Publish error when using generic class for DataType of DataTemplate in XAML

[SokoFromNZ]

Description

When publishing (build a release) for Android I get an Microsoft.Android.Sdk.Aot.targets(91,5): error : Precompiling failed error. I'm using Visual Studio 2022 v17.8.1 Enterprise on an Win11 machine. Running the code as Debug build from VS works.

Steps to Reproduce

1. Create a default MAUI App via File ->New Project
2. Add this new class in the root directory:

   public class MyGenericItem<T>
   {
   public string Label { get;}
   public T Data { get; }
   public MyGenericItem(T data, string label)
   {
       Data = data;
       Label = label;
   }
   }

3. Add the following resource to existing MainPage.xaml ContentPage:

   ...
   <ContentPage.Resources>
       <DataTemplate x:Key="dataTemplate" x:DataType="local:MyGenericItem`1">
           <Grid>
               <Label Text="{Binding Label}" />
           </Grid>
       </DataTemplate>
   </ContentPage.Resources>
   ...

4. Run this publish command from a CLI in the root:

   dotnet.exe publish -c:Release -f:net8.0-android

5. publish exits with this error

   C:\Program Files\dotnet\packs\Microsoft.Android.Sdk.Windows\34.0.43\targets\Microsoft.Android.Sdk.Aot.targets(109,5): error : Precompiling failed for c:\Users\soko\Desktop\MauiPublishAotError\obj\Release\net8.0-android\android-x86\linked\MauiPublishAotError.dll with exit code -1073740791. [c:\Users\soko\Desktop\MauiPublishAotError\MauiPublishAotError.csproj::TargetFramework=net8.0-android]
   C:\Program Files\dotnet\packs\Microsoft.Android.Sdk.Windows\34.0.43\targets\Microsoft.Android.Sdk.Aot.targets(109,5): error : Mono Ahead of Time compiler - compiling assembly c:\Users\soko\Desktop\MauiPublishAotError\obj\Release\net8.0-android\android-x86\linked\MauiPublishAotError.dll [c:\Users\soko\Desktop\MauiPublishAotError\MauiPublishAotError.csproj::TargetFramework=net8.0-android]
   C:\Program Files\dotnet\packs\Microsoft.Android.Sdk.Windows\34.0.43\targets\Microsoft.Android.Sdk.Aot.targets(109,5): error : AOTID B4FB99BD-7643-9D12-5EF1-5DC1A60D8760 [c:\Users\soko\Desktop\MauiPublishAotError\MauiPublishAotError.csproj::TargetFramework=net8.0-android]
   C:\Program Files\dotnet\packs\Microsoft.Android.Sdk.Windows\34.0.43\targets\Microsoft.Android.Sdk.Aot.targets(109,5): error : * Assertion at D:\a\_work\1\s\src\mono\mono\mini\method-to-ir.c:6465, condition `!sig->has_type_parameters' not met [c:\Users\soko\Desktop\MauiPublishAotError\MauiPublishAotError.csproj::TargetFramework=net8.0-android]
   C:\Program Files\dotnet\packs\Microsoft.Android.Sdk.Windows\34.0.43\targets\Microsoft.Android.Sdk.Aot.targets(109,5): error : Unable to compile method 'object MauiPublishAotError.MainPage/<InitializeComponent>_anonXamlCDataTemplate_0:LoadDataTemplate ()' due to: 'Method with open type while not compiling gshared'. [c:\Users\soko\Desktop\MauiPublishAotError\MauiPublishAotError.csproj::TargetFramework=net8.0-android]

Link to public reproduction project repository

No response

Version with bug

8.0.3

Is this a regression from previous behavior?

Not sure, did not test other versions

Last version that worked well

Unknown/Other

Affected platforms

Android, I was not able test on other platforms

Affected platform versions

No response

Did you find any workaround?

No workaround I can think of.

Relevant log output

No response

[drasticactions]

You should be able to work around it by disabling XamlC on the given page.

[XamlCompilation(XamlCompilationOptions.Skip)]
public partial class MainPage : ContentPage

Issues with Mono AOT have been mentioned in the past for Xamarin.Forms (https://github.com/xamarin/Xamarin.Forms/issues/1497) as well. I'm not sure if it's Linker related, or XamlC, maybe both? CC @jonathanpeppers @StephaneDelcroix

[SokoFromNZ]

Is there any disadvantage of your work around compared to a working Mono AOT compilation?

