Use IReadOnlyCollection.

global.json

@@ -1,5 +1,5 @@
 {
   "msbuild-sdks": {
-    "WixToolset.Sdk": "4.0.0-build-0209"
+    "WixToolset.Sdk": "4.0.0-build-0210"
   }
 }

src/wixext/NetfxExtensionFactory.cs

@@ -1,4 +1,4 @@
-// Copyright (c) .NET Foundation and contributors. All rights reserved. Licensed under the Microsoft Reciprocal License. See LICENSE.TXT file in the project root for full license information.
+// Copyright (c) .NET Foundation and contributors. All rights reserved. Licensed under the Microsoft Reciprocal License. See LICENSE.TXT file in the project root for full license information.
 
 namespace WixToolset.Netfx
 {
@@ -8,7 +8,7 @@ namespace WixToolset.Netfx
 
     public class NetfxExtensionFactory : BaseExtensionFactory
     {
-        protected override IEnumerable<Type> ExtensionTypes => new[]
+        protected override IReadOnlyCollection<Type> ExtensionTypes => new[]
         {
             typeof(NetfxCompiler),
             typeof(NetfxExtensionData),

src/wixext/NetfxWindowsInstallerBackendExtension.cs

@@ -8,6 +8,6 @@ namespace WixToolset.Netfx
 
     public class NetfxWindowsInstallerBackendBinderExtension : BaseWindowsInstallerBackendBinderExtension
     {
-        public override IEnumerable<TableDefinition> TableDefinitions => NetfxTableDefinitions.All;
+        public override IReadOnlyCollection<TableDefinition> TableDefinitions => NetfxTableDefinitions.All;
     }
 }