[regression/8.0.0-rc.1.9171] Build error in .NET 8 RC1 - "XamlC error XFC0126: Resources in ResourceDictionary require a x:Key attribute" for resources that have a key

[brunck]

Description

Using version 8.0.100-rc.1.23455.8
Building a .NET MAUI app created using RC1 and VS 2022 preview via File -> New Project and adding the given lines errors with XamlC error XFC0126: Resources in ResourceDictionary require a x:Key attribute.

This appears to be a regression; the same code worked in .NET 8 preview 7.

Steps to Reproduce

1. Create a File -> New .NET MAUI app
2. Add these lines to the App.xaml file:

<Color x:Key="LightSmoke">#4FFFFFFF</Color>

<OnPlatform x:Key="EntryBackgroundColor" x:TypeArguments="Color">
    <On Platform="iOS" Value="{StaticResource LightSmoke}" />
    <On Platform="Android" Value="Transparent" />
</OnPlatform>

3. Build fails with the given error message.

Or, use the solution in the given reproduction repository in the MauiResourceDictionaryBuildError folder.

Link to public reproduction project repository

https://github.com/brunck/reproductions.git

Version with bug

8.0.0-rc.1.9171

Is this a regression from previous behavior?

Yes, this used to work in .NET MAUI

Last version that worked well

8.0.0-preview.7.8842

Affected platforms

I was not able test on other platforms

Affected platform versions

n/a

Did you find any workaround?

Removing the related lines causes the build to pass.

Relevant log output

No response

[tentom]

I get this same issue if I try to use in my App.Xaml file.
Error XFC0126: Resources in ResourceDictionary require a x:Key attribute. (XFC0126) (AppName) XamlC

But it seems to work if I do use a static resource as the value as the following works:

<OnPlatform x:Key="MediumFontFamily"
                    x:TypeArguments="x:String"
                    >
            <On Platform="Android" Value="sans-serif-medium"/>
            <On Platform="iOS" Value="Arial"/>
</OnPlatform>

While this does not work and gives the error mentioned before.

<OnPlatform x:Key="RegularFontFamily"
                    x:TypeArguments="x:String"
                    >
    <On Platform="Android" Value="{StaticResource AppFontFamily}"/>
    <On Platform="iOS" Value="{StaticResource AppFontFamily}"/>
</OnPlatform>

Another side note that might be related is that when using a merged resource dictionary another error appears when using namely:
Error: Collection was modified; enumeration operation may not execute. XamlC

So in my App.xaml file I have

<ResourceDictionary.MergedDictionaries>
    <local:MyAppTheme />
</ResourceDictionary.MergedDictionaries>

and in a separate file MyAppTheme

<ResourceDictionary>
    <OnPlatform x:Key="ImageHighlightedColor" x:TypeArguments="Color" Default="{ StaticResource AccentColor }" />
</ResourceDictionary>

Im Running on a Mac and compiling for iOS

[samhouts]

I was not able to reproduce this with the given project. It builds and runs with RC1. Is there more required?

[ghost]

Hi @brunck. We have added the "s/needs-info" label to this issue, which indicates that we have an open question for you before we can take further action. This issue will be closed automatically in 7 days if we do not hear back from you by then - please feel free to re-open it if you come back to this issue after that time.

[brunck]

@samhouts It definitely does not build for me using the latest VS 2022 preview. Just open and try to build the solution in the MauiResourceDictionaryBuildError folder. No other action is required.
What other info do you need from me?

[XamlTest]

Verified this on Visual Studio Enterprise 17.8.0 Preview 2.0(8.0.0-rc.1.9171). Repro with the given project:
MauiRc1App.zip

[StephaneDelcroix]

the Collection was modified error is fixed. this might be related to OnPlatform optimisation, checking if I can repro and unit test

[StephaneDelcroix]

I can not reproduce this. which platform are you compiling for ?

[brunck]

This is single-project MAUI…how do you compile for just one platform? Android I guess?

[samhouts]

Oh, that's strange. If I hit F5, it builds and runs successfully. If I just do Build, it fails.

Confirmed that this regressed between 8.0.0-preview.7.8842 and 8.0.0-rc.1.9171. #5611 looks sus, @StephaneDelcroix

[brunck]

That’s very strange! I don’t think I’ve ever seen or heard of this type of thing happening before.

[StephaneDelcroix]

trying again to produce a unit test...

[StephaneDelcroix]

I think I understand what's going on. SimplifyOnPlatform was meant to simplify code generation, but using it as a Resource was never a use case. I might need to rethink this. Depending on the deadlines, we might have to revert #5611. @samhouts your call