"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, Jul 11 '12 at 20:49

score 8 · Answer 1 · edited May 23 '17 at 11:43

8

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

Read:

edited May 23 '17 at 11:43

Community

1
1

answered Jul 11 '12 at 19:57

Rafal

12,391
32
54

Thank you, Rafal. The first link helped a ton with understanding the problem: The thread that was creating a new MyParticipant was being default set to MTA, and so inside MyParticipant, that MTA thread was calling new VideoRenderer, which inherits an Image. MTA thread constructing a UI control results in this exception. – user1518816 Jul 12 '12 at 14:34

score 0 · Accepted Answer · answered Jul 12 '12 at 14:35

0

Solved, thanks to Rafal's post:

The problem is that the thread that was creating a new MyParticipant was being default set to MTA, and so inside MyParticipant, that MTA thread was calling new VideoRenderer, which inherits an Image. An MTA thread constructing a UI control results in this exception.

answered Jul 12 '12 at 14:35

user1518816

215
3
12

Why is this still not fixed? – user99999991 Mar 03 '14 at 22:36

score 0 · Answer 3 · answered Jan 23 '18 at 11:06

0

In (WPF Application) Project Properties make sure that Startup object is set to (Not Set). That solved problem in my case.

answered Jan 23 '18 at 11:06

Roske

1

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

3 Answers3

Linked