Conditional XAML not working in ResourceDictionaries

[michael-hawker]

Describe the bug
Been having trouble trying to add more complex conditional XAML to our Resource Dictionaries (like just copying resources from the WinUI repo here that is using the same syntax is failing in our C# UWP Class Library).

The latest message I've received is Type 'TemplateBinding' used after '{' must be a Markup Extension. Error code 0x09cc. which seems similar to this old thread here.

Steps to reproduce the bug

1. Clone WCT repo
2. Open a ResourceDictionary for a control like (TokenizingTextbox.AutoSuggestBox.xaml)
3. Paste in a system style for AutoSuggestBox (grabbed from Generic.xaml from 18362 for instance)
4. Use VS Quick-Fix tip to apply conditional XAML for Description property.
5. Try and compile. This should be valid XAML, but it fails.

I noticed this as well when trying to apply CornerRadius conditionally as well.

Expected behavior
Compile and run.

Screenshots

Version Info
Class Library Min: 16299, Target 17763

Windows 10 version	Saw the problem?
May 2020 Update (19041)	Yes
November 2019 Update (18363)
May 2019 Update (18362)
October 2018 Update (17763)
April 2018 Update (17134)
Fall Creators Update (16299)
Creators Update (15063)

Device form factor	Saw the problem?
Desktop	Yes
Mobile
Xbox
Surface Hub
IoT

VS 2019 16.5.5

[robloo]

This is a really bad one to work around. I believe it's the same as reported by @MartinZikmund here:

https://developercommunity.visualstudio.com/content/problem/937077/conditional-xaml-does-not-work-with-setters.html

I haven't found a separate bug report.

[michael-hawker]

@ranjeshj @MikeHillberg can we get this triaged? It's really confusing why this is working in some cases and not others. Is there some other thing we should be doing here to get this to work?

[ranjeshj]

@fabiant3 @stevenbrix can you take a look ?

[fabiant3]

Actually @evelynwu-msft might be in a better position to advise.

[evelynwu-msft]

Hi @michael-hawker, would it be feasible for you to change the project's target platform version to 19041? This is one of several conditional XAML bugs known to exist prior to the 19041 SDK and there unfortunately isn't any workaround.

[michael-hawker]

@evelynwu-msft I think we're updating our target for our next milestone, we were still staying back on our minor release, so I'll test that out now that we can bump it. Thanks!