Fix ASP.NET WebSite projects to copy netstandard.dll facade when required

documentation/wiki/ChangeWaves.md

@@ -29,6 +29,9 @@ Change wave checks around features will be removed in the release that accompani
 
 ## Current Rotation of Change Waves
 
+### 18.7
+- [Fix ASP.NET WebSite projects to resolve netstandard2.0 dependencies](https://github.com/dotnet/msbuild/pull/13058) - Pass TargetFrameworkVersion to RAR task and copy netstandard.dll facade for .NET Framework 4.7.1+ web projects.
+
 ### 18.6
 - [AbsolutePath.GetCanonicalForm optimization - avoid expensive Path.GetFullPath calls when paths don't need canonicalization](https://github.com/dotnet/msbuild/pull/13369)
 - [TaskHostTask forwards request-level global properties (e.g. MSBuildRestoreSessionId) to out-of-proc TaskHost in -mt mode](https://github.com/dotnet/msbuild/pull/13443)

src/Build/Construction/Solution/SolutionProjectGenerator.cs

@@ -1,4 +1,4 @@
-// Licensed to the .NET Foundation under one or more agreements.
+// Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System;
@@ -418,6 +418,7 @@ private static void AddTasksToCopyAllDependenciesIntoBinDir(
             string copyLocalFilesItemName = referenceItemName + "_CopyLocalFiles";
             string targetFrameworkDirectoriesName = GenerateSafePropertyName(project, "_TargetFrameworkDirectories");
             string fullFrameworkRefAssyPathName = GenerateSafePropertyName(project, "_FullFrameworkReferenceAssemblyPaths");
+            string dependsOnNetStandardPropertyName = GenerateSafePropertyName(project, "_DependsOnNETStandard");
             string destinationFolder = String.Format(CultureInfo.InvariantCulture, @"$({0})\Bin\", GenerateSafePropertyName(project, "AspNetPhysicalPath"));
 
             // This is a bit of a hack.  We're actually calling the "Copy" task on all of
@@ -451,15 +452,61 @@ private static void AddTasksToCopyAllDependenciesIntoBinDir(
             rarTask.SetParameter("FindSerializationAssemblies", "true");
             rarTask.SetParameter("FindRelatedFiles", "true");
             rarTask.SetParameter("TargetFrameworkMoniker", project.TargetFrameworkMoniker);
+
+            if (ChangeWaves.AreFeaturesEnabled(ChangeWaves.Wave18_7))
+            {
+                // Parse the target framework version to determine if we should pass it to RAR.
+                // Only pass TargetFrameworkVersion for .NET Framework 4.7.1+ which has netstandard 2.0 support.
+                // For older frameworks, omitting this preserves the existing warning behavior (MSB3268)
+                // when netstandard dependencies cannot be resolved.
+                try
+                {
+                    var frameworkName = new FrameworkName(project.TargetFrameworkMoniker);
+                    var minVersionForNetstandard = new Version(4, 7, 1);
+
+                    if (frameworkName.Version >= minVersionForNetstandard)
+                    {
+                        // Pass TargetFrameworkVersion so RAR can find netstandard facades
+                        rarTask.SetParameter("TargetFrameworkVersion", $"v{frameworkName.Version}");
+                        // Pass web.config as AppConfigFile so RAR can detect assembly binding redirects
+                        rarTask.SetParameter("AppConfigFile", "$(WebConfigFileName)");
+                    }
+                }
+                catch (ArgumentException)
+                {
+                    // If the TargetFrameworkMoniker is malformed, skip setting TargetFrameworkVersion/AppConfigFile.
+                    // This preserves existing behavior while avoiding an unhandled exception.
+                }
+            }
+
             rarTask.AddOutputItem("CopyLocalFiles", copyLocalFilesItemName, null);
 
+            if (ChangeWaves.AreFeaturesEnabled(ChangeWaves.Wave18_7))
+            {
+                // Capture whether RAR detected a dependency on netstandard
+                rarTask.AddOutputProperty("DependsOnNETStandard", dependsOnNetStandardPropertyName, null);
+            }
+
             // Copy all the copy-local files (reported by RAR) to the web project's "bin"
             // directory.
             ProjectTaskInstance copyTask = target.AddTask("Copy", conditionDescribingValidConfigurations, null);
             copyTask.SetParameter("SourceFiles", "@(" + copyLocalFilesItemName + ")");
+            copyTask.SetParameter("SkipUnchangedFiles", "true");
             copyTask.SetParameter(
                 "DestinationFiles",
                 String.Format(CultureInfo.InvariantCulture, @"@({0}->'{1}%(DestinationSubDirectory)%(Filename)%(Extension)')", copyLocalFilesItemName, destinationFolder));
+
+            if (ChangeWaves.AreFeaturesEnabled(ChangeWaves.Wave18_7))
+            {
+                // If any references depend on netstandard, copy netstandard.dll from the Facades folder.
+                // .NET Framework 4.7.1+ has netstandard 2.0 support in the Facades folder.
+                string netstandardFacadePath = String.Format(CultureInfo.InvariantCulture, @"$({0})Facades\netstandard.dll", targetFrameworkDirectoriesName);
+                string copyFacadesCondition = String.Format(CultureInfo.InvariantCulture, "'$({0})' == 'true' AND Exists('{1}')", dependsOnNetStandardPropertyName, netstandardFacadePath);
+                ProjectTaskInstance copyFacadesTask = target.AddTask("Copy", copyFacadesCondition, null);
+                copyFacadesTask.SetParameter("SourceFiles", netstandardFacadePath);
+                copyFacadesTask.SetParameter("SkipUnchangedFiles", "true");
+                copyFacadesTask.SetParameter("DestinationFolder", destinationFolder);
+            }
         }
 
         /// <summary>
@@ -1242,6 +1289,16 @@ private ProjectInstance CreateMetaproject(ProjectInstance traversalProject, Proj
                     "AspNetCompiler.UnsupportedMSBuildVersion",
                     project.ProjectName);
 #else
+                // Set WebConfigFileName property if web.config exists (for RAR AppConfigFile parameter)
+                if (ChangeWaves.AreFeaturesEnabled(ChangeWaves.Wave18_7))
+                {
+                    string webConfigPath = Path.Combine(project.AbsolutePath, "web.config");
+                    if (File.Exists(webConfigPath))
+                    {
+                        metaprojectInstance.SetProperty("WebConfigFileName", webConfigPath);
+                    }
+                }
+
                 AddMetaprojectTargetForWebProject(traversalProject, metaprojectInstance, project, null);
                 AddMetaprojectTargetForWebProject(traversalProject, metaprojectInstance, project, "Clean");
                 AddMetaprojectTargetForWebProject(traversalProject, metaprojectInstance, project, "Rebuild");

src/Framework/ChangeWaves.cs

@@ -34,7 +34,8 @@ internal static class ChangeWaves
         internal static readonly Version Wave18_4 = new Version(18, 4);
         internal static readonly Version Wave18_5 = new Version(18, 5);
         internal static readonly Version Wave18_6 = new Version(18, 6);
-        internal static readonly Version[] AllWaves = [Wave17_10, Wave17_12, Wave17_14, Wave18_3, Wave18_4, Wave18_5, Wave18_6];
+        internal static readonly Version Wave18_7 = new Version(18, 7);
+        internal static readonly Version[] AllWaves = [Wave17_10, Wave17_12, Wave17_14, Wave18_3, Wave18_4, Wave18_5, Wave18_6, Wave18_7];
 
         /// <summary>
         /// Special value indicating that all features behind all Change Waves should be enabled.