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POXAML Button not garbage collected after eliminating references
 

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  1. POXAML Button not garbage collected after eliminating references
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    <p>I wrote a test program wherein a single <code>Button</code> is defined in XAML as the content of a <code>Window</code>. Upon the window's loading, the <code>Button</code> is programmatically replaced as the window's content, and the field referring to it is also replaced, both by another <code>Button</code> which I created programmatically. I thereafter track both <code>Button</code> objects using weak references, and poll at 1/10th second intervals the <code>IsAlive</code> property of each. Before the first check of <code>IsAlive</code> in the first call to the polling method, I wipe out rooting references to the programmatically-defined <code>Button</code> as well.</p> <p>The expectation in running this code would be that, despite nondeterminism in the timing of C# garbage collection, both <code>Button</code> objects would eventually be reported as garbage collected. Although the programmatically-defined <code>Button</code> shows this behavior, typically within a 1/2 minute, the XAML <code>Button</code> is never collected. I've left the program running for more than ten minutes seeing this behavior.</p> <p>Can anyone tell me why the XAML <code>Button</code> object isn't collected? <strong>In particular, I'd like to know where the garbage collection-blocking reference is</strong>, whether it is in my code or it is in the WPF implementation. Perhaps it's in a XAML loading object. Am I looking at some sort of memory leak?</p> <p>The program described above is included below for reference.</p> <p>MainWindow.xaml :</p> <pre><code>&lt;Window x:Class="Test.MainWindow" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Width="300" Height="150" Loaded="Window_Loaded"&gt; &lt;Button Name="btn" /&gt; &lt;/Window&gt; </code></pre> <p>MainWindow.xaml.cs :</p> <pre><code>namespace Test { public partial class MainWindow : System.Windows.Window { private System.WeakReference wr_xamlBtn, wr_programmaticBtn; public MainWindow() { InitializeComponent(); } private void Window_Loaded(object sender, System.Windows.RoutedEventArgs e) { // Immediately upon the window's loading, create a weak reference to the // button %btn defined in XAML. wr_xamlBtn = new System.WeakReference(btn); // Replace references in %btn and this.Content to the XAML button with // references to a programmatically-defined button. This would be // expected to free the XAML button for garbage collection. btn = new System.Windows.Controls.Button(); Content = btn; // Create a weak reference to the programmatically-defined button, so that // when (strong) references to it are removed, it will be eligible for // garbage collection. wr_programmaticBtn = new System.WeakReference(btn); // Provides a polling mechanism to see whether either the XAML button or // the programmatically-defined button has been collected. var dt = new System.Windows.Threading.DispatcherTimer(); dt.Tick += Poll; dt.Interval = System.TimeSpan.FromMilliseconds(100); dt.Start(); } void Poll(object sender, System.EventArgs e) { // If the programmatically-defined button hasn't had its references // removed yet, this does so. This makes it eligible for garbage // collection. if (btn != null) Content = btn = null; // Uses the console to show a timestamp and the state of collection of the // XAML button and the programmatically-defined button. System.Console.WriteLine( string.Format( "XAML button {0}, Programmatically-defined button {1} @ {2}", wr_xamlBtn.IsAlive ? "Alive" : "Collected", wr_programmaticBtn.IsAlive ? "Alive" : "Collected", System.DateTimeOffset.Now)); } } } </code></pre> <p>App.xaml :</p> <pre><code>&lt;Application x:Class="Test.App" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" StartupUri="MainWindow.xaml" /&gt; </code></pre>
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    1. COForce a garbage collection for ALL generations. Does this solve the problem ?
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    2. COCalling `System.GC.Collect();` within the `Poll(.)` method only had the effect of causing the programmatically-defined Button to be collected right away. I ran the code with this method call added for five minutes but the desired effect on the XAML Button was not produced.
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