Microsoft.WinFX.targets Unknown build error Could not find assembly 'mscorlib'

• .NET Core Version: Version: 3.1.301, Commit: 7feb845744 Base Path: C:\Program Files\dotnet\sdk\3.1.301
  Host (useful for support): Version: 3.1.5, Commit: 65cd789777
• Windows version: Win 10 (1909) 10.0.18363
• Does the bug reproduce also in WPF for .NET Framework 4.8?: Not when creating a classic WPF full Framework project
• Is this bug related specifically to tooling in Visual Studio (e.g. XAML Designer, Code editing, etc…)? no, dotnet build fails (without VS being involved)

Problem description: WPF core app doesn’t build when adding a direct <Reference .../> to an old .Net 2.0 framework dll. (Why would you do that? Expensive, proprietary dll from 3rd party, we can’t live without it, author has retired)

Actual behavior: dotnet build fails with

C:\Program Files\dotnet\sdk\3.1.301\Sdks\Microsoft.NET.Sdk.WindowsDesktop\targets\Microsoft.WinFX.targets(225,9):
error MC1000: Unknown build error, 'Could not find assembly 'mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=969db8053d3322ac'. Either explicitly load this assembly using a method such as LoadFromAssemblyPath() or use a MetadataAssemblyResolver that returns a valid assembly.' [D:\Downloads\DotnetWpfCoreProblem\DotnetWpfCoreProblem.csproj]

Expected behavior: It builds fine.

Minimal repro: Create a new WPF core project: dotnet new wpf which will target netcoreapp3.1 at the time of writing. Add a library reference to an old .Net Framework 2.0 WinCE library. Something like

<Reference Include="SomethingOld" HintPath="SomethingOld.dll" />

Try to build. Unfortunately I can’t publish the proprietary dll here. It is built for .Net Framework 2 with WinCE compatibility.

Related Issues I found similar topics where the build failed with the same error (mscorlib not found), but they seem to be resolved. https://github.com/dotnet/wpf/issues/1451 https://github.com/dotnet/wpf/issues/2274

[DanFTRX]

@wldevries If you just updated Visual Studio you are more likely hitting this: https://github.com/dotnet/sdk/issues/12710

[nzain]

Switching the TargetFramework to net5.0 doesn’t make a difference. @ryalanms commented Please reactivate, if you see it again. but I cannot change the ticket status.