最简单的带有DependencyProperty的WPF UserControl中的内存泄漏

Question

So I am trying to create a WPF control that has one new dependency property and uses this DP. The control's XAML is as simple as this:

<UserControl x:Class="DPTest.TestControl"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:local="clr-namespace:DPTest">
<TextBlock Text="{Binding RelativeSource={RelativeSource AncestorType=local:TestControl}, Path=TestText}"/>

Just a TextBlock displaying the new property. Codebehind declares the dependency property like this:

using System.Windows;

namespace DPTest
{
  public partial class TestControl
  {
    public static readonly DependencyProperty TestTextProperty = 
      DependencyProperty.Register("TestText", typeof(string), typeof(TestControl), new PropertyMetadata(default(string)));

    public TestControl()
    {
      InitializeComponent();
    }

    public string TestText
    {
      get => (string) GetValue(TestTextProperty);
      set => SetValue(TestTextProperty, value);
    }
  }
}

Everything is ok up to this point. But, being created this control never gets collected by GC. To demonstrate this I use a Window which simply creates tons of TestControls on loading:

using System.Windows;

namespace DPTest
{
  public partial class MainWindow
  {
    public MainWindow()
    {
      InitializeComponent();
      Loaded += OnLoaded;
    }

    private void OnLoaded(object sender, RoutedEventArgs e)
    {
      while (true)
      {
        var uc = new TestControl();
      }
    }
  }
}

Opening this window shows RAM rapidly increasing, while GC firing quite often. Memory profiling shows all the instances of TestControl sitting in RAM.

If I replace binding inside the control with constant value so it is just like this:

<TextBlock Text="hello"/>

RAM does not increase and the unused control instances get collected successfully.

Looks like I'm doing something wrong but the example is so simple and common so I got stuck. Please help.

Answer 1

You're dominating the dispatcher thread, which prevents the data binding engine from doing anything. If I were to hazard a guess, something is being queued up to be handled by the data binding engine at the DataBind dispatcher priority, and your loop is preventing that from happening.

If you rewrite it like so, you will still generate an endless number of test controls, but you won't see your memory grow:

private void OnLoaded(object sender, RoutedEventArgs e)
{
    InstantiateNewControl();
}

private void InstantiateNewControl()
{
    var tc = new TestControl();
    this.Dispatcher.BeginInvoke(
        DispatcherPriority.Background,
        new Action(InstantiateNewControl));
}

This will give the data binding engine an opportunity to do some work between each instantiation.

If you comment out the line specifying DispatcherPriority.Background , you'll see the memory start to grow again. That's because the default priority is the highest ( Normal ), which takes precedence over layout, input, data binding, etc. By flooding the dispatcher at a high priority, you're preventing other work from being done.

---

Update

After investigating with a memory profiler, it looks like the memory growth is stemming from the new BindingExpression instances being kept alive in DataBindManager . Specifically, there is a dictionary that maps the binding expressions to their most recent data binding operations, and it looks like you're preventing that dictionary from being properly maintained. The expressions on the newly instantiated test controls never get removed.

Answer 2

this has nothing to do with your control at all. your while-loop in the loaded event runs forever. the garbage collector simply cannot collect as fast as you are creating new controls.

try this with a foreach-loop and a few thousand iterations - that should at least get you up and running