Question

EDIT: The sample code is far off from the original goal, but there’s a legitimate answer to the question that I actually posed, so I’ve edited the question and title.

I am writing a multithread PowerShell script with a WinForms GUI and would like to update the GUI from one of the other threads in the script. I’m having no luck doing this at all. In fact, I can’t even get BeginInvoke to work.

Here’s a very stripped down example. This is sample code, not production, so it’s not pretty, but it’s replicating the issue:

$script:SynchronizedUIHash = [Hashtable]::Synchronized(@{});

[reflection.assembly]::LoadWithPartialName( "System.Windows.Forms")
$form= New-Object Windows.Forms.Form
$label = New-Object Windows.Forms.Label
$label.Text = 'original'
$script:SynchronizedUIHash.label = $label

$form.Controls.add($label)
$form.show()
[System.Windows.Forms.Application]::DoEvents()

Read-Host

$label.BeginInvoke(
    [Action[string]] {
        param($Message)
        [Console]::Beep(500,300)
        $l = $SynchronizedUIHash.label
        $l.Text = $Message
        [System.Windows.Forms.Application]::DoEvents()
    },
    'changed'
)

Read-Host

$form.close()

I don’t even hear the console beep, so it’s as if the BeginInvoke code isn’t being executed at all.

If I throw in a [System.Windows.Forms.Application]::DoEvents() after the Read-Host then I see the change so it seems to be a timing issue, but I don’t want to go into a spin loop or something silly like that – I want to fire and forget the BeginInvoke and have the results visible as appropriate. I see other answers like the one to PowerShell: Job Event Action with Form not executed but that needs to be in a check, sleep, check loop.

There’s got to be a simpler way that I’m missing… what is it?

What is the right solution here?

EDIT: Keith’s comment made me realize I missed a key point going from production to the stripped sample code – launching a job is indeed multi-process, but the sample code isn’t, so the sample doesn’t reflect real life, and I’ll have to rethink the whole thing. That all said – the sample code doesn’t use jobs, and it doesn’t work as I expect either, so I’d like to understand why, even if that doesn’t help me in this specific case.

Answer 1

You can’t really have a Console and a Windows run on the same thread. I suggest you create another thread to host the window, like this (It’s a C# console app, but it should be easy to translate):

class Program
{
    static void Main(string[] args)
    {
        Form form = new Form();
        Label label = new Label();
        label.Text = "original";
        form.Controls.Add(label);

        // run this form on its own thread, so it can work properly
        Thread t = new Thread((state) => Application.Run((Form)state));
        t.Start(form);

        Console.ReadLine();

        // note you *could* do label.Text = "other" here, but BeginInvoke is always recommended
        label.BeginInvoke((Action)delegate() { label.Text = "other"; });

        Console.ReadLine();
        Application.Exit();
    }
}

EDIT: It took me a while to cook a Powershell version, but here it is. The trick is: you just can’t create a standard .NET thread from the Powershell environement. Period. As soon as you try to do it, you will crash Powershell. The underlying error that causes the crash is this:

System.Management.Automation.PSInvalidOperationException: There is no
Runspace available to run scripts in this thread. You can provide one
in the DefaultRunspace property of the
System.Management.Automation.Runspaces.Runspace type

And, you can’t really try to set this DefaultRunspace variable from another thread because it’s marked threadstatic (one copy per thread)! Now, if you google for multi-threading Powershell, you will find many articles about Powershell Jobs. But these allow multi-tasking, but it’s not really multi-threading, it’s more heavyweight.

So… turns out the solution is really to use Powershell features, that is: Powershell runspace. Here it is:

[reflection.assembly]::LoadWithPartialName( "System.Windows.Forms")
$form = New-Object System.Windows.Forms.Form
$label = New-Object System.Windows.Forms.Label
$label.Text = 'original'
$form.Controls.add($label)

$ps = [powershell]::create()
$ps.AddScript(
     {
     [System.Windows.Forms.Application]::Run($form)
     })
$ps.Runspace.SessionStateProxy.SetVariable("form", $form)
$ps.BeginInvoke()

Read-Host

$label.BeginInvoke(
    [Action[string]] {
        param($Message)
        $label.Text = $Message
    },
    'changed'
)

Read-Host

[System.Windows.Forms.Application]::Exit()
$ps.Dispose()