ObservePropertyChanged extension method (INotifyPropertyChanged.Property => IObservable)

Extensions.cs

public static class Extensions
{
    public static IObservable<TProperty> ObservePropertyChanged<TNotifier, TProperty>(
        this TNotifier notifier,
        Expression<Func<TNotifier, TProperty>> propertyAccessor,
        bool startWithCurrent = false)

        where TNotifier : INotifyPropertyChanged
    {
        // Parse the expression to find the correct property name.
        MemberExpression member = (MemberExpression)propertyAccessor.Body;
        string name = member.Member.Name;

        // Compile the expression so we can run it to read the property value.
        var reader = propertyAccessor.Compile();

        var propertyChanged = Observable.FromEvent<PropertyChangedEventHandler, PropertyChangedEventArgs>(
            handler => (sender, args) => handler(sender, args),
            x => notifier.PropertyChanged += x,
            x => notifier.PropertyChanged -= x);

        // Filter the events to the correct property name, then select the value of the property from the notifier.
        var newValues = from p in propertyChanged
                        where p.EventArgs.PropertyName == name
                        select reader(notifier);

        // If the caller wants the current value as well as future ones, use Defer() so that the current value is read when the subscription
        // is added, rather than right now. Otherwise just return the above observable.
        return startWithCurrent ? Observable.Defer(() => Observable.Return(reader(notifier)).Concat(newValues)) : newValues;
    }
}

ExtensionsTests.cs

[TestClass]
public class ExtensionsTests
{
    [TestMethod]
    public void ObserveFuturePropertyChanges()
    {
        PropertyChanger p = new PropertyChanger { Amount = 6 };
        var observable = p.ObservePropertyChanged(x => x.Amount);
        p.Amount = 0;
        var ints1 = Store(observable);
        p.Amount = 1;
        var ints2 = Store(observable);
        p.Amount = 2;
        p.Amount = 3;
            
        AssertEnumerablesEqual(new []{1,2,3}, ints1);
        AssertEnumerablesEqual(new []{2,3}, ints2);

        ints1.Dispose();
        ints2.Dispose();
    }

    [TestMethod]
    public void ObserveCurrentAndFuturePropertyChanges()
    {
        PropertyChanger p = new PropertyChanger { Amount = 6 };
        var observable = p.ObservePropertyChanged(x => x.Amount, true);
        p.Amount = 0;
        var ints1 = Store(observable);
        p.Amount = 1;
        var ints2 = Store(observable);
        p.Amount = 2;
        p.Amount = 3;
        AssertEnumerablesEqual(new[] { 0, 1, 2, 3 }, ints1);
        AssertEnumerablesEqual(new[] { 1, 2, 3 }, ints2);

        ints1.Dispose();
        ints2.Dispose();
    }

    public static void AssertEnumerablesEqual<T>(IEnumerable<T> expected, IEnumerable<T> actual)
    {
        if (!expected.SequenceEqual(actual))
        {
            Assert.Fail(string.Format("Expected [{0}], but was [{1}]", string.Join(", ", expected), string.Join(", ", actual)));
        }
    }

    public class PropertyChanger : INotifyPropertyChanged
    {
        int amount;

        public int Amount
        {
            get { return amount; }
            set
            {
                amount = value;
                PropertyChangedEventHandler handler = PropertyChanged;
                if (handler != null)
                {
                    handler(this, new PropertyChangedEventArgs("Amount"));
                }
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;
    }
}