Add support for reading the extern alias

src/Tasks/Common/MetadataKeys.cs

@@ -99,6 +99,8 @@ internal static class MetadataKeys
         public const string DestinationSubPath = "DestinationSubPath";
         public const string AssetType = "AssetType";
 
-        public const string ReferenceOnly = "ReferenceOnly";  
+        public const string ReferenceOnly = "ReferenceOnly";
+
+        public const string Aliases = "Aliases";
     }
 }

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

@@ -6,7 +6,6 @@
 using System.Diagnostics;
 using System.IO;
 using System.Linq;
-using System.Reflection.Metadata;
 using System.Security.Cryptography;
 using System.Text;
 using Microsoft.Build.Evaluation;
@@ -869,7 +868,15 @@ private void WriteCompileTimeAssemblies()
             {
                 WriteItems(
                     _compileTimeTarget,
-                    package => package.CompileTimeAssemblies);
+                    package => package.CompileTimeAssemblies,
+                    filter: null,
+                    writeMetadata: (package, asset) =>
+                    {
+                        if (asset.Properties.TryGetValue(LockFileItem.AliasesProperty, out var aliases))
+                        {
+                            WriteMetadata(MetadataKeys.Aliases, aliases);
+                        }
+                    });
             }
 
             private void WriteContentFilesToPreprocess()

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

@@ -0,0 +1,249 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using FluentAssertions;
+using Microsoft.NET.TestFramework;
+using Microsoft.NET.TestFramework.Assertions;
+using Microsoft.NET.TestFramework.Commands;
+using Microsoft.NET.TestFramework.ProjectConstruction;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.IO;
+using System.Linq;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace Microsoft.NET.Build.Tests
+{
+    public class GivenThatWeHaveAPackageReferenceWithAliases : SdkTest
+    {
+
+        public GivenThatWeHaveAPackageReferenceWithAliases(ITestOutputHelper log) : base(log)
+        { }
+
+        [RequiresMSBuildVersionFact("16.7.0")]
+        public void CanBuildProjectWithPackageReferencesWithConflictingTypes()
+        {
+            var targetFramework = "netcoreapp5.0";
+            var packageReferences = GetPackageReferencesWithConflictingTypes(targetFramework, "A", "B");
+
+            TestProject testProject = new TestProject()
+            {
+                Name = "Project",
+                IsSdkProject = true,
+                IsExe = false,
+                TargetFrameworks = targetFramework,
+            };
+
+            testProject.PackageReferences.Add(packageReferences.First());
+            testProject.PackageReferences.Add(
+                new TestPackageReference(
+                    packageReferences.Last().ID,
+                    packageReferences.Last().Version,
+                    packageReferences.Last().NupkgPath,
+                    packageReferences.Last().PrivateAssets,
+                    aliases: "Special"));
+            var packagesPaths = packageReferences.Select(e => Path.GetDirectoryName(e.NupkgPath));
+
+            testProject.AdditionalProperties.Add("RestoreSources",
+                                     "$(RestoreSources);" + string.Join(";", packagesPaths));
+
+            //  Use a test-specific packages folder
+            testProject.AdditionalProperties["RestorePackagesPath"] = @"$(MSBuildProjectDirectory)\..\pkg";
+            testProject.SourceFiles[$"{testProject.Name}.cs"] = ConflictingClassLibUsage;
+            var testAsset = _testAssetsManager.CreateTestProject(testProject);
+
+            var buildCommand = new BuildCommand(Log, Path.Combine(testAsset.TestRoot, testProject.Name));
+            buildCommand.Execute()
+                .Should()
+                .Pass();
+        }
+
+        [RequiresMSBuildVersionFact("16.7.0")]
+        public void CanBuildProjectWithMultiplePackageReferencesWithAliases()
+        {
+            var targetFramework = "netcoreapp5.0";
+
+            var packageReferenceA = GetPackageReference(targetFramework, "A", ClassLibClassA);
+            var packageReferenceB = GetPackageReference(targetFramework, "B", ClassLibClassB);
+
+            TestProject testProject = new TestProject()
+            {
+                Name = "Project",
+                IsSdkProject = true,
+                IsExe = false,
+                TargetFrameworks = targetFramework,
+            };
+
+            testProject.PackageReferences.Add(
+               new TestPackageReference(
+                   packageReferenceA.ID,
+                   packageReferenceA.Version,
+                   packageReferenceA.NupkgPath,
+                   packageReferenceA.PrivateAssets,
+                   aliases: "First"));
+            testProject.PackageReferences.Add(
+               new TestPackageReference(
+                   packageReferenceB.ID,
+                   packageReferenceB.Version,
+                   packageReferenceB.NupkgPath,
+                   packageReferenceB.PrivateAssets,
+                   aliases: "Second"));
+
+            testProject.AdditionalProperties.Add("RestoreSources",
+                                     "$(RestoreSources);" + Path.GetDirectoryName(packageReferenceA.NupkgPath) + ";" + Path.GetDirectoryName(packageReferenceB.NupkgPath));
+
+            //  Use a test-specific packages folder
+            testProject.AdditionalProperties["RestorePackagesPath"] = @"$(MSBuildProjectDirectory)\..\pkg";
+            testProject.SourceFiles[$"{testProject.Name}.cs"] = ClassLibAandBUsage;
+            var testAsset = _testAssetsManager.CreateTestProject(testProject);
+
+            var buildCommand = new BuildCommand(Log, Path.Combine(testAsset.TestRoot, testProject.Name));
+            buildCommand.Execute()
+                .Should()
+                .Pass();
+        }
+
+        [RequiresMSBuildVersionFact("16.7.0")]
+        public void CanBuildProjectWithAPackageReferenceWithMultipleAliases()
+        {
+            var targetFramework = "netcoreapp5.0";
+
+            var packageReferenceA = GetPackageReference(targetFramework, "A", ClassLibMultipleClasses);
+
+            TestProject testProject = new TestProject()
+            {
+                Name = "Project",
+                IsSdkProject = true,
+                IsExe = false,
+                TargetFrameworks = targetFramework,
+            };
+
+            testProject.PackageReferences.Add(
+               new TestPackageReference(
+                   packageReferenceA.ID,
+                   packageReferenceA.Version,
+                   packageReferenceA.NupkgPath,
+                   packageReferenceA.PrivateAssets,
+                   aliases: "First,Second"));
+
+            testProject.AdditionalProperties.Add("RestoreSources",
+                                     "$(RestoreSources);" + Path.GetDirectoryName(packageReferenceA.NupkgPath));
+
+            //  Use a test-specific packages folder
+            testProject.AdditionalProperties["RestorePackagesPath"] = @"$(MSBuildProjectDirectory)\..\pkg";
+            testProject.SourceFiles[$"{testProject.Name}.cs"] = ClassLibAandBUsage;
+            var testAsset = _testAssetsManager.CreateTestProject(testProject);
+
+            var buildCommand = new BuildCommand(Log, Path.Combine(testAsset.TestRoot, testProject.Name));
+            buildCommand.Execute()
+                .Should()
+                .Pass();
+        }
+
+        private IEnumerable<TestPackageReference> GetPackageReferencesWithConflictingTypes(string targetFramework, params string[] packageNames)
+        {
+            foreach (var packageName in packageNames)
+            {
+                yield return GetPackageReference(targetFramework, packageName, ClassLibConflictingMethod);
+            }
+        }
+
+        private TestPackageReference GetPackageReference(string targetFramework, string packageName, string projectFileContent)
+        {
+            var project = GetProject(targetFramework, packageName, projectFileContent);
+            var packCommand = new PackCommand(Log, _testAssetsManager.CreateTestProject(project).TestRoot, packageName);
+
+            packCommand
+                .Execute()
+                .Should()
+                .Pass();
+            return new TestPackageReference(packageName, "1.0.0", packCommand.GetNuGetPackage(packageName));
+        }
+
+        private static TestProject GetProject(string targetFramework, string referenceProjectName, string projectFileContent)
+        {
+            var project = new TestProject()
+            {
+                Name = referenceProjectName,
+                TargetFrameworks = targetFramework,
+                IsSdkProject = true
+            };
+            project.SourceFiles[$"{referenceProjectName}.cs"] = projectFileContent;
+            return project;
+        }
+
+        private static string ClassLibConflictingMethod = @"
+using System;
+public class ClassLib
+{
+    public void ConflictingMethod()
+    {
+    }
+}
+";
+
+        private static string ClassLibClassA = @"
+using System;
+public class A
+{
+    public void AMethod()
+    {
+    }
+}
+";
+
+        private static string ClassLibMultipleClasses = @"
+using System;
+public class A
+{
+    public void AMethod()
+    {
+    }
+}
+
+public class B
+{
+    public void BMethod()
+    {
+    }
+}
+";
+
+        private static string ClassLibClassB = @"
+using System;
+public class B
+{
+    public void BMethod()
+    {
+    }
+}
+";
+
+        private static string ClassLibAandBUsage = @"
+extern alias First;
+extern alias Second;
+using System;
+public class ClassLibUsage
+{
+    public void UsageMethod()
+    {
+        new First.A().AMethod();
+        new Second.B().BMethod();
+    }
+}
+";
+
+        private static string ConflictingClassLibUsage = @"
+extern alias Special;
+using System;
+public class ClassLibUsage
+{
+    public void UsageMethod()
+    {
+        new Special.ClassLib().ConflictingMethod();
+    }
+}
+";
+    }
+}

