Implement an empty SGen .NET Core task (dotnet#5212)

The SGen task has not been implemented on .NET Core and building a project with `<GenerateSerializationAssemblies>` is failing with what looks like an infrastructure error.

This PR is adding an empty implementation of the task which throws a nicer error pointing the user to the build-time serialization mechanism supported on .NET Core - Microsoft.XmlSerializer.Generator. The changes here are analogous to how the ResolveComReference task has been implemented to allow erroring out on Core.

Fixes dotnet#3583

ref/Microsoft.Build.Tasks.Core/netstandard/Microsoft.Build.Tasks.Core.cs

@@ -682,6 +682,32 @@ public void CleanupTask(Microsoft.Build.Framework.ITask task) { }
         public Microsoft.Build.Framework.TaskPropertyInfo[] GetTaskParameters() { throw null; }
         public bool Initialize(string taskName, System.Collections.Generic.IDictionary<string, Microsoft.Build.Framework.TaskPropertyInfo> parameterGroup, string taskBody, Microsoft.Build.Framework.IBuildEngine taskFactoryLoggingHost) { throw null; }
     }
+    public partial class SGen : Microsoft.Build.Tasks.ToolTaskExtension
+    {
+        public SGen() { }
+        [Microsoft.Build.Framework.RequiredAttribute]
+        public string BuildAssemblyName { get { throw null; } set { } }
+        [Microsoft.Build.Framework.RequiredAttribute]
+        public string BuildAssemblyPath { get { throw null; } set { } }
+        public bool DelaySign { get { throw null; } set { } }
+        public string KeyContainer { get { throw null; } set { } }
+        public string KeyFile { get { throw null; } set { } }
+        public string Platform { get { throw null; } set { } }
+        public string[] References { get { throw null; } set { } }
+        public string SdkToolsPath { get { throw null; } set { } }
+        [Microsoft.Build.Framework.OutputAttribute]
+        public Microsoft.Build.Framework.ITaskItem[] SerializationAssembly { get { throw null; } set { } }
+        public string SerializationAssemblyName { get { throw null; } }
+        [Microsoft.Build.Framework.RequiredAttribute]
+        public bool ShouldGenerateSerializer { get { throw null; } set { } }
+        protected override string ToolName { get { throw null; } }
+        public string[] Types { get { throw null; } set { } }
+        public bool UseKeep { get { throw null; } set { } }
+        [Microsoft.Build.Framework.RequiredAttribute]
+        public bool UseProxyTypes { get { throw null; } set { } }
+        public override bool Execute() { throw null; }
+        protected override string GenerateFullPathToTool() { throw null; }
+    }
     public abstract partial class TaskExtension : Microsoft.Build.Utilities.Task
     {
         internal TaskExtension() { }

src/Tasks.UnitTests/Microsoft.Build.Tasks.UnitTests.csproj

@@ -80,6 +80,7 @@
 
   <ItemGroup Condition="'$(MonoBuild)' == 'true'">
     <Compile Remove="LC_Tests.cs" />
+    <Compile Remove="SGen_Tests.cs" />
   </ItemGroup>
 
   <ItemGroup Condition="$(TargetFrameworkIdentifier) != '.NETFramework'">
@@ -126,7 +127,6 @@
     <Compile Remove="ResolveComReference_Tests.cs" />
     <Compile Remove="ResolveSDKReference_Tests.cs" />
     <Compile Remove="SdkToolsPathUtility_Tests.cs" />
-    <Compile Remove="SGen_Tests.cs" />
     <Compile Remove="TlbImp_Tests.cs" />
     <Compile Remove="VisualBasicParserUtilitites_Tests.cs" />
     <Compile Remove="VisualBasicTokenizer_Tests.cs" />

src/Tasks.UnitTests/SGen_Tests.cs

@@ -7,12 +7,46 @@
 using Microsoft.Build.Utilities;
 using Microsoft.Build.Shared;
 using System.IO;
+using System.Linq;
 using Xunit;
+using Xunit.Abstractions;
+using Shouldly;
 
