Flag usage of non-AnyCPU values for PlatformTarget in modern .NET applications

[baronfel]

Is your feature request related to a problem? Please describe.

The PlatformTarget field (and therefore Platform in MSbuild) only applies to .NET Framework and doesn't apply to modern .NET applications. For these applications, Platform can (should) always be AnyCPU. We should guide users to removing customizations to this property for projects that do not need it.

From Jan Kotas in #42344:

PlatformTarget is left-over from .NET Framework. (PlatformTarget can only represent architectures that .NET Framework targets. It does not represent full range of platforms that .NET Core can target.)

If you are targeting current .NET, building everything as AnyCPU (ie hardcoding PlatformTarget to AnyCPU) all the time should work just fine. It is what we are doing in dotnet/runtime repo. We hardcode PlatformTarget to AnyCPU, even for platform-specific binaries.

[jkotas]

Also, you may want to stop inferring the PlatformTarget in the SDK when targeting modern .NET applications:

sdk/src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.RuntimeIdentifierInference.targets

Lines 124 to 157 in 88c6473

	<!-- Determine PlatformTarget (if not already set) from runtime identifier. -->
	<Choose>
	<When Condition="'$(PlatformTarget)' != '' or '$(RuntimeIdentifier)' == ''" />
	<When Condition="$(RuntimeIdentifier.EndsWith('-x86')) or $(RuntimeIdentifier.Contains('-x86-'))">
	<PropertyGroup>
	<PlatformTarget>x86</PlatformTarget>
	</PropertyGroup>
	</When>
	<When Condition="$(RuntimeIdentifier.EndsWith('-x64')) or $(RuntimeIdentifier.Contains('-x64-'))">
	<PropertyGroup>
	<PlatformTarget>x64</PlatformTarget>
	</PropertyGroup>
	</When>
	<When Condition="$(RuntimeIdentifier.EndsWith('-arm')) or $(RuntimeIdentifier.Contains('-arm-'))">
	<PropertyGroup>
	<PlatformTarget>arm</PlatformTarget>
	</PropertyGroup>
	</When>
	<When Condition="$(RuntimeIdentifier.EndsWith('-arm64')) or $(RuntimeIdentifier.Contains('-arm64-'))">
	<PropertyGroup>
	<PlatformTarget>arm64</PlatformTarget>
	</PropertyGroup>
	</When>
	<Otherwise>
	<PropertyGroup>
	<PlatformTarget>AnyCPU</PlatformTarget>
	</PropertyGroup>
	</Otherwise>
	</Choose>

[baronfel]

@jkotas this issue is for an MSBuild BuildCheck for usage in customer projects, so I extracted your comment to a new issue for the SDK to stop setting it in the first place.

[MattKotsenas]

We should also flag anyone setting <ResolveAssemblyWarnOrErrorOnTargetArchitectureMismatch> as that's a way to suppress MSB3270 warnings, which don't make sense in this new world.