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Dave Haylett

Drawing on Timer tick

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I'm getting frustrated because I can't proceed with my project development without getting this (seemingly simple) problem out of the way, so I need help.


The below WPF will simply put a dot on the screen at the click of a mouse. However, I want a timer to run (say every two seconds), and call the OnTimedEvent() to carry out the vital task of plotting a dot at the top-left of the image. This is of course a rather slimmed-down example of my project.


If I uncomment the SetTimer() line, then the code will throw the "The calling thread cannot access this object because a different thread owns it" exception. I kind of understand that the main app thread owns the WriteableBitmap, and I kind of understand that the timer is a separate thread. But I don't understand how to just get it to work and do as it's told. I've spent all evening googling and read all about async/await and DispatchTimer and all sorts but I just can't work it out.


Could someone kindly put me on the right track.

using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Input;
using System.Timers;

namespace MyFail
    class Program
        private static System.Timers.Timer aTimer;
        static WriteableBitmap writeableBitmap = BitmapFactory.New(1024, 768);
        static Window w;
        static Image i;

        static void Main(string[] args)
            i = new Image();
            w = new Window();
            w.Content = i;
            w.Width = 1024;
            w.Height = 768;

            writeableBitmap = new WriteableBitmap(

            i.Source = writeableBitmap;
            i.Stretch = Stretch.None;
            i.HorizontalAlignment = HorizontalAlignment.Left;
            i.VerticalAlignment = VerticalAlignment.Top;
            i.MouseLeftButtonDown += new MouseButtonEventHandler(i_MouseLeftButtonDown);


            Application app = new Application();

        static void DrawPixel(int x, int y)
            byte[] ColorData = { 255, 255, 255, 0 }; // B G R
            Int32Rect rect = new Int32Rect(x, y, 1, 1);
            writeableBitmap.WritePixels(rect, ColorData, 4, 0);

        static void i_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
            DrawPixel((int)e.GetPosition(i).X, (int)e.GetPosition(i).Y);

        private static void SetTimer()
            aTimer = new System.Timers.Timer(2000);
            aTimer.Elapsed += OnTimedEvent;
            aTimer.AutoReset = true;
            aTimer.Enabled = true;

        private static void OnTimedEvent(Object source, ElapsedEventArgs e)
            DrawPixel(0, 0);



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Good point; The DispatcherTimer class looks like a combination of Timer and Dispatcher.Invoke.

It's extremely important to know how Dispatcher.Invoke works all by itself, in case you want to do cross-thread stuff without a timer.

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Yesss! Thankyou Nypren and Postie. The DispatcherTimer itself seems to be exactly what I wanted. Now I can start the project properly! :) Thankyou again.

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