WhenAnyValue with null propogation, how to init and monitor for other values?

[sinatrocious]

There is a proposed workaround for null propagation. However it doesn't trigger (at all) if expression has null value in the chain. Which leads to 2 problems:

• it doesn't provide initial value (can be worked around with StartWith() >< )
• it will not be able to monitor for changes of other observables.

I made a simple test to demonstrate the issue.

Show code

public class Program
{
    static void Main(string[] args)
    {
        var parent = new Parent();

        parent.WhenAnyValue(o => o.Child, o => o.Trigger, o => o.Child!.Property)
            .Subscribe(o => Console.WriteLine($"A: {o.Item1}"));

        parent.WhenAnyValue(o => o.Child, o => o.Trigger, o => o.Child!.Property)
            .StartWith((null, false, null))
            .Subscribe(o => Console.WriteLine($"B: {o.Item1}"));

        parent.WhenAnyValue(o => o.Trigger)
            .CombineLatest(parent.WhenAnyValue(o => o.Child, o => o.Child!.Property))
            .Subscribe(o => Console.WriteLine($"C: {o.Second.Item1}"));

        parent.WhenAnyValue(o => o.Child, o => o.Trigger)
            .Subscribe(o => Console.WriteLine($"D: {o.Item1}"));

        Console.WriteLine("Trigger");

        parent.Trigger = true;

        Console.ReadKey();
    }
}

public class Parent : ReactiveObject
{
    [Reactive]
    public Child? Child { get; set; }

    [Reactive]
    public bool Trigger { get; set; }

    public override string ToString() => $"{Trigger} {Child}";
}

public class Child : ReactiveObject
{
    [Reactive]
    public string? Property { get; set; }

    public override string ToString() => $"Child {Property}";
}

I have to monitor for Trigger and Child.Property properites.

Run the test and the output will be:

B:
D:
Trigger
D:

In other words:

• A never works.
• B with StartWith workaround only able to init value, but then doesn't work.
• C (attempt to use CombineLatest) never works either
• only D (doesn't have null propagation) works, but it's not able to catch Child.Property changes.

Question

How to monitor for Trigger and Child.Property (where Child may always be null)?