"The calling thread must be STA, because many UI components require this" error when creating a WPF pop-up Window in thread

Question

I have a WPF application in which a thread checks some value. In certain cases, I show a pop-up Window in order to display a message. When I create this pop-up window in the thread, an exception is thrown by the pop-up window's constructor:

"The calling thread must be STA, because many UI components require this."

How do I resolve this error?

This is my code for creating the pop-up window:

// using System.Threading;
// using System.Windows.Threading;
Thread Messagethread = new Thread(new ThreadStart(delegate()
{
    DispatcherOperation DispacherOP = 
        frmMassenger.Dispatcher.BeginInvoke(
            DispatcherPriority.Normal,
            new Action(delegate()
            {
                frmMassenger.Show();
            }));
}));
Messagethread.Start();

I found Problem Root. I have a timer which in the each timer.tick , check for new message`s and then show each new message. Timer thread make my work so hard. — Rev, Apr 18 '10 at 10:00
Off-topic: In WPF, windows are called just that: "windows"; not "forms". That latter term is used in another UI framework, Windows Forms. — stakx - no longer contributing, Aug 20 '13 at 07:48
Possible duplicate of [The calling thread must be STA, because many UI components require this](http://stackoverflow.com/questions/2329978/the-calling-thread-must-be-sta-because-many-ui-components-require-this) — naXa stands with Ukraine, Sep 17 '16 at 20:27

score 16 · Answer 1 · edited Jul 18 '18 at 00:00

16

For the thread that you're trying to start the GUI element in, you need to set the apartment state of the thread to STA BEFORE you start it.

Example:

myThread.SetApartmentState(ApartmentState.STA);
myThread.Start();

edited Jul 18 '18 at 00:00

archer

262
2
15

answered Apr 17 '10 at 04:21

Brian R. Bondy

339,232
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@Rev: Let me know, if it doesn't get past your error let me know the error message and I'll provide the extra info you need. I think that's all you'll need though. – Brian R. Bondy Apr 17 '10 at 16:58
thx for help but that doesn't work. this problem related to Wpf and Threading.I think We Should use Dispatcher for solving that. Regards Rev – Rev Apr 18 '10 at 04:44
2

@Rev: Probably you have 2 issues. The threading STA and the dispatcher. If you want help with the dispatcher let me know and I can show you how. – Brian R. Bondy Apr 18 '10 at 04:47
I think dispatcher can help me. ;) – Rev Apr 18 '10 at 09:56

score 10 · Accepted Answer · edited Apr 29 '20 at 11:40

Absolutely Dispatcher is only way to do something (in specific Thread) when we work with multi-threading in WPF!

But for work with Dispatcher we must know 2 things:

Too many way to use Dispatcher like Dispatcher_Operation , [window.dispatcher] or etc.
We must call dispatcher in the main thread of app (that thread is must be STA thread)

So for example: if we want show other window[wpf] in another thread, we can use this code:

Frmexample frmexample = new Frmexample();
            frmexample .Dispatcher.BeginInvoke //Updated the variable name
                (System.Windows.Threading.DispatcherPriority.Normal,
                (Action)(() =>
                {
                    frmexample.Show();
                    //---or do any thing you want with that form
                }
                ));

Tip: Remember - we can't access any fields or properties from out dispatcher, so use that wisely

"The calling thread must be STA, because many UI components require this" error when creating a WPF pop-up Window in thread

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