Microsoft-ui-xaml: Proposal: Make UWP XAML able to set struct fields in XAML markup

[jevansaks]

Proposal: Make UWP XAML able to set and bind struct fields in XAML markup

Summary

This was reported by a customer issue on the doc repo. Filing it here so that we can track making this improvement in the product.

Customer said:
This doesn't seem possible and it's a fundamental oversight. We should be able to set them directly in XAML and with bindings. Once again, WPF was the pinnacle of XAML It has been downhill since :(

What they wanted to do:

<TextBlock.Margin>
  <Thickness Left="10"
             Top="10"
             Right="10"
             Bottom="{Binding Path='BottomMargin', Mode=OneWay" />
</TextBlock.Margin>

Source issue: https://github.com/MicrosoftDocs/winrt-api/issues/549

Rationale

Functional Requirements

Important Notes

Open Questions

[MikeHillberg]

Love this. Couple of things we need to be careful about ...

The interesting thing about structs is that they're value types. So you can't modify Margin.Left (it generates a compiler error in C#). Instead you read out the Margin, modify the Thickness, and write the whole thing back.

That's not something that Binding can handle, which is part of why WPF doesn't support Binding on struct fields. It is something that x:Bind can support, but it will require special handling to teach it the copy/modify/write pattern. (Which gets even more fun with nested structs, which maybe aren't worth supporting.)

The other interesting thing about being a value type is if you put an x:Name on it in markup, what you get in code behind is a _copy_ of the struct. So if you x:Name the struct T1 there's one copy of the Thickness struct in the TextBlock, another in copy in this.T1. WPF does the right thing here and doesn't let you x:Name a value type.

[martind0]

or make Thickness a class