RC2: Triggers and MultiBindings do not inherit their parent datatype

[daltzctr]

Description

Triggers such as DataTrigger and Multibindings do not inherit their parent datatype. This is likely due to #24513. This triggers an XC0022 unless explicitly declaring datatype in the binding itself.

These bindings did not trigger any warnings in .NET 9 RC 1

Steps to Reproduce

<MultiBinding Mode="TwoWay" StringFormat="{}Version ({0})">
    <Binding Path="Version.Version"/>
</MultiBinding>

or

<DataTrigger
    Binding="{Binding MyObject.IsDoingAction}"
    TargetType="Button"
    Value="True">
    <Setter Property="Text" Value="DOING ACTION" />
    <Setter Property="BackgroundColor" Value="Green" />
</DataTrigger>

triggers an XC0022

Link to public reproduction project repository

No response

Version with bug

9.0.0-rc.1.24453.9

Is this a regression from previous behavior?

No, this is something new

Last version that worked well

Unknown/Other

Affected platforms

iOS, Android, Windows, macOS

Affected platform versions

No response

Did you find any workaround?

Explicitly declare the DataType in the bindings inside of MultiBinding or DataTrigger

Relevant log output

No response

[simonrozsival]

I had to remind myself, but this is by design: https://github.com/dotnet/maui/pull/22023/files#diff-ef95be4a1744c9832aea582efb20521a574f8e82ad631936b9dc2ffd4f2770f7R426-R432

Now the question is if the restriction makes sense or if it should be removed. If I remember correctly, my reasoning was that the x:DataType should always match the current BindingContext of the current scope and when you pass a binding for the Picker component, the binding context for that binding will be each individual item in the list of items to choose from. This is similar to how we have a warning when you don't set x:DataType on DataTemplate and instead the data type from the parent scope would be inherited by the template. That is likely an oversight and should be fixed.

In the cases mentioned in this issue, DataTrigger and MultiBinding, the same reasoning doesn't apply, I think. It makes sense for the binding to inherit the parent binding context type.

Looking at this design again after some time, I don't find it intuitive and rather confusing. Especially since the warning doesn't mention that there is a special rule for "standalone" bindings. Unless we are able to distinguish the "Picker" scenario from the "Multibinding" scenario, I think we should remove the restriction. If we do that, we should also revisit #24513 to match the behavior in runtime xaml parsing.

/cc @StephaneDelcroix