[drasticactions]

Is there any disadvantage of your work around compared to a working Mono AOT compilation?

XamlC is compiled XAML. The Linker is blowing up on the compiled XAML since, I think, it's trying to access parts that are being linked away. My code turns off XamlC, so that I believe would cause your XAML to be built at runtime (I could be wrong on that, @StephaneDelcroix would know more), so I imagine there could a performance hit depending on the complexity of the page and/or code you have it on. But in terms of specific drop off, I don't know, it depends.

That said, I do think there's a legit issue with how that code is being handled by XamlC and/or AOT.

The other thing you could do is turn off XamlC specifically for Android on that page

#if ANDROID
[XamlCompilation(XamlCompilationOptions.Skip)]
#endif
public partial class MainPage : ContentPage

So it would keep the other platforms using the compiled xaml

[PureWeen]

@jonathanpeppers thoughts ?

[jonathanpeppers]

@SokoFromNZ can you share a .binlog of the error? https://aka.ms/binlog

Does turning off XamlC fix it? or not? That is also a huge clue.

[SokoFromNZ]

msbuild.zip here you are. the attribute fixes the issue

[jonathanpeppers]

Looking at the error, I suspect the assembly somehow has invalid IL:

C:\Program Files\dotnet\packs\Microsoft.Android.Sdk.Windows\34.0.43\targets\Microsoft.Android.Sdk.Aot.targets(109,5): Precompiling failed for C:\Users\soko\Desktop\MauiPublishAotError\obj\Release\net8.0-android\android-arm\linked\MauiPublishAotError.dll with exit code -1073740791.
Mono Ahead of Time compiler - compiling assembly C:\Users\soko\Desktop\MauiPublishAotError\obj\Release\net8.0-android\android-arm\linked\MauiPublishAotError.dll
AOTID D36F7652-F57B-BFE8-C96C-CAFE7BF3AC1B
* Assertion at D:\a\_work\1\s\src\mono\mono\mini\method-to-ir.c:6465, condition `!sig->has_type_parameters' not met
Unable to compile method 'object MauiPublishAotError.MainPage/<InitializeComponent>_anonXamlCDataTemplate_0:LoadDataTemplate ()' due to: 'Method with open type while not compiling gshared'. [C:\Users\soko\Desktop\MauiPublishAotError\MauiPublishAotError.csproj]

Can you share C:\Users\soko\Desktop\MauiPublishAotError\obj\Release\net8.0-android\android-arm\linked\MauiPublishAotError.dll?

It's possible some platforms accept this form of slightly invalid IL, while the Mono AOT compiler does not.

In either case, I think this points to a XamlC issue if turning that off for one page fixes it.

[SokoFromNZ]

MauiPublishAotError.zip sure

[jonathanpeppers]

The method itself seems like it is missing a type parameter:

I think this should say MyGenericItem<T> P_0, and the method should also have a type parameter?

The generic type itself looks OK:

[SokoFromNZ]

So the question stands which source code generates the first screenshot out of the second one. There is the nasty bug :)

[jonathanpeppers]

@StephaneDelcroix is probably best to answer that, it's likely just the code in XamlC that generates typed bindings.

Honestly, I don't see how this is even useable from a developer's perspective: x:DataType="local:MyGenericItem`1"

Is this something that should even work? How would a regular Joe know to put "`1"?

[SokoFromNZ]

A regular Joe I am not ;) https://learn.microsoft.com/en-us/dotnet/api/system.codedom.codetypereference.basetype?view=dotnet-plat-ext-8.0&redirectedfrom=MSDN#representation-of-generic-types Generic types are formatted with the name of the type followed by a grave accent ("`") followed by a count of the generic type arguments.

Also here: https://stackoverflow.com/questions/10005187/how-to-reference-a-generic-type-in-the-datatype-attribute-of-a-datatemplate/70021008#70021008

Actually I'm curious why a debug build works fine?! As it doesn't seem to be an AOT issue and more an XamlC issue. Is there a different XamlC for Debug/Release?

[jonathanpeppers]

In Debug builds, XamlC is "validate-only" mode, so it doesn't write any IL.

[ghost]

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