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)?

[sinatrocious]

Found a workaround!

For null propagated property it's absolutely necessary to create an extra property to monitor only for value changes of that property and then use that extra property instead of null-forgiving operator. Then WhenAnyValue works for all scenarios.

To fix code from first post:

1. Add extra property to parent (it can be anywhere, but makes the most sense to be there):

[ObservableAsProperty]
public string? ChildProperty { get; }

2. Use 2 subscriptions:

// this subscription only monitors for child property value
// it will stop working as any WhenAnyValue with null-forgiving operator when parent in expression is null
// but we don't care, as we only need correct value when parent is not null
parent.WhenAnyValue(o => o.Child, o => o.Child!.Property, (child, property) => property)
    .ToPropertyEx(parent, o => o.ChildProperty);

// this subscription is bullet proof and will correctly rise on any change!
parent.WhenAnyValue(o => o.Child, o => o.Trigger, o => o.ChildProperty)
    .Subscribe(o => Console.WriteLine($"E: {o.Item1}"));

3. Test it, it should trigger after any change:

parent.Trigger = !parent.Trigger; // E:

parent.Child = new(); // E: Child

parent.Child.Property = "123"; // E: Child 123

parent.Child = null; // E: