Add reference to empty Microsoft.NETCore.App package

src/Tasks/Microsoft.NET.Build.Tasks/ResolveFrameworkReferences.cs

@@ -48,6 +48,9 @@ public class ResolveFrameworkReferences : TaskBase
         [Output]
         public ITaskItem[] PackagesToDownload { get; set; }
 
+        [Output]
+        public ITaskItem[] LegacyFrameworkPackages { get; set; }
+
         [Output]
         public ITaskItem[] RuntimeFrameworks { get; set; }
 
@@ -80,6 +83,7 @@ protected override void ExecuteCore()
             var frameworkReferenceMap = FrameworkReferences.ToDictionary(fr => fr.ItemSpec);
 
             List<ITaskItem> packagesToDownload = new List<ITaskItem>();
+            List<ITaskItem> legacyFrameworkPackages = new List<ITaskItem>();
             List<ITaskItem> runtimeFrameworks = new List<ITaskItem>();
             List<ITaskItem> targetingPacks = new List<ITaskItem>();
             List<ITaskItem> runtimePacks = new List<ITaskItem>();
@@ -91,6 +95,21 @@ protected override void ExecuteCore()
             {
                 frameworkReferenceMap.TryGetValue(knownFrameworkReference.Name, out ITaskItem frameworkReference);
 
+                if (frameworkReference != null)
+                {
+                    if (!string.IsNullOrEmpty(knownFrameworkReference.LegacyFrameworkPackages))
+                    {
+                        foreach (var packageAndVersion in knownFrameworkReference.LegacyFrameworkPackages.Split(';'))
+                        {
+                            var items = packageAndVersion.Split('/');
+                            TaskItem packageToReference = new TaskItem(items[0]);
+                            packageToReference.SetMetadata(MetadataKeys.Version, items[1]);
+
+                            legacyFrameworkPackages.Add(packageToReference);
+                        }
+                    }
+                }
+
                 //  Get the path of the targeting pack in the targeting pack root (e.g. dotnet/ref)
                 TaskItem targetingPack = new TaskItem(knownFrameworkReference.Name);
                 targetingPack.SetMetadata(MetadataKeys.PackageName, knownFrameworkReference.TargetingPackName);
@@ -182,6 +201,11 @@ protected override void ExecuteCore()
                 PackagesToDownload = packagesToDownload.ToArray();
             }
 
+            if (legacyFrameworkPackages.Any())
+            {
+                LegacyFrameworkPackages = legacyFrameworkPackages.ToArray();
+            }
+
             if (runtimeFrameworks.Any())
             {
                 RuntimeFrameworks = runtimeFrameworks.ToArray();
@@ -352,6 +376,19 @@ public KnownFrameworkReference(ITaskItem item)
 
             public string RuntimePackRuntimeIdentifiers => _item.GetMetadata("RuntimePackRuntimeIdentifiers");
 
+            public string LegacyFrameworkPackages
+            {
+                get
+                {
+                    var packages = _item.GetMetadata("LegacyFrameworkPackages");
+                    if (string.IsNullOrEmpty(packages))
+                    {
+                        packages = _item.GetMetadata("PackagesToReference");
+                    }
+                    return packages;
+                }
+            }
+
             public NuGetFramework TargetFramework { get; }
         }
     }

src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Sdk.DefaultItems.targets

@@ -181,23 +181,6 @@ Copyright (c) .NET Foundation. All rights reserved.
       <PackageReference Condition="'%(PackageReference.Identity)' == 'Microsoft.NETCore.App'">
         <Version Condition="'%(PackageReference.Version)' == ''">$(_TargetFrameworkVersionWithoutV)</Version>
       </PackageReference>
-
-      <!-- Reference the Platforms package in order to supply the RID graph to NuGet.
-           This can be removed once we do https://github.com/dotnet/cli/issues/10528 or
-           https://github.com/NuGet/Home/issues/7351 
-
-           We only do this here when there are PackageReferences. If the only references 
-           are FrameworkReferences, then we do not need to provide this package to NuGet. -->
-      <PackageReference Include="Microsoft.NETCore.Platforms"
-                        Version="$(BundledNETCorePlatformsPackageVersion)"
-                        IsImplicitlyDefined="true"
-                        Condition="'$(DisableImplicitFrameworkReferences)' != 'true'
-                                    and '$(DisableImplicitNETCorePlatformsReference)' != 'true'
-                                    and '$(TargetFrameworkIdentifier)' == '.NETCoreApp'
-                                    and '$(_TargetFrameworkVersionWithoutV)' != ''
-                                    and '$(_TargetFrameworkVersionWithoutV)' >= '3.0'"
-                        PrivateAssets="All"
-                        Publish="true" />
     </ItemGroup>
   </Target>
 

src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Sdk.TargetingPackResolution.targets

