UserControls are not found with the .NET 6 SDK if the AssemblyName is changed with $(MSBuildProjectName).

[loop8ack]

• .NET Core Version: .net6.0-windows
• .NET Core SDK Version: 6.0.100
• Windows Version: 10.0.19043.1288
• Visual Studio Version: 17.1.0 Preview 1.0
• Does the bug reproduce also in WPF for .NET Framework 4.8?: Not tested
• Is this bug related specifically to tooling in Visual Studio? In the XAML editor and during build

Problem description:
When I change the AssemblyName based on the MSBuildProjectName property, controls in the same project are no longer found. This error occurs both in the XAML editor and when building. When I change the sdk-Version (in csproj and global.json) to 5.0.400, everything works fine.
F12 (Go to definition) from XAML editor works as expected.

Actual behavior:
Build will fail with error code "MC3074"
Description: The tag 'UserControl1' does not exist in XML namespace 'clr-namespace:WpfApp1'. Line 10 Position 10.

Minimal repro:

1. Create a new WPF project template "WPF Application"
2. Select .NET 6.0 as target framework
3. Create a new UserControl and use it in the MainWindow
4. Run the project
   => This should build and start as expected
5. Add the following line to a PropertyGroup in the csproj-file:
   <AssemblyName>test.$(MsBuildProjectName)</AssemblyName>
6. Run the project
   => Build will fail with error code "MC3074"
   => XAML-Editor should show the same error inline
7. Change the target framework to "net5.0-windows".
8. Create global.json next to the sln file with the following content:
   { "sdk": { "version": "5.0.400" } }
9. Run the project
   => This should build and start as expected

This error only occurs when MSBuild properties are used. I tested it with $(MSBuildProjectName), $(AssemblyName) and $(RootNamespace).
But if I use $(Nullable) for example, it works as expected.
If I use a fixed value (e.g. "test.project"), it also works as expected.

[Nefarion]

@loop8ack Had the exact same error!

What worked for me was to explicitly set
<IncludePackageReferencesDuringMarkupCompilation>false</IncludePackageReferencesDuringMarkupCompilation>
in the csproj of my wpf...

This was changed from Default = false to Default = true between net5 and net6

[loop8ack]

You are right, it works with that, thank you.
Very strange...

But it still sounds like a bug, so I'll leave this issue open.