.NET SDK 5.0.200 and 5.0.104 can't load default styles of custom control

[huoyaoyuan]

Update:
This has been fixed by .NET SDK 5.0.202 which is shipped with VS 16.9.4.

Confirmed that there's a problematic CsWinRT version included by SDK 5.0.104/200/201. Force downgrading it resolves the issue.

Workarounds
Option A: in your global.json:

{
  "sdk": {
    "version": "5.0.103",
    "rollForward": "disable"
  }
}

However, Windows Update can update your installed 5.0.103 SDK to 5.0.104, which is also buggy.

Option B: in your project file, the managed exe project:

<ItemsGroup>
  <!-- If you are targeting 18362 -->
  <FrameworkReference Update="Microsoft.Windows.SDK.NET.Ref" RuntimeFrameworkVersion="10.0.18362.13" />

  <!-- Or -->
  <!-- If you are targeting 19041 -->
  <FrameworkReference Update="Microsoft.Windows.SDK.NET.Ref" RuntimeFrameworkVersion="10.0.19041.13" />
</ItemsGroup>

.14 are the buggy verions.

---

Describe the bug
I'm surprised that my previously working app showing blank screen. After investigating with Live Visual Tree, the subtree of a templated control is totally empty. I compared with several commits and realized that the only difference should be .NET SDK update.

I'll try to get a repro later. You may also try to reproduce it yourself.
The control in a control library. It's default style is in a separated resource dictionary, merged by Generic.xaml. The control resource dictionary uses x:Bind and has code behind. When I set breakpoint in the code behind constructor, it's not invoked.

Version Info

NuGet package version:

[Microsoft.WinUI 3.0.0-preview3.201113.0]
[Microsoft.WinUI 3.0.0-preview4.210210.4]

Windows app type:

UWP	Win32
	Yes

Windows 10 version	Saw the problem?
Insider Build (xxxxx)
October 2020 Update (19042)	Yes
May 2020 Update (19041)
November 2019 Update (18363)
May 2019 Update (18362)
October 2018 Update (17763)
April 2018 Update (17134)
Fall Creators Update (16299)
Creators Update (15063)

Device form factor	Saw the problem?
Desktop	Yes
Xbox
Surface Hub
IoT

[StephenLPeters]

@bartekk8 I'm pretty surprised that the .net sdk could affect our resource lookups... do you think that is possible?

@huoyaoyuan is suspect the .net sdk is a red herring.

[huoyaoyuan]

Yes, installing back 5.0.103 solves the issue immediately.
However, I want a workaround other than installing old SDK, since there's installer issue in this version.

[huoyaoyuan]

Copying the WinRT.Runtime.dll from 5.0.103 output also fixes the issue. There's something broken with CsWinRT.
@manodasanW can you help here?

[manodasanW]

@huoyaoyuan Just to confirm I am following, with 5.0.103 your scenario works, but with 5.0.200 it doesn't. I assume you downloaded 5.0.200 as part of yesterday's release right? If so, we might be looking at a CsWinRT issue.

Can you provide a repro so I can look into it? What type of value are you binding to?

[huoyaoyuan]

Yes.
The issue is about templated control. The involved type, if any, could be System.Type and RuntimeType.

[huoyaoyuan]

SdkTemplateRepro.zip
Minimal repro.
It just creating a custom control using IDE template into a black application, without any trick.
Changing the SDK version in global.json to 5.0.200 will cause the custom control not loaded.

[manodasanW]

Thanks @huoyaoyuan I was able to get a repro and I see the issue. I opened a CsWinRT bug for it.

[huoyaoyuan]

@anawishnoff Could you highlight this issue and its workaround for a month? It has very wide impact. The fix could only be available after the .NET SDK update at next month.