Setter showing a Build Error when using XAML OnPlatform Markup Extension

[vsfeedback]

This issue has been moved from a ticket on Developer Community.

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[severity:I'm unable to use this version]
If I try to use an On Platform XAML Markup Extension and Build the project I get an error, "Missing Value For Setter". This does not stop the project running in debug mode, where the error disappears from the list.
However this does stop Publishing because the project will not Build (Failed to create App archive 'ProjectPlannerMAUI'. Please ensure your project builds before archiving it.).
The following shows a snippet where the first Setter is OK but the second Setter gives an error:

Error Position 43:18. Missing Value for Setter ProjectPlannerMAUI C:\VSProjects\ProjectPlannerMAUI\ProjectPlannerMAUI\App.xaml 43

<Style x:Key="controlStyleSmallAndroid" TargetType="Label">
                <Setter Property="FontSize" Value="18"/>
            </Style>
            <Style x:Key="controlStyleLargeAndroid" TargetType="Label">
                <Setter Property="FontSize" Value="{OnPlatform iOS=18, Android=24, WinUI=18}"/>
            </Style>

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Original Comments

Feedback Bot on 11/30/2022, 06:35 PM:

(private comment, text removed)

Glenn Taylor on 12/2/2022, 06:23 AM:

(private comment, text removed)

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Original Solutions

(no solutions)

[GDT48]

Don't understand this. This is now flagged as Closed - Duplicate. Does this mean the reported issue is closed? Without a reply or solution? Not sure what the added area/Xaml </> and area/tooling means.

[ghost]

We've moved this issue to the Backlog milestone. This means that it is not going to be worked on for the coming release. We will reassess the backlog following the current release and consider this item at that time. To learn more about our issue management process and to have better expectation regarding different types of issues you can read our Triage Process.

[jsuarezruiz]

cc @StephaneDelcroix

[Hooterr]

This issue still exists. If you use {OnPlatform} in a app-level resource dictionary it causes build error in Visual Studio on windows.
Weirdly enough on VS for mac it works fine...
cc: @jsuarezruiz

[jinxinjuan]

Could you provide a sample project to repro this issue? Also other more info? Thanks.

[GDT48]

I have created a small sample project available at https://github.com/GDT48/SetterError
Added a Picker to MainPage.
Added a Style to App.XML which contains a conditional property.
Project runs in Debug without issue.
However will not build, errors:
Error Position 14:18. Missing Value for Setter
Error Position 14:18. Missing Value for Setter

[Hooterr]

@GDT48 looks like the repo is private.

[GDT48]

Thanks, my apologies. I have now made public.

[BretJohnson]

The test project targets iOS (and thus builds for it, even on Windows), but OnPlatform just specifies values for Android and WinUI, not iOS. Adding iOS to OnPlatform, or adding a Default value, looks to make it work.
@GDT48 - That should fix things for you, but please let us know if there's another issue here you see.
Meanwhile, on our side, we should really update the error message to be more specific on what's missing, to be less confusing. I'll take a look at fixing that.

[GDT48]

Hi Bret, Thanks for getting back to me. I tried adding iOS=#FFfafafe, but still get the error – note get only one error message rather than 2. I tried adding default, this builds without issue. Does not need iOS. For completeness I also tried Value="{OnIdiom Tablet=#FFfafafe, Desktop=#FFfdfdfe}". This, without default, builds without issue which makes sense because you don’t build for a specific Idiom. This suggests, for OnPlatform, you always need Default. Maybe if you referenced all possible properties you may not need Default – I tried MacCatalyst, which also gave an error without default – but I did not try all options. Suggest also looking at the documentation to better explain the need for a default with OnPlatform. Regards, Glenn From: Bret Johnson ***@***.***> Sent: 24 August 2023 21:19 To: dotnet/maui ***@***.***> Cc: GDT48 ***@***.***>; Mention ***@***.***> Subject: Re: [dotnet/maui] Setter showing a Build Error when using XAML OnPlatform Markup Extension (Issue #12064) This works if there's a Default value present in OnPlatform, so that's a workaround. I believe we just need to update the error message to be more specific on what's missing. — Reply to this email directly, view it on GitHub <#12064 (comment)> , or unsubscribe <https://github.com/notifications/unsubscribe-auth/AOOXYQDUAWSBSZFRGGXBUX3XW6ZJXANCNFSM6AAAAAAS5ZC2CU> . You are receiving this because you were mentioned. <https://github.com/notifications/beacon/AOOXYQBYFD5FMFSUDLJ7YP3XW6ZJXA5CNFSM6AAAAAAS5ZC2CWWGG33NNVSW45C7OR4XAZNMJFZXG5LFINXW23LFNZ2KUY3PNVWWK3TUL5UWJTTE345KY.gif> Message ID: ***@***.*** ***@***.***> >

[BretJohnson]

Update: I looked more at OnPlatform functionality in Xamarin.Forms and confirmed that when the platform doesn't match, there should just be no property set, not an error. So while setting a default is a workaround, we really should fix this. Looking at that now.

[BretJohnson]

Another update: With this issue, when XAML is compiled it results in the "Missing Value For Setter" XamlC error, but when XAML compilation is turned off, the result is a runtime NullReferenceException crash. So that's all the more reason to fix it, which my PR here #17061 does.

[mtarros]

I have created a small sample project available at https://github.com/GDT48/SetterError Added a Picker to MainPage. Added a Style to App.XML which contains a conditional property. Project runs in Debug without issue. However will not build, errors: Error Position 14:18. Missing Value for Setter Error Position 14:18. Missing Value for Setter

Try removing the ResourceDictionary tags in App.XML and see if that fixes your issue.

I have a similar issue were debug mode runs just fine but in release build I get missing key errors and removing the ResourceDictionary tags solved my issue.