WiX v5 uses wixext5 NuGet package subdirectory.

src/ext/WixExt.props

@@ -10,7 +10,7 @@
 
   <ItemGroup>
     <Content Include="$(MSBuildProjectName).targets" PackagePath="build" />
-    <Content Include="$(TargetPath)" PackagePath="wixext4" />
+    <Content Include="$(TargetPath)" PackagePath="wixext5" />
 
     <PackageReference Include="WixToolset.Extensibility" PrivateAssets="all" />
   </ItemGroup>

src/wix/WixToolset.Core.ExtensionCache/ExtensionCacheManager.cs

@@ -29,12 +29,16 @@ public ExtensionCacheManager(IMessaging messaging, IExtensionManager extensionMa
         {
             this.Messaging = messaging;
             this.ExtensionManager = extensionManager;
+
+            this.WixVersion = typeof(ExtensionCacheManager).Assembly.GetName().Version.Major.ToString();
         }
 
         private IMessaging Messaging { get; }
 
         private IExtensionManager ExtensionManager { get; }
 
+        public string WixVersion { get; }
+
         public async Task<bool> AddAsync(bool global, string extension, CancellationToken cancellationToken)
         {
             if (String.IsNullOrEmpty(extension))
@@ -212,11 +216,16 @@ private async Task<bool> DownloadAndExtractAsync(bool global, string id, string
                         {
                             stream.Position = 0;
 
-                            Directory.CreateDirectory(extensionFolder);
-
                             using (var archive = new PackageArchiveReader(stream))
                             {
                                 var files = archive.GetFiles(extensionPackageRootFolderName);
+                                if (!files.Any())
+                                {
+                                    this.Messaging.Write(ExtensionCacheWarnings.MissingExtensionPackageRootFolder(id, nugetVersion.ToString(), extensionPackageRootFolderName, this.WixVersion));
+                                    return false;
+                                }
+
+                                Directory.CreateDirectory(extensionFolder);
                                 await archive.CopyFilesAsync(extensionFolder, files, this.ExtractProgress, logger, cancellationToken);
                             }
 
@@ -266,6 +275,7 @@ private bool ExtensionFileExists(string baseFolder, string extensionId, string e
             var extensionFolder = Path.Combine(baseFolder, extensionId, extensionVersion, packageRootFolderName);
             if (!Directory.Exists(extensionFolder))
             {
+                this.Messaging.Write(ExtensionCacheWarnings.MissingExtensionPackageRootFolder(extensionId, extensionVersion, packageRootFolderName, this.WixVersion));
                 return false;
             }
 

src/wix/WixToolset.Core.ExtensionCache/ExtensionCacheWarnings.cs

@@ -11,6 +11,11 @@ public static Message NugetException(string extensionId, string exceptionMessage
             return Message(new SourceLineNumber(extensionId), Ids.NugetException, "{0}", exceptionMessage);
         }
 
+        public static Message MissingExtensionPackageRootFolder(string extensionId, string packageVersion, string packageRootFolderName, string wixVersion)
+        {
+            return Message(new SourceLineNumber(extensionId), Ids.MissingExtensionPackageRootFolder, "Could not find expected package root folder {0}. Ensure {1}/{2} is compatible with WiX v{3}.", packageRootFolderName, extensionId, packageVersion, wixVersion);
+        }
+
         private static Message Message(SourceLineNumber sourceLineNumber, Ids id, string format, params object[] args)
         {
             return new Message(sourceLineNumber, MessageLevel.Warning, (int)id, format, args);
@@ -19,6 +24,7 @@ private static Message Message(SourceLineNumber sourceLineNumber, Ids id, string
         public enum Ids
         {
             NugetException = 6100,
+            MissingExtensionPackageRootFolder = 6101,
         } // last available is 6499. 6500 is ExtensionCacheErrors.
     }
 }

