Proposal: Support setting/changing Application.Current.RequestedTheme after app constructor (rather than throw exception)

Describe the bug Setting Application.Current.RequestedTheme at run-time to alter the app’s light/dark theme throws this exception:

System.NotSupportedException
  HResult=0x80131515
  Message=Specified method is not supported.
  Source=WinRT.Runtime
  StackTrace:
   at WinRT.ExceptionHelpers.ThrowExceptionForHR(Int32 hr)
   at ABI.Microsoft.UI.Xaml.IApplication.global::Microsoft.UI.Xaml.IApplication.set_RequestedTheme(ApplicationTheme value)
   at Microsoft.UI.Xaml.Application.set_RequestedTheme(ApplicationTheme value)
   at TestApp.RootWindow.OnThemeMenuFlyoutItemClick(Object sender, RoutedEventArgs e) in S:\Code\TestApp\Views\RootWindow.xaml.cs:line 71
   at ABI.Microsoft.UI.Xaml.RoutedEventHandler.<>c__DisplayClass10_0.<Do_Abi_Invoke>b__0(RoutedEventHandler invoke)
   at WinRT.ComWrappersSupport.MarshalDelegateInvoke[T](IntPtr thisPtr, Action`1 invoke)
   at ABI.Microsoft.UI.Xaml.RoutedEventHandler.Do_Abi_Invoke(IntPtr thisPtr, IntPtr sender, IntPtr e)

Setting the RequestedTheme property on the root Grid in my Window succeeds. But I want to change it app-wide, not only on this window.

Steps to reproduce the bug

Use this code to reproduce:

Application.Current.RequestedTheme = ApplicationTheme.Dark;

Expected behavior

No exception and the app theme should toggle to the selected theme.

Version Info

NuGet package version: [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)
May 2020 Update (19041)	Yes
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

[billhenn]

@codendone Thanks, I apologize I didn’t see that remark before. The more descriptive error message would certainly help prevent confusion on this, and as you said, error messages really need to be more detailed across the board.

I would really ask that you improve this to support app-wide theme changes without having to restart the app. All popular modern apps support instant live theme changes between variations of light and dark themes without having to restart, and it’s kind of an expected app capability at this point. Not having support for that in a “modern” UI platform is very disappointing. Thanks for listening!

[jtorjo]

Guys, can you please please please add just a tad of description in the exception, when you throw it? I wanted to bang my head against the wall on this one (Until i figured out i should set that in the constructor). The (insanely generic) exception is beyond annoying.