Class not registered exception: BitmapImage - xunit

Question

When I try to create a BitmapImage instance (under WinUI 3) I get an exception that a necessary class is not registered. This is taking place within Visual Studio 2022, within an xUnit based test project.
The line throwing the exception is simply:
var retVal = new BitmapImage();
Given how simple the line is I'm wondering if there's something wrong with my xUnit project setup. Here it is:
<Project Sdk="Microsoft.NET.Sdk">
<PropertyGroup>
<TargetFramework>net6.0-windows10.0.19041.0</TargetFramework>
<RuntimeIdentifiers>win10-x64</RuntimeIdentifiers>
<Nullable>enable</Nullable>
<IsPackable>false</IsPackable>
</PropertyGroup>
<ItemGroup>
<PackageReference Include="FluentAssertions" Version="6.6.0" />
<PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.1.0" />
<PackageReference Include="xunit" Version="2.4.1" />
<PackageReference Include="xunit.runner.visualstudio" Version="2.4.3">
<IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
<PrivateAssets>all</PrivateAssets>
</PackageReference>
<PackageReference Include="coverlet.collector" Version="3.1.2">
<IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
<PrivateAssets>all</PrivateAssets>
</PackageReference>
</ItemGroup>
<ItemGroup>
<ProjectReference Include="..\MapLibraryWinApp\MapLibraryWinApp.csproj" />
</ItemGroup>
</Project>
Here's the project file for the library containing the code I'm trying to test:
<Project Sdk="Microsoft.NET.Sdk">
<PropertyGroup>
<TargetFramework>net6.0-windows10.0.19041.0</TargetFramework>
<TargetPlatformMinVersion>10.0.17763.0</TargetPlatformMinVersion>
<RuntimeIdentifiers>win10-x86;win10-x64;win10-arm64</RuntimeIdentifiers>
<UseWinUI>true</UseWinUI>
<Nullable>enable</Nullable>
<RootNamespace>J4JSoftware.MapLibrary</RootNamespace>
<AssemblyName>J4JSoftware.MapLibraryWinApp</AssemblyName>
<AssemblyTitle>Map Library (Windows Apps)</AssemblyTitle>
<Version>0.5.0</Version>
<Copyright>Copyright © Mark A. Olbert</Copyright>
</PropertyGroup>
<ItemGroup>
<PackageReference Include="Microsoft.WindowsAppSDK" Version="1.0.3" />
<PackageReference Include="Microsoft.Windows.SDK.BuildTools" Version="10.0.22000.197" />
<PackageReference Include="Microsoft.AspNetCore.WebUtilities" Version="2.2.0" />
</ItemGroup>
<ItemGroup>
<ProjectReference Include="..\..\J4JLogging\J4JLogging\J4JLogging.csproj" />
<ProjectReference Include="..\MapLibrary\MapLibrary.csproj" />
</ItemGroup>
</Project>
I thought perhaps the test project might need to contain the <UseWinUI> tag, but adding it did not solve the problem.

Answer 1

You need to run an actual packaged WinUI app or deploy the Windows App SDK runtime along with a non-MSIX-packaged app to be able to use the WinUI types.
I am afraid it's not as simple as simply referencing the WinUI code from a standalone testrunner app.
Please refer to this blog post for an example of how to set up an actual app that runs the tests. Or avoid creating UI specific stuff in your unit tests.