Enable netstandard2.1 libraries to build (#10614)

Directory.Build.targets

@@ -81,9 +81,12 @@
 
   <ItemGroup>
     <KnownFrameworkReference Update="Microsoft.NETCore.App">
-      <LatestRuntimeFrameworkVersion>$(MicrosoftNETCoreAppPackageVersion)</LatestRuntimeFrameworkVersion>
-      <DefaultRuntimeFrameworkVersion Condition="'$(IsServicingBuild)' != 'true'">$(MicrosoftNETCoreAppPackageVersion)</DefaultRuntimeFrameworkVersion>
-      <TargetingPackVersion Condition="'$(IsServicingBuild)' != 'true'">$(MicrosoftNETCoreAppPackageVersion)</TargetingPackVersion>
+      <!-- Always update the 'latest version', whether the repo is servicing or not. -->
+      <LatestRuntimeFrameworkVersion Condition="'%(TargetFramework)' == '$(DefaultNetCoreTargetFramework)'">$(MicrosoftNETCoreAppPackageVersion)</LatestRuntimeFrameworkVersion>
+      <!-- Only update the default runtime version for preview builds. -->
+      <DefaultRuntimeFrameworkVersion Condition="'%(TargetFramework)' == '$(DefaultNetCoreTargetFramework)' and '$(IsServicingBuild)' != 'true'">$(MicrosoftNETCoreAppPackageVersion)</DefaultRuntimeFrameworkVersion>
+      <!-- Only update the targeting pack version for preview builds. -->
+      <TargetingPackVersion Condition="'%(TargetFramework)' == '$(DefaultNetCoreTargetFramework)' and '$(IsServicingBuild)' != 'true'">$(MicrosoftNETCoreAppPackageVersion)</TargetingPackVersion>
     </KnownFrameworkReference>
 
     <KnownFrameworkReference Condition="'$(UseAspNetCoreSharedRuntime)' != 'true'" Remove="Microsoft.AspNetCore.App" />
@@ -95,6 +98,10 @@
       <TargetingPackVersion Condition="'$(IsServicingBuild)' != 'true'">$(SharedFxVersion)</TargetingPackVersion>
     </KnownFrameworkReference>
 
+    <KnownFrameworkReference Update="NETStandard.Library">
+      <TargetingPackVersion Condition="'%(TargetFramework)' == 'netstandard2.1' and '$(IsServicingBuild)' != 'true'">$(NETStandardLibraryRefPackageVersion)</TargetingPackVersion>
+    </KnownFrameworkReference>
+
   </ItemGroup>
 
   <!-- Copied from https://github.com/dotnet/arcade/blob/9d0fd805448082c8d55e2434607b481bca70a146/src/Microsoft.DotNet.Arcade.Sdk/tools/RepositoryInfo.targets#L12-L38 -->

eng/SharedFramework.External.props

@@ -83,7 +83,7 @@
     These compilation references are necessary to compile netstandard2.0 assemblies which are in the shared framework.
     This references are part of Microsoft.NETCore.App, so are listed here as references to be used during compilation only.
   -->
-  <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0'">
+  <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0' or '$(TargetFramework)' == 'netstandard2.1'">
     <_CompilationOnlyReference Include="System.Buffers" />
     <_CompilationOnlyReference Include="System.ComponentModel.Annotations" />
     <_CompilationOnlyReference Include="System.Runtime.CompilerServices.Unsafe" />

eng/Version.Details.xml

@@ -377,6 +377,10 @@
       <Uri>https://github.com/dotnet/core-setup</Uri>
       <Sha>20426e8c486d8715337cb6438ec70bc3619a514d</Sha>
     </Dependency>
+    <Dependency Name="NETStandard.Library.Ref" Version="2.1.0-preview6-27723-08" CoherentParentDependency="Microsoft.Extensions.Logging">
+      <Uri>https://github.com/dotnet/core-setup</Uri>
+      <Sha>20426e8c486d8715337cb6438ec70bc3619a514d</Sha>
+    </Dependency>
   </ProductDependencies>
   <ToolsetDependencies>
     <!-- Listed explicitly to workaround https://github.com/dotnet/cli/issues/10528 -->

eng/Versions.props

@@ -25,6 +25,7 @@
     <!-- Packages from dotnet/core-setup -->
     <MicrosoftExtensionsDependencyModelPackageVersion>3.0.0-preview6-27723-08</MicrosoftExtensionsDependencyModelPackageVersion>
     <MicrosoftNETCoreAppPackageVersion>3.0.0-preview6-27723-08</MicrosoftNETCoreAppPackageVersion>
+    <NETStandardLibraryRefPackageVersion>2.1.0-preview6-27723-08</NETStandardLibraryRefPackageVersion>
     <!-- Packages from dotnet/corefx -->
     <MicrosoftCSharpPackageVersion>4.6.0-preview6.19273.5</MicrosoftCSharpPackageVersion>
     <MicrosoftWin32RegistryPackageVersion>4.6.0-preview6.19273.5</MicrosoftWin32RegistryPackageVersion>