[illink] Type registrations fallback switch

src/Mono.Android/ILLink/ILLink.Substitutions.xml

@@ -0,0 +1,10 @@
+<linker>
+  <assembly fullname="Mono.Android">
+    <type fullname="Java.Interop.TypeManager" feature="Java.Interop.TypeManager.TypeRegistrationFallbackIsEnabled" featurevalue="false">
+      <method signature="System.Boolean get_TypeRegistrationFallbackIsEnabled()" body="stub" value="false" />
+      <method signature="System.Type TypeRegistrationFallback()" body="stub" value="null" />
+      <method signature="System.Void RegisterPackage(System.String,System.Converter`2&lt;System.String,System.Type&gt;)" body="stub" />
+      <method signature="System.Void RegisterPackages(System.String[],System.Converter`2&lt;System.String,System.Type&gt;[])" body="stub" />
+    </type>
+  </assembly>
+</linker>

src/Mono.Android/Java.Interop/TypeManager.cs

@@ -212,6 +212,10 @@ static Exception CreateJavaLocationException ()
 		[MethodImplAttribute(MethodImplOptions.InternalCall)]
 		static extern Type monodroid_typemap_java_to_managed (string java_type_name);
 
+		static bool TypeRegistrationFallbackIsEnabled { get; } = InitializeTypeRegistrationFallbackIsEnabled ();
+		static bool InitializeTypeRegistrationFallbackIsEnabled () =>
+		    !AppContext.TryGetSwitch ("Java.Interop.TypeManager.TypeRegistrationFallbackIsEnabled", out bool isEnabled) || isEnabled;
+
 		internal static Type? GetJavaToManagedType (string class_name)
 		{
 			Type? type = monodroid_typemap_java_to_managed (class_name);
@@ -225,10 +229,18 @@ static Exception CreateJavaLocationException ()
 				return null;
 			}
 
+			if (TypeRegistrationFallbackIsEnabled)
+				return TypeRegistrationFallback (class_name);
+
+			return null;
+		}
+
+		internal static Type? TypeRegistrationFallback (string class_name)
+		{
 			__TypeRegistrations.RegisterPackages ();
 
-			type = null;
-			int ls      = class_name.LastIndexOf ('/');
+			Type? type = null;
+			int ls = class_name.LastIndexOf ('/');
 			var package = ls >= 0 ? class_name.Substring (0, ls) : "";
 			if (packageLookup.TryGetValue (package, out var mappers)) {
 				foreach (Converter<string, Type?> c in mappers) {
@@ -353,10 +365,18 @@ public static void RegisterType (string java_class, Type t)
 			}
 		}
 
-		static Dictionary<string, List<Converter<string, Type?>>> packageLookup = new Dictionary<string, List<Converter<string, Type?>>> ();
+		static Dictionary<string, List<Converter<string, Type?>>>? packageLookup;
+
+		static void LazyInitPackageLookup ()
+		{
+			if (packageLookup == null)
+				packageLookup = new Dictionary<string, List<Converter<string, Type?>>> (StringComparer.Ordinal);
+		}
 
 		public static void RegisterPackage (string package, Converter<string, Type> lookup)
 		{
+			LazyInitPackageLookup ();
+
 			lock (packageLookup) {
 				if (!packageLookup.TryGetValue (package, out var lookups))
 					packageLookup.Add (package, lookups = new List<Converter<string, Type?>> ());
@@ -366,6 +386,8 @@ public static void RegisterPackage (string package, Converter<string, Type> look
 
 		public static void RegisterPackages (string[] packages, Converter<string, Type?>[] lookups)
 		{
+			LazyInitPackageLookup ();
+
 			if (packages == null)
 				throw new ArgumentNullException ("packages");
 			if (lookups == null)

src/Mono.Android/Mono.Android.csproj

@@ -97,6 +97,9 @@
     <EmbeddedResource Include="ILLink/ILLink.LinkAttributes.xml">
       <LogicalName>ILLink.LinkAttributes.xml</LogicalName>
     </EmbeddedResource>
+    <EmbeddedResource Include="ILLink/ILLink.Substitutions.xml">
+      <LogicalName>ILLink.Substitutions.xml</LogicalName>
+    </EmbeddedResource>
   </ItemGroup>
 
   <ItemGroup>

src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.ILLink.targets

@@ -22,13 +22,18 @@ This file contains the .NET 5-specific targets to customize ILLink
     <UseSystemResourceKeys Condition="'$(UseSystemResourceKeys)' == ''">true</UseSystemResourceKeys>
     <HttpActivityPropagationSupport Condition="'$(HttpActivityPropagationSupport)' == ''">false</HttpActivityPropagationSupport>
     <StartupHookSupport Condition="'$(StartupHookSupport)' == ''">false</StartupHookSupport>
+    <XATypeRegistrationFallback Condition="'$(XATypeRegistrationFallback)' == ''">false</XATypeRegistrationFallback>
   </PropertyGroup>
 
   <Target Name="_PrepareLinking"
       Condition=" '$(PublishTrimmed)' == 'true' "
       AfterTargets="ComputeResolvedFilesToPublishList"
       DependsOnTargets="GetReferenceAssemblyPaths;_CreatePropertiesCache">
     <ItemGroup>
+      <RuntimeHostConfigurationOption Include="Java.Interop.TypeManager.TypeRegistrationFallbackIsEnabled"
+          Condition="'$(XATypeRegistrationFallback)' != ''"
+          Value="$(XATypeRegistrationFallback)"
+          Trim="true" />
       <!-- Mark all assemblies to be linked for AndroidLinkMode=Full -->
       <ResolvedFileToPublish
           Update="@(ResolvedFileToPublish)"

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/Tasks/LinkerTests.cs

@@ -394,5 +394,28 @@ public unsafe bool MyMethod (Android.OS.IBinder windowToken, [global::Android.Ru
 				}
 			}
 		}
+
+		[Test]
+		public void TypeRegistrationsFallback ([Values (true, false)] bool enabled)
+		{
+			if (!Builder.UseDotNet)
+				Assert.Ignore ("Test only valid on .NET 6");
+
+			var proj = new XamarinAndroidApplicationProject () { IsRelease = true };
+			if (enabled)
+				proj.SetProperty (proj.ActiveConfigurationProperties, "XATypeRegistrationFallback", "true");
+
+			using (var b = CreateApkBuilder ()) {
+				Assert.IsTrue (b.Build (proj), "Build should have succeeded.");
+				var assemblyFile = "Mono.Android.dll";
+				var assemblyPath = BuildTest.GetLinkedPath (b, true, assemblyFile);
+				using (var assembly = AssemblyDefinition.ReadAssembly (assemblyPath)) {
+					Assert.IsTrue (assembly != null);
+
+					var td = assembly.MainModule.GetType ("Java.Interop.__TypeRegistrations");
+					Assert.IsTrue ((td != null) == enabled);
+				}
+			}
+		}
 	}
 }