WinUI Packaged projects throws Markup.XamlParseException: 'XAML parsing failed.'

[strfast365]

Describe the bug

Single-project MSIX throws Markup.XamlParseException: 'XAML parsing failed.' when using templated control from ClassLibrary referenced by dll.

Steps to reproduce the bug

Steps to reproduce:

1. Create WinUI class library
2. Add empty custom control to the project
3. Create a new Single-project MSIX project
4. Add a new reference to dll of the class library
5. Add custom control from class library to MainWindow.xaml
6. Run project

WinUIRes.zip

Expected behavior

You can also create a new solution with a separate packaging project and repeat step 3,4,5,6 - it works fine

Screenshots

No response

NuGet package version

No response

Windows app type

• UWP
• Win32

Device form factor

Desktop

Windows version

May 2021 Update (19043)

Additional context

No response

[StephenLPeters]

@RealTommyKlein and @evelynwu-msft FYI

[jeromelaban]

A workaround for this issue is to add this to the library:

<PropertyGroup>
   <DisableEmbeddedXbf>false</DisableEmbeddedXbf>
</PropertyGroup>

Though I'm not sure why this is disabled in debug in the first place. Maybe it breaks some hot reload scenarios.

Digging deeper, it seems that this issue is related to the way XBFs are embedded in the PRI file. In debug, this property is set to false:

  <PropertyGroup Condition="'$(_OverriddenDisableXbf)' == 'false'">
    <DisableEmbeddedXbf>true</DisableEmbeddedXbf>
    <DisableEmbeddedXbf Condition="'$(Configuration)'!='Debug'">false</DisableEmbeddedXbf>
  </PropertyGroup>

causing XBF files to be generated on the side.

In this mode, and using EnablePreviewMsixTooling to true, additional library payload files (xbf or XAML) are not published to the resulting payload, causing the app to fail to load anything non-embedded referenced in the PRI properly. This is not happening with the .wapproj which copies the payload file properly.

[sam-wheat]

What is the status on this ticket please? Suggested workaround does not work - are there any others?

Thank you.

[dotMorten]

I don’t think this could work. You need to use either a project reference or pack your output as a nuget package and reference that to get the resources as well. The DLL alone doesn’t carry enough in itself. This was also a requirement in the UWP days

[sam-wheat]

or pack your output as a nuget package and reference that to get the resources as well.

@dotMorten In my case my control works fine when referenced as a project however when I create a nuget package and reference that instead I get "XAML parsing failed".

Why would it not work? Seems like the standard use case for nuget packages.

[dotMorten]

@sam-wheat see the issue linked a few messages up. It takes a bit of hacking to get it to make a usable package

[Felix-CodingClimber]

Same problem here...

[pmmachado1971]

Hello. Same problem here. Anyone have a solution?
I need to develop and App that loads plugins that have WinUI 3 pages/windows and always get xaml parsing failed is dll loaded externely. If class library is referenced directly from the project everything works ok, but I need to allow others to develop plugins with user interface and load the dynamically.
Any help will be appreciated.

[haonanttt]

I've created a discussion thread giving the workaround about how to solve the 'XAML parsing failed.' issue in this case. The link is #10268.

In short, when generating the nuget package, we could either:

1. Set DisableEmbeddedXbf property to false when packing a 'Debug' nuget package
   or
2. Pack a 'Release' nuget package instead

The workaround is same to what @jeromelaban had mentioned, and in a similar issue #7830 I've tried it worked