src/wix/WixToolset.Core/ExtensibilityServices/ExtensionManager.cs

@@ -15,8 +15,8 @@ namespace WixToolset.Core.ExtensibilityServices
 
     internal class ExtensionManager : IExtensionManager
     {
-        // This value needs to stay in sync with the Property in "wix.targets" with the same name.
-        private const string WixToolsetExtensionPackageFolder = "wixext4";
+        // This value needs to stay in sync with the Property in "wix.props" with the same name.
+        private const string WixToolsetExtensionPackageFolder = "wixext5";
 
         private const string UserWixFolderName = ".wix";
         private const string MachineWixFolderName = "WixToolset";

src/wix/WixToolset.Sdk/tools/wix.props

@@ -9,7 +9,7 @@
     <WixToolsetPatchVersion>$$WixToolsetPatchVersion$$</WixToolsetPatchVersion>
     <WixToolsetMajorMinorVersion>$$WixToolsetMajorMinorVersion$$</WixToolsetMajorMinorVersion>
     <WixToolsetVersion>$$WixToolsetVersion$$</WixToolsetVersion>
-    <WixToolsetExtensionPackageFolder>wixext4</WixToolsetExtensionPackageFolder>
+    <WixToolsetExtensionPackageFolder>wixext5</WixToolsetExtensionPackageFolder>
   </PropertyGroup>
 
   <ImportGroup>

src/wix/test/WixToolsetTest.CoreIntegration/ExtensionFixture.cs

@@ -12,6 +12,8 @@ namespace WixToolsetTest.CoreIntegration
     using WixToolset.Data;
     using WixToolset.Data.Symbols;
     using Xunit;
+    using System.Diagnostics;
+    using System.Reflection;
 
     public class ExtensionFixture
     {
@@ -290,7 +292,7 @@ public void CannotBuildWithMissingVersionedExtension()
             }
         }
 
-        [Fact]
+        [Fact(Skip = "Depends on a v5 extension being available, which isn't true for nuget.org yet or this early in the build.")]
         public void CanManipulateExtensionCache()
         {
             var currentFolder = Environment.CurrentDirectory;
@@ -304,7 +306,7 @@ public void CanManipulateExtensionCache()
 
                     var result = WixRunner.Execute(new[]
                     {
-                        "extension", "add", "WixToolset.UI.wixext"
+                        "extension", "add", "WixToolset.UI.wixext",
                     });
 
                     result.AssertSuccess();
@@ -318,8 +320,12 @@ public void CanManipulateExtensionCache()
                     });
 
                     result.AssertSuccess();
+
                     var output = result.Messages.Select(m => m.ToString()).Single();
-                    Assert.StartsWith("WixToolset.UI.wixext 4.", output);
+                    var executingAssembly = Assembly.GetExecutingAssembly();
+                    var fileVersion = FileVersionInfo.GetVersionInfo(executingAssembly.Location);
+
+                    Assert.StartsWith($"WixToolset.UI.wixext {fileVersion.FileMajorPart}", output);
                     Assert.DoesNotContain("damaged", output);
 
                     result = WixRunner.Execute(new[]
@@ -337,6 +343,33 @@ public void CanManipulateExtensionCache()
             }
         }
 
+        [Fact]
+        public void TryingToAddAV4ExtensionFails()
+        {
+            var currentFolder = Environment.CurrentDirectory;
+
+            try
+            {
+                using (var fs = new DisposableFileSystem())
+                {
+                    var folder = fs.GetFolder(true);
+                    Environment.CurrentDirectory = folder;
+
+                    var result = WixRunner.Execute(new[]
+                    {
+                        "extension", "add", "WixToolset.UI.wixext/4.0.4",
+                    });
+
+                    Assert.Equal(2, result.ExitCode);
+                    Assert.Equal(6101, result.Messages.Single().Id);
+                }
+            }
+            finally
+            {
+                Environment.CurrentDirectory = currentFolder;
+            }
+        }
+
         private static void Build(string[] args)
         {
             var result = WixRunner.Execute(args);