Msbuild: Update 16.7 error message when targeting 5.0

[marcpopMSFT]

Issue
If a customer created a net 5 app or got one from a coworker, opens it in 16.7, we have a bad error experience today as we decide it must be a framework application.
https://github.com/dotnet/core/issues/5311

We don’t recognize the TFM so we assume it’s a .net framework version and the message matches that.

Proposal
Update MSBuild 16.7 with a new message in this specific scenario in the below code path.

@RainerSigwald, any concerns just adding a check here for if the version is >= to 5.0 and providing a different error? That seems like the narrowest way to solve this with the least risk. I wouldn’t bother with 16.4.
https://github.com/dotnet/msbuild/blob/29665241066669d6c59f931faabf94a555bcce85/src/Tasks/GetReferenceAssemblyPaths.cs#L249

Other questions

• How does 16.7 MSBuild parse TFMs with a platform provided (ie net5.0-windows) and will the value for version above be accurate?
• Check precompiled apps

[marcpopMSFT]

@BenVillalobos mind prepping a 16.7 fix for this? No rush as we have a week or so before they lock down. Talk to Kathleen about the specific message to show and talk to Rainer about the app types he's concerned about. Test 5.0-windows.

[BenVillalobos]

Sure. What is the expected value of frameworkmoniker.Identifier when we're on net5.0? Is it net or netcoreapp?

[BenVillalobos]

Created PR for the rest of the discussion here: https://github.com/dotnet/msbuild/pull/5826

[BenVillalobos]

@marcpopMSFT Targeting net5.0-windows results in an SDK error:

"C:\src\repros\5820-16.7-errormsg\5820-16.7-errormsg.csproj" (default target) (1) ->
(_CheckForUnsupportedTargetFramework target) ->
  C:\Program Files\dotnet\sdk\3.1.403\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.TargetFrameworkInference.targets(93
,5): error NETSDK1013: The TargetFramework value 'net5.0-windows' was not recognized. It may be misspelled. If not, th
en the TargetFrameworkIdentifier and/or TargetFrameworkVersion properties must be specified explicitly. [C:\src\repros
\5820-16.7-errormsg\5820-16.7-errormsg.csproj]

[marcpopMSFT]

That seems like a reasonable message. I don't think we want to try to special case every possible targetframework that we don't recognize. @KathleenDollard as an FYI.

[Forgind]

Any reason this is still open?

[BenVillalobos]

@Forgind I'll keep this open until we also merge into 16.8

[marcpopMSFT]

May want to test if this makes sense in 16.8 as it may not apply after 16.7.

[mc0re]

Still gives an error on msbuild version 16.8.2+25e4d540b. Is there any workaround?

[BenVillalobos]

@mc0re do you have a global.json file that's lowering the sdk you use while targeting your app to net5.0?

[mc0re]

The error gone away after updating to 16.9.2. Now I have other problems, but they are more click-once related :-)