src/Tests/Microsoft.NET.TestFramework/ProjectConstruction/TestProject.cs

@@ -154,6 +154,10 @@ internal void Create(TestAsset targetTestAsset, string testProjectsSourceFolder,
                 {
                     packageReferenceElement.Add(new XAttribute("PrivateAssets", packageReference.PrivateAssets));
                 }
+                if (packageReference.Aliases != null)
+                {
+                    packageReferenceElement.Add(new XAttribute("Aliases", packageReference.Aliases));
+                }
                 packageReferenceItemGroup.Add(packageReferenceElement);
             }
 

src/Tests/Microsoft.NET.TestFramework/TestPackageReference.cs

@@ -11,19 +11,20 @@ namespace Microsoft.NET.TestFramework
 {
     public class TestPackageReference
     {
-        public TestPackageReference(string id, string version = null, string nupkgPath = null, string privateAssets = null)
+        public TestPackageReference(string id, string version = null, string nupkgPath = null, string privateAssets = null, string aliases = null)
         {
             ID = id;
             Version = version;
             NupkgPath = nupkgPath;
             PrivateAssets = privateAssets;
+            Aliases = aliases;
         }
 
         public string ID { get; private set; }
         public string Version { get; private set; }
         public string NupkgPath { get; private set; }
         public string PrivateAssets { get; private set; }
-
+        public string Aliases { get; private set; }
         public bool NuGetPackageExists()
         {
             return File.Exists(Path.Combine(this.NupkgPath, String.Concat(this.ID + "." + this.Version + ".nupkg")));