TemplateBinding and TemplatedParent Binding return different values (UnsetValue vs null) on failure to resolve

[HppZ]

using TemplateBinding in VisualState.Setters will make app crash while using TemplatedParent won't.

[jevansaks]

They should be behaviorally equivalent, with the Binding to TemplateParent enabling more complex paths and converters. But I wouldn't have expected TemplateBinding to crash. What is the call stack and error message you see? (you would need to enable native debugging and break on first chance WinRT originate error).

[evelynwu-msft]

@HppZ if you could provide a repro project that would be ideal as it would ensure that we are investigating the right thing.

[HppZ]

https://github.com/HppZ/TemplateParentBug

[evelynwu-msft]

Thanks for the repro! So it seems like your template binding (whether as {TemplateBinding} or {Binding RelativeSource={RelativeSource TemplatedParent}}) doesn't resolve to a value. However, there is a subtle difference in what the two markup expressions return: in the former case it's unset value whereas in the latter case it's null. VSM setters raise an error (when a debugger is attached) if Setter.Value is unset value, but null is considered a valid value.

Having said that, starting with the 1903 release VSM Setters should be permitting unset value on Setter.Value, so I suspect we have a bug of the backwards-logic sort in the framework. I would also expect your binding to successfully resolve, so that's probably a bug as well.

[HppZ]

since it is a bug, please fix it, thanks!

[fabiant3-zz]

Changed title, we'll be considering this issue in the future (bug, not a question anymore). Thanks for reporting and providing the sample code HppZ!

[evelynwu-msft]

Created issues #1104 and #1105 to track different aspects of the overall reported failure.

[marcelwgn]

@StephenLPeters Should this be marked as WinUI 3 since this is currently part of OS XAML?

[StephenLPeters]

I think it should actually just be closed, Winui 3 wont have OS version specific code and from the comments it looks like this was fixed in 1903 so it should already be fixed in Winui 3. @alwu-msft can you confirm and close?

[anawishnoff]

This issue should be fixed as of the latest Windows App SDK 0.8 release, so I'm closing this. Let me know if it still persists and we can re-open.