Custom Control not working in WinUI 3 preview 3 (desktop)

[wbokkers]

Describe the bug
After adding and using a Custom Control, I get a XAML error:
WMC0610, The XAML Binary Format (XBF) generator reported syntax error '0x03e9'

Steps to reproduce the bug

1. Get and install the WinUI 3 preview 3 templates
2. Open Visual Studio 2019 Community Preview (Version 16.9.0 Preview 1.0)
3. Create an empty WinUI (C#) project for Desktop
4. Add a custom control.
5. Use the custom control in MainWindow.xaml
6. Try to run the app (F5)

Expected behavior
The project should compile and run.

Screenshots

Version Info

NuGet package version:
[Microsoft.WinUI 3.0.0-preview3.201113.0]

Windows app type:

UWP	Win32
	Yes

Windows 10 version	Saw the problem?
May 2020 Update (19041)	Yes

|

Device form factor	Saw the problem?
Desktop	Yes

Additional context
I can compile and run when moving the XAML namespace declaration of the custom control style to the ResourceDictionary, but then I get this runtime error:

In the output window I see the following errors:
D:\a\1\s\dev\MRTCore\mrt\Core\src\MRM.cpp(276)\MRM.dll!7C1ADEA1: (caller: 7C1AF6A8) ReturnHr(1) tid(65e4) 80073B17 NamedResource Not Found.
D:\a\1\s\dev\MRTCore\mrt\Core\src\MRM.cpp(403)\MRM.dll!7C1AF6C5: (caller: 571BBF9E) ReturnHr(2) tid(65e4) 80073B17 NamedResource Not Found.
D:\a\1\s\dev\MRTCore\mrt\Core\src\MRM.cpp(755)\MRM.dll!7C1AF8E0: (caller: 571BBF9E) ReturnHr(3) tid(65e4) 80073B17 NamedResource Not Found.
Exception thrown at 0x76AE9AB2 (KernelBase.dll) in App1.exe: WinRT originate error - 0x80073B17 : 'NamedResource Not Found.'.
onecore\com\combase\winrt\error\restrictederror.cpp(1016)\combase.dll!768623ED: (caller: 767C5126) ReturnHr(1) tid(65e4) 8007007E The specified module could not be found.
Exception thrown at 0x76AE9AB2 in App1.exe: Microsoft C++ exception: winrt::hresult_error at memory location 0x02DDE268.

[stevenbrix]

Adding @alwu for FYI.

what does the controls namespace map to? this worked fine for me locally, although I did hit the first chance exception you see. You can disable this exception, and your app should still run:

[wbokkers]

This is my solution structure:

There is no controls namespace. It's called local2 and it maps as follows:
xmlns:local2="using:App1.Controls"

[wbokkers]

Not breaking on the exception works for me. But I can't leave the namespace declaration in the control.
So, I need to specify it as follows:

The auto generated code (below) does not work:

[stevenbrix]

Just so I understand this properly, you're saying that the auto-generated code results in the following compiler error?

And you're able to workaround it by moving the local2 namespace declaration to the top of the file?

[wbokkers]

Yes, exactly!

[stevenbrix]

Thanks @wbokkers, I'm glad you have a workaround, even though I'm sure it's annoying.

Do you know if this is a regression in WinUI3, or if this happens in WinUI2.x as well?

[wbokkers]

Never saw this happen with UWP, but I never tested with WinUI2, unfortunately.

[stevenbrix]

@wbokkers I just tested this with a UWP project and it does look like it's happening, although maybe there was some regression in UWP that was missed?

[KonstantinKhabarlak]

The problem is still present in 1.0 release. The proposed workaround also works

Not breaking on the exception works for me. But I can't leave the namespace declaration in the control. So, I need to specify it as follows:

The auto generated code (below) does not work:

[clembu]

Still relevant for 1.2

[iamhazel]

Still relevant for 1.3 Preview1

[TekuSP]

Can someone explain to me the workaround?
I end up on

[TekuSP]

If I ignore exception I end with

[ruimaciel]

I've just experienced this exact problem after adding a ContentDialog to an existing WinUi3 project.