@@ -91,6 +91,7 @@ Copyright (c) .NET Foundation. All rights reserved.
                                 EnableTargetingPackDownload="$(EnableTargetingPackDownload)">
 
       <Output TaskParameter="PackagesToDownload" ItemName="_PackageToDownload" />
+      <Output TaskParameter="LegacyFrameworkPackages" ItemName="_LegacyFrameworkPackage" />
       <Output TaskParameter="RuntimeFrameworks" ItemName="RuntimeFramework" />
       <Output TaskParameter="TargetingPacks" ItemName="TargetingPack" />
       <Output TaskParameter="RuntimePacks" ItemName="RuntimePack" />
@@ -141,6 +142,16 @@ Copyright (c) .NET Foundation. All rights reserved.
                         ExcludeAssets="all" />
     </ItemGroup>
 
+    <!-- Only add these "legacy" package references if there are already existing package references,
+         as they are only needed in those scenarios (for example supplying the RID graph to
+         NuGet, or preventing older versions of Framework packages from being used). -->
+    <ItemGroup Condition="'@(PackageReference)' != '' and '$(DisableImplicitFrameworkReferences)' != 'true'">
+      <PackageReference Include="@(_LegacyFrameworkPackage)"
+                        IsImplicitlyDefined="true"
+                        PrivateAssets="all"
+                        ExcludeAssets="all" />
+    </ItemGroup>
+
   </Target>
 
   <UsingTask TaskName="GetPackageDirectory" AssemblyFile="$(MicrosoftNETBuildTasksAssembly)" />

src/Tests/Microsoft.NET.Build.Tests/GivenThatWeWantToBuildANetCoreApp.cs

@@ -616,5 +616,68 @@ static void Main(string[] args)
                 .Should()
                 .Pass();
         }
+
+        [Fact(Skip = "https://github.com/dotnet/sdk/issues/3044")]
+        public void ReferenceLegacyContracts()
+        {
+            var testProject = new TestProject()
+            {
+                Name = "ReferencesLegacyContracts",
+                TargetFrameworks = "netcoreapp3.0",
+                IsSdkProject = true,
+                IsExe = true,
+                RuntimeIdentifier = EnvironmentInfo.GetCompatibleRid("netcoreapp3.0")
+            };
+
+            //  Dependencies on contracts from different 1.x "bands" can cause downgrades when building
+            //  with a RuntimeIdentifier.
+            testProject.PackageReferences.Add(new TestPackageReference("System.IO.FileSystem", "4.0.1"));
+            testProject.PackageReferences.Add(new TestPackageReference("System.Reflection", "4.3.0"));
+
+
+            var testAsset = _testAssetsManager.CreateTestProject(testProject, testProject.Name)
+                .Restore(Log, testProject.Name);
+
+            var buildCommand = new BuildCommand(Log, Path.Combine(testAsset.TestRoot, testProject.Name));
+
+            buildCommand
+                .Execute()
+                .Should()
+                .Pass();
+        }
+
+        [Fact]
+        public void ItHasNoPackageReferences()
+        {
+            var testProject = new TestProject()
+            {
+                Name = "NoPackageReferences",
+                TargetFrameworks = "netcoreapp3.0",
+                IsSdkProject = true,
+                IsExe = true
+            };
+
+            var testAsset = _testAssetsManager.CreateTestProject(testProject, testProject.Name)
+                .Restore(Log, testProject.Name);
+
+            string testDirectory = Path.Combine(testAsset.TestRoot, testProject.Name);
+
+            var getPackageReferences = new GetValuesCommand(
+               Log,
+               testDirectory,
+               testProject.TargetFrameworks,
+               "PackageReference",
+               GetValuesCommand.ValueType.Item);
+
+            getPackageReferences.Execute().Should().Pass();
+
+            List<string> packageReferences = getPackageReferences.GetValues();
+
+            packageReferences
+                .Should()
+                .BeEmpty();
+
+
+        }
     }
 }