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

I am using http://www.codeproject.com/KB/IP/Facebook_API.aspx

I am trying to call the XAML which is created using WPF. But it gives me an error:

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

I don't know what to do. I am trying to do this:

FacebookApplication.FacebookFriendsList ffl = new FacebookFriendsList();

But it is giving me that error.

I added a background worker:

static BackgroundWorker bw = new BackgroundWorker();

static void Main(string[] args)
{
    bw.DoWork += bw_DoWork;
    bw.RunWorkerAsync("Message to worker");
    Console.ReadLine();
}

static void bw_DoWork(object sender, DoWorkEventArgs e)
{
    // This is called on the worker thread
    FacebookApplication.FacebookFriendsList ffl = new FacebookFriendsList();

    Console.WriteLine(e.Argument);        // Writes "Message to worker"

    // Perform time-consuming task...
}

Try to invoke your code from the dispatcher:

Application.Current.Dispatcher.Invoke((Action)delegate{
      // your code
});

If you make the call from the main thread, you must add the STAThread attribute to the Main method, as stated in the previous answer.

If you use a separate thread, it needs to be in a STA (single-threaded apartment), which is not the case for background worker threads. You have to create the thread yourself, like this:

Thread t = new Thread(ThreadProc);
t.SetApartmentState(ApartmentState.STA);

t.Start();

with ThreadProc being a delegate of type ThreadStart.

You can also try this

// create a thread  
Thread newWindowThread = new Thread(new ThreadStart(() =>  
{  
    // create and show the window
    FaxImageLoad obj = new FaxImageLoad(destination);  
    obj.Show();  
    
    // start the Dispatcher processing  
    System.Windows.Threading.Dispatcher.Run();  
}));  

// set the apartment state  
newWindowThread.SetApartmentState(ApartmentState.STA);  

// make the thread a background thread  
newWindowThread.IsBackground = true;  

// start the thread  
newWindowThread.Start();  

I suspect that you are getting a callback to a UI component from a background thread. I recommend that you make that call using a BackgroundWorker as this is UI thread aware.

For the BackgroundWorker, the main program should be marked as [STAThread].

Just mark your program Main method with the [STAThread] attribute and the error goes away! it's magic :)

Example:

class Program {
    [STAThread]
    static void Main(string[] args) {
    // My code here
    }
}

For me, this error occurred because of a null parameter being passed. Checking the variable values fixed my issue without having to change the code. I used BackgroundWorker.

If the Application.Current is null for example by unit test, you can try this:

 System.Windows.Threading.Dispatcher.CurrentDispatcher.Invoke( YOUR action )

In my case, I wanted to launch a WPF window from a console app. Simply setting the Main method with [STAThread] didn't work.

A combination of Timores' and Mohammad's answer worked for me:

private static void StaThreadWrapper(Action action)
{
	var t = new Thread(o =>
	{
		action();
		System.Windows.Threading.Dispatcher.Run();
	});
	t.SetApartmentState(ApartmentState.STA);
	t.Start();
}

Example usage:

StaThreadWrapper(() =>
{
	var mainWindow = new MainWindow();
	mainWindow.Show();
});

Another situation if you may meet, choosing which Window to new and show.
Don't make the choice in App.xaml.cs' App() or OnStartup(), instead, make the choice in Startup event.

// App.xaml.cs
        private App()
        {
            Window window = CheckSession() ? new InstallWindow() : (Window)new LoginWindow();
            window.Show(); // bad 1
        }

        protected override void OnStartup(StartupEventArgs e)
        {
            Window window = CheckSession() ? new InstallWindow() : (Window)new LoginWindow();
            window.Show(); // bad 2

            base.OnStartup(e);
        }

Below should be good

// App.xaml.cs
        private App()
        {
            Startup += Application_Startup;
        }

        private void Application_Startup(object sender, StartupEventArgs e)
        {
            Window window = CheckSession() ? new InstallWindow() : (Window)new LoginWindow();
            window.Show();  // good
        }

Also remember to remove the StartupUri from App.xaml

<!--App.xaml-->
<Application StartupUri="MainWindow">
<!--remove StartupUri-->
</Application>

OR add the event here is OK too.

<!--App.xaml-->
<Application Startup="Application_Startup">

</Application>

// App.xaml.cs
        private App()
        {
            
        }

        private void Application_Startup(object sender, StartupEventArgs e)
        {
            Window window = CheckSession() ? new InstallWindow() : (Window)new LoginWindow();
            window.Show();  // good
        }

If you call a new window UI statement in an existing thread, it throws an error. Instead of that create a new thread inside the main thread and write the window UI statement in the new child thread.