gistfile1.cs

    public class TestViewModel : ReactiveObject
    {
        const int _intVal = 1;
        public int IntVal { get { return _intVal; } }

        ObservableAsPropertyHelper<int> _MyReactiveVal;
        public int MyReactiveVal
        {
            get { return _MyReactiveVal.Value; }
        }

        ObservableAsPropertyHelper<int> _MyReactiveVal2;
        public int MyReactiveVal2
        {
            get { return _MyReactiveVal2.Value; }
        }

        public TestViewModel()
        {
            // Test 1 - No initial value set explicitly
            // This will raise a propertyChanged event in the 'future', IMO shouldn't
            this.WhenAny(x => x.IntVal, x => x.Value)
                .Select(x => x)
                .ToProperty(this, x => x.MyReactiveVal);

            // Expecting MyReactiveVal == 1 here, but getting 0 instead
            if (MyReactiveVal != IntVal)
                ShowMessage(String.Format("MyReactiveVal {0} != Intval {1}", MyReactiveVal, IntVal));

            Task.Factory.StartNew(
                () => {
                    // Here it works - we are getting 1 now
                    Thread.Sleep(5000);
                    if (MyReactiveVal != IntVal)
                        ShowMessage(String.Format("MyReactiveVal {0} != Intval {1}. I shouldn't be raised", MyReactiveVal, IntVal));
                });


            // Test 2 - An initial value explicitly set
            // This will raise a propertyChanged event in the 'future', IMO definitely shouldn't, because explicitly set initialvalue is equal to calculatedvalue
            this.WhenAny(x => x.IntVal, x => x.Value)
                .Select(x => x)
                .ToProperty(this, x => x.MyReactiveVal2, IntVal);

            // Expecting MyReactiveVal2 == 1 here, and indeed getting 1 because of initialvalue
            if (MyReactiveVal2 != IntVal) ShowMessage(String.Format("MyReactiveVal2 {0} != Intval {1}", MyReactiveVal2, IntVal));

            // However, now even though both the initial and calculated value are equal,
            // a PropertyChanged event will be raised. And not once, but twice!

            // Attach to PropertyChanged events and show MessageBox when raised
            PropertyChanged +=
                (sender, args) => ShowMessage(args.PropertyName + ": I shouldn't be raised! Especially not the second round");
        }

        private void ShowMessage(string message)
        {
            Dispatcher.CurrentDispatcher.BeginInvoke(
                () => MessageBox.Show(message));
        }
    }