消除引用后，XAML按钮未被垃圾回收

Question

我编写了一个测试程序，其中一个单一的按钮被定义为Window内容的XAML。在窗口加载时，该Button被程序替换为窗口的内容，引用它的字段也被替换，都是由我通过编程创建的另一个Button。之后使用弱引用跟踪这两个Button对象，并以1/10秒的间隔轮询每个对象的IsAlive属性。在第一次调用轮询方法之前，我会将对编程定义的Button的根引用全部清除。

运行此代码的期望是，尽管C#垃圾回收的时间不确定性，但两个Button对象最终都将被报告为垃圾收集。尽管编程定义的Button表现出这种行为，通常在半分钟内，但XAML Button从未被收集。我已经让程序运行了十多分钟，仍然看到这种行为。

有人能告诉我为什么XAML Button对象没有被回收吗？特别是，我想知道垃圾回收阻塞引用在哪里，无论它是在我的代码还是在WPF实现中。也许它在XAML加载对象中。我是否正在查看某种内存泄漏？

以下是上述程序的参考代码：

MainWindow.xaml:

<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">
    <Button Name="btn" />
</Window>

MainWindow.xaml.cs :

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));
        }
    }
}

App.xaml：

<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" />

- 80386 DX

---

强制进行所有代的垃圾回收。这样能解决问题吗？ - Riki

在 Poll(.) 方法中调用 System.GC.Collect(); 只会导致编程定义的按钮立即被回收。我加入了这个方法调用并运行了五分钟，但对 XAML 按钮的期望效果没有产生。 - 80386 DX

Answer 1

The button is not collected because it was strongly referenced within the Window namescope:

But it shouldn't be recognized as memory leak, because you should reregister your new button within the scope:

//...
INameScope scope = NameScope.GetNameScope(this);
scope.UnregisterName("btn");
btn = new System.Windows.Controls.Button();
Content = btn;
scope.RegisterName("btn", btn);
//...

- DmitryG