[Maui 10 preview6] When starting up the app crashes: System.ArgumentException: 'We only support xmlns aggregation in ...

[softlion]

Description

I migrated a small net9 app to net10. The tfm is windows.

When starting up, the app crashes:
System.ArgumentException: 'We only support xmlns aggregation in http://schemas.microsoft.com/dotnet/maui/global (Parameter 'target')

I tracked down the issue in that code:
https://github.com/dotnet/maui/blob/10.0.0-preview.6.25359.8/src/Controls/src/Core/XmlnsDefinitionAttribute.cs

I added debug info for the generated code, as there is no XmlnsDefinition attribute in my own code, nor in the obj folder:

<EmitCompilerGeneratedFiles>true</EmitCompilerGeneratedFiles>
CompilerGeneratedFilesOutputPath>.\obj\DebugGeneratedFile</CompilerGeneratedFilesOutputPath>

A quick search shows that one of those generated file DOES have the offending XmlnsDefinition which triggers the crash.

Get-ChildItem -Recurse -File | Where-Object {$_.Extension -ne ".dll"} | Select-String "XmlnsDefinition" | ForEach-Object {
>>     Write-Host "$($_.Path):$($_.LineNumber)" -ForegroundColor Cyan
>>     Write-Host "  $($_.Line.Trim())" -ForegroundColor Yellow

Among the results:

.\obj\DebugGeneratedFile\Microsoft.Maui.Controls.SourceGen\Microsoft.Maui.Controls.SourceGen.CodeBehindGenerator\GlobalXmlns.g.cs:12
  [assembly: global::Microsoft.Maui.Controls.XmlnsDefinition("http://schemas.microsoft.com/dotnet/maui/global", "http://schemas.microsoft.com/dotnet/2021/maui")]

And looking at that file, it appears that the value for the XmlnsDefinition is incorrect. It can't be "http://schemas.microsoft.com/dotnet/2021/maui" as that triggers the crash.

//------------------------------------------------------------------------------
// <auto-generated>
//     This code was generated by a .NET MAUI source generator.
//
//     Changes to this file may cause incorrect behavior and will be lost if
//     the code is regenerated.
// </auto-generated>
//------------------------------------------------------------------------------


[assembly: global::Microsoft.Maui.Controls.XmlnsDefinition("http://schemas.microsoft.com/dotnet/maui/global", "http://schemas.microsoft.com/dotnet/2021/maui")]
[assembly: global::Microsoft.Maui.Controls.XmlnsPrefix("http://schemas.microsoft.com/dotnet/maui/global", "global")]

#if MauiAllowImplicitXmlnsDeclaration
[assembly: global::Microsoft.Maui.Controls.Xaml.Internals.AllowImplicitXmlnsDeclaration]
#endif

I verified that the app is using the preview 6 version of that Microsoft.Maui.Controls nuget:

Steps to Reproduce

I tried with an empty app, but that does not trigger the issue. Not sure why.

Link to public reproduction project repository

No response

Version with bug

10.0.0-preview.6

Is this a regression from previous behavior?

Yes, this used to work in .NET MAUI

Last version that worked well

9.0.90 SR9

Affected platforms

Windows

Affected platform versions

net10.0-windows10.0.19041.0

Did you find any workaround?

Rollback to net9.

Relevant log output

see above

[jfversluis]

Without any form of reproduction it will be near impossible to track down what might be going wrong here.

[dotnet-policy-service]

Hi @@softlion. 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.