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POWPF Event: BitmapImage PropertyChanged: "Calling Thread Cannot access"
 

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  1. POWPF Event: BitmapImage PropertyChanged: "Calling Thread Cannot access"
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    <p>I'm trying to understand what it is about the following code that <b>is</b> perfectly happy with loading a text file and displaying its contents, but <b>isn't</b> happy with loading a BitmapImage and displaying it on a timer.Elapsed event handler. </p> <p>I understand it has to do with the UI thread. </p> <p>But why is this not a problem for the textfile example?</p> <p>First, the XAML:</p> <pre><code>&lt;Window x:Class="WpfApplication7.Window1" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Title="Window1" Height="300" Width="300"&gt; &lt;StackPanel Orientation="Vertical"&gt; &lt;TextBlock Text="{Binding Path=Message, UpdateSourceTrigger=PropertyChanged}" FontSize="20" Height="40" Width="300" Background="AliceBlue" /&gt; &lt;Image Source="{Binding Path=Image,UpdateSourceTrigger=PropertyChanged}" Height="100" Width="100"/&gt; &lt;/StackPanel&gt; &lt;/Window&gt; </code></pre> <p>and the C#, which raises a PropertyChangedEventHandler on a timer:</p> <pre><code>using System; using System.ComponentModel; using System.Timers; using System.Windows; using System.IO; using System.Windows.Threading; using System.Windows.Media.Imaging; </code></pre> <p>and</p> <pre><code>namespace WpfApplication7 { public partial class Window1 : Window, INotifyPropertyChanged { public BitmapImage Image { get; private set; } public string Message { get; set; } public event PropertyChangedEventHandler PropertyChanged = delegate { }; private Timer timer; public Window1() { InitializeComponent(); this.DataContext = this; this.timer = new Timer { Enabled = true, Interval = 100 }; this.timer.Elapsed += (s, e) =&gt; { //---happy loading from text file. UI updates :) this.Message = File.ReadAllText(@"c:\windows\win.ini").Substring(0, 20); PropertyChanged(this, new PropertyChangedEventArgs("Message")); //---not happy loading a BitmapImage. PropertyChanged unhappy :( // (Don't make me have to: ! ) //Application.Current.Dispatcher.Invoke( //DispatcherPriority.Send, new Action(delegate //{ this.Image = new BitmapImage(new Uri(@"C:\WINDOWS\Web\Wallpaper\Ascent.jpg")); //Edit --Ah hah, thanks Daniel ! // DependencyObject-&gt; Freezable-&gt; Animatable-&gt; // ImageSource-&gt; BitmapSource-&gt; BitmapImage this.Image.Freeze(); //&lt;--- this will fix it, no need for Dispatcher //Without Dispatcher or Freeze() ... right here: //"The calling thread cannot access this object because a different thread owns it." PropertyChanged(this, new PropertyChangedEventArgs("Image")); //})); }; } } } </code></pre> <p>I know I can fix this with a "Application.Current.Dispatcher.Invoke". So fixing it isn't the problem. Not understanding why I should have to is the problem :)</p> <p><i>Similar</i> questions</p> <ul> <li><p><a href="https://stackoverflow.com/questions/36748/asynchronously-loading-a-bitmapimage-in-c-using-wpf">asynchronously loading a BitmapImage in C# using WPF</a></p></li> <li><p><a href="https://stackoverflow.com/questions/590590/making-sure-onpropertychanged-is-called-on-ui-thread-in-mvvm-wpf-app">Making sure OnPropertyChanged() is called on UI thread in MVVM WPF app</a></p></li> </ul>
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