 namespace Microsoft.Build.UnitTests
 {
     public class SGen_Tests
     {
+#if RUNTIME_TYPE_NETCORE
+        [Fact]
+        public void TaskFailsOnCore()
+        {
+            using (TestEnvironment testenv = TestEnvironment.Create())
+            {
+                MockLogger logger = ObjectModelHelpers.BuildProjectExpectFailure(@$"
+<Project>
+    <Target Name=""MyTarget"">
+        <SGen
+            BuildAssemblyName=""Foo""
+            BuildAssemblyPath=""Foo""
+            ShouldGenerateSerializer=""true""
+            UseProxyTypes=""true""
+            UseKeep=""true""
+            References=""Foo""
+            KeyContainer=""Foo""
+            KeyFile=""Foo""
+            DelaySign=""true""
+            SerializationAssembly=""Foo""
+            SdkToolsPath=""Foo""
+            Platform=""Foo""
+            Types=""Foo""
+        />
+    </Target>
+</Project>");
+                logger.ErrorCount.ShouldBe(1);
+                logger.Errors.First().Code.ShouldBe("MSB3474");
+            }
+        }
+#else
         internal class SGenExtension : SGen
         {
             internal string CommandLine()
@@ -56,6 +90,7 @@ public void TestKeepFlagTrue()
 
             Assert.True(commandLine.IndexOf("/keep", StringComparison.OrdinalIgnoreCase) >= 0);
         }
+
         [Fact]
         public void TestKeepFlagFalse()
         {
@@ -242,5 +277,6 @@ public void TestReferencesCommandLine()
 
             Assert.Equal(targetCommandLine, commandLine);
         }
+#endif
     }
 }

src/Tasks/Microsoft.Build.Tasks.csproj

@@ -484,6 +484,9 @@
     <Compile Include="RoslynCodeTaskFactory\RoslynCodeTaskFactoryCodeType.cs" />
     <Compile Include="RoslynCodeTaskFactory\RoslynCodeTaskFactoryCompilers.cs" />
     <Compile Include="RoslynCodeTaskFactory\RoslynCodeTaskFactoryTaskInfo.cs" />
+    <Compile Include="SGen.cs" Condition="'$(MonoBuild)' != 'true'">
+      <ExcludeFromStyleCop>true</ExcludeFromStyleCop>
+    </Compile>
     <Compile Include="System.Design.cs" />
     <Compile Include="system.design\stronglytypedresourcebuilder.cs" />
     <Compile Include="TaskExtension.cs">
@@ -627,9 +630,6 @@
     <Compile Include="RequiresFramework35SP1Assembly.cs" Condition="'$(MonoBuild)' != 'true'">
       <ExcludeFromStyleCop>true</ExcludeFromStyleCop>
     </Compile>
-    <Compile Include="SGen.cs">
-      <ExcludeFromStyleCop>true</ExcludeFromStyleCop>
-    </Compile>
     <Compile Include="SignFile.cs" Condition="'$(MonoBuild)' != 'true'">
       <ExcludeFromStyleCop>true</ExcludeFromStyleCop>
     </Compile>

src/Tasks/Resources/Strings.resx

@@ -1967,6 +1967,10 @@
     <value>MSB3473: Path for "{0}" is invalid. {1}</value>
     <comment>{StrBegin="MSB3473: "}</comment>
   </data>
+  <data name="SGen.TaskNotSupported">
+    <value>MSB3474: The task "{0}" is not supported on the .NET Core version of MSBuild. Use the Microsoft XML Serializer Generator package instead. See https://go.microsoft.com/fwlink/?linkid=858594 for more information.</value>
+    <comment>{StrBegin="MSB3474: "}</comment>
+  </data>
   <!--
         The SignFile message bucket is: MSB3481 - MSB3490