Dependency property re-entrancy (or: why does this work?)

Question

Put simply, I can create 2 dependency properties in a WPF control and put code in each property change notification to change the other property (i.e PropA change sets PropB and PropB change sets PropA).

I would expect this to disappear up its own backside but WPF seems to handle it nicely. That's actually very handy for my purposes but I can't find this behaviour documented anywhere.

So what's going on? Does the WPF dependency property change notification system guard against reentrancy?

Representative code follows:

XAML:

<Window x:Class="WPFReentrancy1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <TextBox Text="{Binding PropB, UpdateSourceTrigger=PropertyChanged}"/>

    </Grid>
</Window>

Code behind:

 public partial class MainWindow : Window
    {

        public string PropA
        {
            get { return (string)GetValue(PropAProperty); }
            set { SetValue(PropAProperty, value); }
        }
        public static readonly DependencyProperty PropAProperty =
                        DependencyProperty.Register("PropA", typeof (string), typeof (MainWindow),new UIPropertyMetadata("0", PropAChanged));


        public string PropB
        {
            get { return (string)GetValue(PropBProperty); }
            set { SetValue(PropBProperty, value); }
        }

        public static readonly DependencyProperty PropBProperty =
            DependencyProperty.Register("PropB", typeof (string), typeof (MainWindow), new UIPropertyMetadata("", PropBChanged));

        private static void PropBChanged(DependencyObject lDependencyObject, DependencyPropertyChangedEventArgs lDependencyPropertyChangedEventArgs)
        {
            ((MainWindow) lDependencyObject).PropA = (string) lDependencyPropertyChangedEventArgs.NewValue;
        }


        private static void PropAChanged(DependencyObject lDependencyObject, DependencyPropertyChangedEventArgs lDependencyPropertyChangedEventArgs)
        {
            ((MainWindow) lDependencyObject).PropB =
                double.Parse((string) lDependencyPropertyChangedEventArgs.NewValue).ToString("0.000");
        }


        public MainWindow()
        {
            InitializeComponent();
            DataContext = this;
            PropA = "1.123";
        }
    }

Answer 1

Those callbacks are only fired if the property changed, your code does not create an infinite loop of different values.

Try this and you will get a SO exception:

private static readonly Random _random = new Random();
private static void PropBChanged(DependencyObject lDependencyObject, DependencyPropertyChangedEventArgs lDependencyPropertyChangedEventArgs)
{
    ((MainWindow)lDependencyObject).PropA = _random.Next().ToString();
}
private static void PropAChanged(DependencyObject lDependencyObject, DependencyPropertyChangedEventArgs lDependencyPropertyChangedEventArgs)
{
    ((MainWindow)lDependencyObject).PropB = _random.Next().ToString();
}