add shared metadata tests for msbuild workspace

src/Workspaces/CoreTest/ServicesTest.csproj

@@ -298,6 +298,9 @@
       <SubType>Designer</SubType>
     </EmbeddedResource>
   </ItemGroup>
+  <ItemGroup>
+    <EmbeddedResource Include="TestFiles\CSharpProject_CSharpProject_ExternAlias2.csproj" />
+  </ItemGroup>
   <ImportGroup Label="Targets">
     <Import Project="..\..\Tools\Microsoft.CodeAnalysis.Toolset.Open\Targets\VSL.Imports.targets" />
     <Import Project="..\..\..\build\VSL.Imports.Closed.targets" />

src/Workspaces/CoreTest/TestFiles/CSharpProject_CSharpProject_ExternAlias2.csproj

+<?xml version="1.0" encoding="utf-8"?>
+<!-- Copyright (c)  Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information. -->
+<Project ToolsVersion="4.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props" Condition="Exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props')" />
+  <PropertyGroup>
+    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
+    <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
+    <ProjectGuid>{74404B73-C865-49CF-A586-C34759F2C95B}</ProjectGuid>
+    <OutputType>WinExe</OutputType>
+    <AppDesignerFolder>Properties</AppDesignerFolder>
+    <RootNamespace>bug13338</RootNamespace>
+    <AssemblyName>bug13338</AssemblyName>
+    <TargetFrameworkVersion>v4.5</TargetFrameworkVersion>
+    <FileAlignment>512</FileAlignment>
+    <ProjectTypeGuids>{60dc8134-eba5-43b8-bcc9-bb4bc16c2548};{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}</ProjectTypeGuids>
+    <WarningLevel>4</WarningLevel>
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
+    <PlatformTarget>AnyCPU</PlatformTarget>
+    <DebugSymbols>true</DebugSymbols>
+    <DebugType>full</DebugType>
+    <Optimize>false</Optimize>
+    <OutputPath>bin\Debug\</OutputPath>
+    <DefineConstants>DEBUG;TRACE</DefineConstants>
+    <ErrorReport>prompt</ErrorReport>
+    <WarningLevel>4</WarningLevel>
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
+    <PlatformTarget>AnyCPU</PlatformTarget>
+    <DebugType>pdbonly</DebugType>
+    <Optimize>true</Optimize>
+    <OutputPath>bin\Release\</OutputPath>
+    <DefineConstants>TRACE</DefineConstants>
+    <ErrorReport>prompt</ErrorReport>
+    <WarningLevel>4</WarningLevel>
+  </PropertyGroup>
+  <ItemGroup>
+    <Reference Include="System">
+      <Aliases>Sys1, Sys3</Aliases>
+    </Reference>
+    <Reference Include="System.Xml" />
+  </ItemGroup>
+  <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
+  <!-- To modify your build process, add your task inside one of the targets below and uncomment it. 
+       Other similar extension points exist, see Microsoft.Common.targets.
+  <Target Name="BeforeBuild">
+  </Target>
+  <Target Name="AfterBuild">
+  </Target>
+  -->
+</Project>

src/Workspaces/CoreTest/WorkspaceTests/MSBuildWorkspaceTests.cs

@@ -77,6 +77,99 @@ public void TestOpenSolution_MultiProjectSolution()
             Assert.Equal(5, compReferences.Count);
         }
 
