"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();

Answer 1

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();

Answer 2

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:

1. Too many way to use Dispatcher like Dispatcher_Operation , [window.dispatcher] or etc.
2. 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