Don't use Targets* helper properties in libs (#64500)

* Don't use Targets* helper properties in libs projs

This change makes it possible to migrate 200+ (ref+src) projects to use
TargetFramework instead of TargetFrameworks which avoids the additional
outer build evaluation and invocation which ultimately makes the overall
build faster.

Targets* properties (i.e. TargetsWindows, TargetsAnyOS, TargetsUnix,
etc.) rely on the TargetFramework property which usually are set
inside a project. The TargetFramework property is only
available before a project specifies it if it's explicitly set in a props
file or if the project is cross-targeting and the outer-build dispatches
into the inner-build. During the dispatch, the TargetFramework property
is passed in as a global property.

Until now that behavior wasn't a problem because every libraries project
cross-targeted (by setting the TargetFrameworks property) even though
many only include a single TargetFramework (i.e. NetCoreAppCurrent).

To allow projects to use the TargetFramework property instead of
TargetFrameworks, the Targets* helper properties can't be calculated
anymore early in a props file as the TargetFramework property isn't set
at that time.

In general, the guidance by the SDK/msbuild team is to not read from the
TargetFramework property before the project sets it
(in a property group). That effectively means that the TargetFramework
property shouldn't be used in props files at all.

Therefore these helper properties can't be used anymore for property
conditions and I'm replacing their usage with TargetPlatformIdentifier
comparisons for both properties and items.
In nearly all cases, the Targets* helper properties can be replaced with
TargetPlatformIdentifier checks on items and in the few cases where
TargetsUnix or TargetsLinux marks multiple tfms as compatible, the exact
tfms must be used instead for the TargetPlatformIdentifier comparison.

Whenever a project needs to condition properties on the platform, I'm
first setting the TargetPlatformIdentifier the same way the SDK sets it
so that the SDK later doesn't need to set it again to avoid the
additional expensive msbuild function call.

* Use TargetFramework singular to avoid outer builds

Use TargetFramework instead of TargetFrameworks property whenever a
projects only targets a single target framework. This avoid unnecessary
outer builds and evaluations and makes the build faster.

1 files changed, 18 insertions, 1 deletions

diff --git a/eng/BeforeTargetFrameworkInference.targets b/eng/BeforeTargetFrameworkInference.targets
index 60b41284548..70de3444b58 100644
--- a/eng/BeforeTargetFrameworkInference.targets
+++ b/eng/BeforeTargetFrameworkInference.targets
@@ -1,5 +1,22 @@
 <Project>
 
-  <Import Project="$(MSBuildThisDirectory)targetframeworksuffix.props" Condition="'$(DesignTimeBuild)' == 'true'" />
+  <PropertyGroup>
+    <TargetPlatformSupported>true</TargetPlatformSupported>
+    <TargetPlatformVersionSupported>true</TargetPlatformVersionSupported>
+
+    <!-- Value of 0.0 produces versionless SupportedOSPlatform attribute.
+         This is required for platforms not expected to have a version,
+         and we currently omit the version for all platforms. -->
+    <SupportedOSPlatformVersion>0.0</SupportedOSPlatformVersion>
+
+    <!-- Disable setting a default Windows platform for .NETStandard and .NET Framework libraries.
+         This ensures that the TargetPlatformIdentifier property is empty for non .NETCoreApp tfms. -->
+    <_EnableDefaultWindowsPlatform>false</_EnableDefaultWindowsPlatform>
+    <_targetPlatformIdentifier Condition="$(TargetFramework.Contains('-'))">$(TargetFramework.SubString($([MSBuild]::Add($(TargetFramework.IndexOf('-')), 1))))</_targetPlatformIdentifier>
+  </PropertyGroup>
+
+  <PropertyGroup Condition="'$(_targetPlatformIdentifier)' != '' and '$(_targetPlatformIdentifier)' != 'windows'">
+    <TargetPlatformVersion>1.0</TargetPlatformVersion>
+  </PropertyGroup>
 
 </Project>