Listen to changes of dependency property

Is there any way to listen to changes of a DependencyProperty? I want to be notified and perform some actions when the value changes but I cannot use binding. It is a DependencyProperty of another class.

This method is definitely missing here:

DependencyPropertyDescriptor
    .FromProperty(RadioButton.IsCheckedProperty, typeof(RadioButton))
    .AddValueChanged(radioButton, (s,e) => { /* ... */ });

If it's a DependencyProperty of a separate class, the easiest way is to bind a value to it, and listen to changes on that value.

If the DP is one you're implementing in your own class, then you can register a PropertyChangedCallback when you create the DependencyProperty. You can use this to listen to changes of the property.

If you're working with a subclass, you can use OverrideMetadata to add your own PropertyChangedCallback to the DP that will get called instead of any original one.

I wrote this utility class:

• It gives DependencyPropertyChangedEventArgs with old & new value.
• The source is stored in a weak reference in the binding.
• Not sure if exposing Binding & BindingExpression is a good idea.
• No leaks.

using System;
using System.Collections.Concurrent;
using System.Windows;
using System.Windows.Data;

public sealed class DependencyPropertyListener : DependencyObject, IDisposable
{
    private static readonly ConcurrentDictionary<DependencyProperty, PropertyPath> Cache = new ConcurrentDictionary<DependencyProperty, PropertyPath>();

    private static readonly DependencyProperty ProxyProperty = DependencyProperty.Register(
        "Proxy",
        typeof(object),
        typeof(DependencyPropertyListener),
        new PropertyMetadata(null, OnSourceChanged));

    private readonly Action<DependencyPropertyChangedEventArgs> onChanged;
    private bool disposed;

    public DependencyPropertyListener(
        DependencyObject source, 
        DependencyProperty property, 
        Action<DependencyPropertyChangedEventArgs> onChanged = null)
        : this(source, Cache.GetOrAdd(property, x => new PropertyPath(x)), onChanged)
    {
    }

    public DependencyPropertyListener(
        DependencyObject source, 
        PropertyPath property,
        Action<DependencyPropertyChangedEventArgs> onChanged)
    {
        this.Binding = new Binding
        {
            Source = source,
            Path = property,
            Mode = BindingMode.OneWay,
        };
        this.BindingExpression = (BindingExpression)BindingOperations.SetBinding(this, ProxyProperty, this.Binding);
        this.onChanged = onChanged;
    }

    public event EventHandler<DependencyPropertyChangedEventArgs> Changed;

    public BindingExpression BindingExpression { get; }

    public Binding Binding { get; }

    public DependencyObject Source => (DependencyObject)this.Binding.Source;

    public void Dispose()
    {
        if (this.disposed)
        {
            return;
        }

        this.disposed = true;
        BindingOperations.ClearBinding(this, ProxyProperty);
    }

    private static void OnSourceChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var listener = (DependencyPropertyListener)d;
        if (listener.disposed)
        {
            return;
        }

        listener.onChanged?.Invoke(e);
        listener.OnChanged(e);
    }

    private void OnChanged(DependencyPropertyChangedEventArgs e)
    {
        this.Changed?.Invoke(this, e);
    }
}

---

using System;
using System.Windows;

public static class Observe
{
    public static IDisposable PropertyChanged(
        this DependencyObject source,
        DependencyProperty property,
        Action<DependencyPropertyChangedEventArgs> onChanged = null)
    {
        return new DependencyPropertyListener(source, property, onChanged);
    }
}

There are multiple ways to achieve this. Here is a way to convert a dependent property to an observable, such that it can be subscribed to using System.Reactive:

public static class DependencyObjectExtensions
{
    public static IObservable<EventArgs> Observe<T>(this T component, DependencyProperty dependencyProperty)
        where T:DependencyObject
    {
        return Observable.Create<EventArgs>(observer =>
        {
            EventHandler update = (sender, args) => observer.OnNext(args);
            var property = DependencyPropertyDescriptor.FromProperty(dependencyProperty, typeof(T));
            property.AddValueChanged(component, update);
            return Disposable.Create(() => property.RemoveValueChanged(component, update));
        });
    }
}

Usage

Remember to dispose the subscriptions to prevent memory leaks:

public partial sealed class MyControl : UserControl, IDisposable 
{
    public MyControl()
    {
        InitializeComponent();
        
        // this is the interesting part 
        var subscription = this.Observe(MyProperty)
                               .Subscribe(args => { /* ... */}));

        // the rest of the class is infrastructure for proper disposing
        Subscriptions.Add(subscription);
        Dispatcher.ShutdownStarted += DispatcherOnShutdownStarted; 
    }

    private IList<IDisposable> Subscriptions { get; } = new List<IDisposable>();

    private void DispatcherOnShutdownStarted(object sender, EventArgs eventArgs)
    {
        Dispose();
    }
    
    Dispose(){
        Dispose(true);
    }

    ~MyClass(){
        Dispose(false);
    }
    
    bool _isDisposed;
    void Dispose(bool isDisposing)
    {
        if(_disposed) return;
        
        foreach(var subscription in Subscriptions)
        {
            subscription?.Dispose();
        }

        _isDisposed = true;
        if(isDisposing) GC.SupressFinalize(this);
    }
}

You could inherit the Control you're trying to listen, and then have direct access to:

protected void OnPropertyChanged(string name)

No risk of memory leak.

Don't be afraid of standard OO techniques.

If that is the case, One hack. You could introduce a Static class with a DependencyProperty. You source class also binds to that dp and your destination class also binds to the DP.