MSBuild 16.7+ No Longer Supports Legacy netcore50 Apps

[benvillalobos]

Context

Due to customers receiving bad error messages when using an older SDK and targeting net5.0, we've decided to improve this error message so that it provides a proper solution to their problem.

Old Message: The reference assemblies for {0} were not found. To resolve this, install the Developer Pack (SDK/Targeting Pack) for this framework version or retarget your application. You can download .NET Framework Developer Packs at https://aka.ms/msbuild/developerpacks

New Message (when targeting net5.0 and on an older SDK): The reference assemblies for "{0}" were not found. You might be using an older SDK to target .NET 5.0 or higher. Update Visual Studio and/or your .NET SDK.

The Problem

Some legacy apps (UWP Windows Store 8/8.1 and potentially others) would target netcore50, not to be confused with the newer net5.0. If one happens to build one of these legacy apps on VS16.7+, they could receive the newer message which doesn't help their scenario.

The Path Forward

After discussions with @richlander , @terrajobst , and @rainersigwald , it's looking like we're okay marking this scenario as no longer supported.

This issue essentially acknowledges this as a breaking change and is meant to host the discussion for this change.

[dsplaisted]

I don't believe that #5826 would change any behavior for apps targeting netcore50. I think the TargetFrameworkIdentifier for those apps would be .NETCore, and not .NETFramework or .NETCoreApp, so the new error message wouldn't kick in.

[benvillalobos]

@dsplaisted is correct. And as @clairernovotny helped me determine, the TargetFramework is listed as ".NETCore,Version=v5.0". This can be seen via the Microsoft.AspNet.SignalR.Client package using ilspy on the binaries in the netcore50 folder.

So there is no breaking change after all 🙂