+        [Fact, Trait(Traits.Feature, Traits.Features.Workspace)]
+        public void Test_SharedMetadataReferences()
+        {
+            CreateFiles(GetMultiProjectSolutionFiles());
+
+            var sol = MSBuildWorkspace.Create().OpenSolutionAsync(GetSolutionFileName("TestSolution.sln")).Result;
+            var p0 = sol.Projects.ElementAt(0);
+            var p1 = sol.Projects.ElementAt(1);
+
+            Assert.NotSame(p0, p1);
+
+            var p0mscorlib = GetMetadataReference(p0, "mscorlib");
+            var p1mscorlib = GetMetadataReference(p1, "mscorlib");
+
+            Assert.NotNull(p0mscorlib);
+            Assert.NotNull(p1mscorlib);
+
+            // metadata references to mscorlib in both projects are the same
+            Assert.Same(p0mscorlib, p1mscorlib);
+        }
+
+        private static MetadataReference GetMetadataReference(Project project, string name)
+        {
+            return project.MetadataReferences.OfType<PortableExecutableReference>().SingleOrDefault(mr => mr.FilePath.Contains(name));
+        }
+
+        private static MetadataReference GetMetadataReferenceByAlias(Project project, string aliasName)
+        {
+            return project.MetadataReferences.OfType<PortableExecutableReference>().SingleOrDefault(mr => 
+            !mr.Properties.Aliases.IsDefault && mr.Properties.Aliases.Contains(aliasName));
+        }
+
+        [Fact, Trait(Traits.Feature, Traits.Features.Workspace), WorkItem(546171, "DevDiv")]
+        public void Test_SharedMetadataReferencesWithAliases()
+        {
+            var projPath1 = @"CSharpProject\CSharpProject_ExternAlias.csproj";
+            var projPath2 = @"CSharpProject\CSharpProject_ExternAlias2.csproj";
+            var files = new FileSet(new Dictionary<string, object>
+            {
+                { projPath1, GetResourceText("CSharpProject_CSharpProject_ExternAlias.csproj") },
+                { projPath2, GetResourceText("CSharpProject_CSharpProject_ExternAlias2.csproj") },
+                { @"CSharpProject\CSharpExternAlias.cs", GetResourceText("CSharpProject_CSharpExternAlias.cs") },
+            });
+
+            CreateFiles(files);
+
+            var fullPath1 = Path.Combine(this.SolutionDirectory.Path, projPath1);
+            var fullPath2 = Path.Combine(this.SolutionDirectory.Path, projPath2);
+            using (var ws = MSBuildWorkspace.Create())
+            {
+                var proj1 = ws.OpenProjectAsync(fullPath1).Result;
+                var proj2 = ws.OpenProjectAsync(fullPath2).Result;
+
+                var p1Sys1 = GetMetadataReferenceByAlias(proj1, "Sys1");
+                var p1Sys2 = GetMetadataReferenceByAlias(proj1, "Sys2");
+                var p2Sys1 = GetMetadataReferenceByAlias(proj2, "Sys1");
+                var p2Sys3 = GetMetadataReferenceByAlias(proj2, "Sys3");
+
+                Assert.NotNull(p1Sys1);
+                Assert.NotNull(p1Sys2);
+                Assert.NotNull(p2Sys1);
+                Assert.NotNull(p2Sys3);
+
+                // same filepath but different alias so they are not the same instance
+                Assert.NotSame(p1Sys1, p1Sys2);
+                Assert.NotSame(p2Sys1, p2Sys3);
+
+                // same filepath and alias so they are the same instance
+                Assert.Same(p1Sys1, p2Sys1);
+
+                var mdp1Sys1 = GetMetadata(p1Sys1);
+                var mdp1Sys2 = GetMetadata(p1Sys2);
+                var mdp2Sys1 = GetMetadata(p2Sys1);
+                var mdp2Sys3 = GetMetadata(p2Sys1);
+
+                Assert.NotNull(mdp1Sys1);
+                Assert.NotNull(mdp1Sys2);
+                Assert.NotNull(mdp2Sys1);
+                Assert.NotNull(mdp2Sys3);
+
+                // all references to System.dll share the same metadata bytes
+                Assert.Same(mdp1Sys1, mdp1Sys2);
+                Assert.Same(mdp1Sys1, mdp2Sys1);
+                Assert.Same(mdp1Sys1, mdp2Sys3);
+            }
+        }
+
+        private Metadata GetMetadata(MetadataReference mref)
+        {
+            var fnGetMetadata = mref.GetType().GetMethod("GetMetadata", BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.FlattenHierarchy);
+            return fnGetMetadata?.Invoke(mref, null) as Metadata;
+        }
+
         [WorkItem(552981, "DevDiv")]
         [Fact, Trait(Traits.Feature, Traits.Features.Workspace)]
         public void TestOpenSolution_DuplicateProjectGuids()