure that we at least have the option to run STA (and preferably make it the default.)

[BrucePay]

PowerShell V1 was MTA because that was the .NET default. We didn’t really think about it much and just decided to go with the default. It turned out that STA was preferred for WinForms and absolutely required for XAML. (In practice, it seems that a fair number of people do build GUIs in PowerShell. Sapien even has a WinForms designer for PowerShell.) Since the ISE is a XAML application, it had to be STA. For consistency in experience we decided to also change the console host to be STA. But there might have been some things that required MTA so we gave you the option to start the console host with either MTA or STA. Likewise in the API, if you use the default behaviour of creating a new thread for each command, you can run individual commands either STA or MTA.