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

Question

I'm encountering an InvalidOperationException with the message "The calling thread must be STA, because many UI components require this." in a WPF application with heavy dependence on a referenced library.

I've tried to pin down where the error is coming from, using dispatchers of various threads and objects, ensured main() has STAthread attribute, tried setting "[STAThread]" on seemingly relevant methods.

Inside MyParticipant constructor, as MyVideoRenderer pic is being constructed, which inherits VideoRenderer, the VideoRenderer constructor itself is throwing this exception, not entering the constructor.

Code:

public class MyParticipant : Participant           //inside MainWindow.xaml.cs
    {
        public enum PictureMode
        {
            Avatar,
            Video
        }

        public PictureMode pictureMode = PictureMode.Avatar;

        public ProgressBar voiceVolume;
        public Label nameLabel;
        public MyVideoRenderer pic;
        public MyVideo video;

        public bool isCachedInClient = false;   
        public string displayName = null;
        public Image avatarImage = null;

        public static int picHeight = 480;
        public static int piclWidth = 640;
        public static int panelHeight = 155;
        public static int panelWidth = 174;

        public static Color liveColor = SystemColors.GradientActiveCaptionColor;
        public static Color nonLiveColor = SystemColors.GradientInactiveCaptionColor;


        public MyParticipant(uint objectId, VideoManager videoManager)
            : base(objectId, videoManager)
        {
            pic = new MyVideoRenderer(videoManagerRef)   
            {
                //Top = 5,
                //Left = 5,
                Height = picHeight,
                Width = piclWidth,
                //SizeMode = PictureBoxSizeMode.StretchImage
            };
...

public class VideoRenderer : System.Windows.Controls.Image         //referenced external class
{
    public VideoRenderer(VideoManagerRoot videoManager)        ///Exception here
    {
        this.videoManagerRef = videoManager;
    }
...

It is entirely unclear on what thread this code might run. The exception however indicates strongly that it is not on the main thread, the one that displays UI. — Hans Passant

Rafal Rafal · Accepted Answer · 2012-07-11T19:57:04

My guess is that you're creating UI elements from a background thread, which is the cause of the exception.

Read:

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

3 Answers