Add extension point to override TargetFramework in consuming project (#6361)

* Add extension point to override TargetFramework in consumer

Author: ViktorHofer

src/Microsoft.DotNet.Build.Tasks.TargetFramework.Sdk/src/ChooseBestP2PTargetFrameworkTask.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using Microsoft.Build.Framework;
+using System;
 using System.IO;
 
 namespace Microsoft.DotNet.Build.Tasks.TargetFramework.Sdk
@@ -34,11 +35,17 @@ public override bool Execute()
                     targetFrameworksValue = projectReference.GetMetadata("TargetFrameworks");
                 }
                 string[] targetFrameworks = targetFrameworksValue.Split(';');
+
+                string referringTargetFramework = projectReference.GetMetadata("ReferringTargetFramework");
+                if (string.IsNullOrWhiteSpace(referringTargetFramework))
+                {
+                    referringTargetFramework = TargetFramework;
+                }
 
-                string bestTargetFramework = targetFrameworkResolver.GetBestSupportedTargetFramework(targetFrameworks, TargetFramework);
+                string bestTargetFramework = targetFrameworkResolver.GetBestSupportedTargetFramework(targetFrameworks, referringTargetFramework);
                 if (bestTargetFramework == null)
                 {
-                    Log.LogError($"Not able to find a compatible supported target framework for {TargetFramework} in Project {Path.GetFileName(projectReference.ItemSpec)}. The Supported Configurations are {string.Join(", ", targetFrameworks)}");
+                    Log.LogError($"Not able to find a compatible supported target framework for {referringTargetFramework} in Project {Path.GetFileName(projectReference.ItemSpec)}. The Supported Configurations are {string.Join(", ", targetFrameworks)}");
                 }
 
                 projectReference.SetMetadata("SetTargetFramework", "TargetFramework=" + bestTargetFramework);                

src/Microsoft.DotNet.Build.Tasks.TargetFramework.Sdk/src/build/Microsoft.DotNet.Build.Tasks.TargetFramework.Sdk.targets

@@ -67,23 +67,25 @@
              Targets="GetTargetFrameworks"
              BuildInParallel="$(BuildInParallel)"
              RemoveProperties="TargetFramework;RuntimeIdentifier"
+             Condition="'%(ProjectReference.SkipGetTargetFrameworkProperties)' != 'true'"
              SkipNonexistentTargets="true">
-      <Output TaskParameter="TargetOutputs" ItemName="_ProjectRefWithTfms"/>
+      <Output TaskParameter="TargetOutputs" ItemName="_ProjectReferenceWithTargetFrameworks" />
     </MSBuild>
-     
+
     <PropertyGroup>
       <_OriginalTargetFramework Condition="'$(_OriginalTargetFramework)' == ''">$(TargetFramework)</_OriginalTargetFramework>
     </PropertyGroup>
 
     <ChooseBestP2PTargetFrameworkTask TargetFramework="$(_OriginalTargetFramework)"
-                                      ProjectReferencesWithTargetFrameworks="@(_ProjectRefWithTfms)"
+                                      ProjectReferencesWithTargetFrameworks="@(_ProjectReferenceWithTargetFrameworks)"
                                       RuntimeGraph="$(RuntimeGraph)" >
-      <Output TaskParameter="AnnotatedProjectReferencesWithSetTargetFramework" ItemName="_ProjectReferencesWithBestTargetFrameworks" />
+      <Output TaskParameter="AnnotatedProjectReferencesWithSetTargetFramework" ItemName="_ProjectReferenceWithBestTargetFramework" />
     </ChooseBestP2PTargetFrameworkTask>
 
     <ItemGroup>
       <ProjectReference Remove="@(ProjectReference)" />
-      <ProjectReference Include="@(_ProjectReferencesWithBestTargetFrameworks)" SkipGetTargetFrameworkProperties="true"/>
+      <ProjectReference Include="@(_ProjectReferenceWithBestTargetFramework)"
+                        SkipGetTargetFrameworkProperties="true" />
     </ItemGroup>
   </Target>