How to cast inside WhenAnyValue expression?

[sinatrocious]

I need to cast inside WhenAnyValue to monitor for Property changes:

this.WhenAnyValue(vm => (vm.Foo as Bar)!.Property)

But I am getting

System.NotSupportedException
Unsupported expression of type 'TypeAs' (o.Foo As Bar)."

Show full exception details

System.NotSupportedException
  HResult=0x80131515
  Message=Unsupported expression of type 'TypeAs' (o.Foo As Bar).
  Source=ReactiveUI
  StackTrace:
   at ReactiveUI.ExpressionRewriter.Visit(Expression node) in /_/src/ReactiveUI/Expression/ExpressionRewriter.cs:line 58
   at System.Linq.Expressions.ExpressionVisitor.VisitMember(MemberExpression node)
   at ReactiveUI.ExpressionRewriter.Visit(Expression node) in /_/src/ReactiveUI/Expression/ExpressionRewriter.cs:line 39
   at ReactiveUI.Reflection.Rewrite(Expression expression) in /_/src/ReactiveUI/Expression/Reflection.cs:line 55
   at ReactiveUI.ReactiveNotifyPropertyChangedMixin.SubscribeToExpressionChain[TSender,TValue](TSender source, Expression expression, Boolean beforeChange, Boolean skipInitial, Boolean suppressWarnings) in /_/src/ReactiveUI/Mixins/ReactiveNotifyPropertyChangedMixin.cs:line 141
   at ReactiveUI.ReactiveNotifyPropertyChangedMixin.ObservableForProperty[TSender,TValue](TSender item, Expression`1 property, Boolean beforeChange, Boolean skipInitial) in /_/src/ReactiveUI/Mixins/ReactiveNotifyPropertyChangedMixin.cs:line 77
   at ReactiveUI.WhenAnyMixin.WhenAny[TSender,TRet,T1](TSender sender, Expression`1 property1, Func`2 selector) in /_/src/ReactiveUI/VariadicTemplates.cs:line 80
   at ReactiveUI.WhenAnyMixin.WhenAnyValue[TSender,TRet](TSender sender, Expression`1 property1) in /_/src/ReactiveUI/VariadicTemplates.cs:line 40
...
   at Test.Program.Main(String[] args) in ...

Am I missing something obvious, doing something wrong or is there a good workaround?

---

I was using something weird and likely memory leaking:

this.WhenAnyValue(vm => vm.Foo).Subscribe(foo =>
{
    if(foo is Bar bar)
        bar.WhenAnyValue(o => o!.Property).Subscribe(property => ExtraProperty = property);
});

But I can't use this approach anymore, because I need to pass resulting IObservable (before subscribing) around as parameter.

[anaisbetts]

Not particularly elegant but if you really have to do the cast:

this.WhenAnyValue(x => x.Foo)
  .Select(x => x != null ? ((Bar)x).WhenAnyValue(y => y => Property) : Observable.Empty())
  .Switch();

[glennawatson]

To give context. WhenAnyValue only allows direct property and field access. So just X.Y etc. you can't do any casting or anything else in it. Which is why Ani solution is to cast in the Select.

Another solution is to have a property that casts it. You can have it private or protected. You have to raise the appropriate INotifyPropertyChanged events

[sinatrocious]

Thanks @anaisbetts. Switch() is cool.

In my case Foo can be of Bar or Baz type, so to avoid unnecessary exception I had to rewrite Select as:

.Select(o => (o as Bar)?.WhenAnyValue(bar => bar.Property) ?? Observable.Empty<string>())

[anaisbetts]

@glennawatson definitely also has a good idea here, you can just ToProperty:

this.WhenAnyValue(x => x.Foo)
  .Select(x => x as Bar)
  .ToProperty(this, x => x.FooAsBar, out _FooAsBar);