[typemaps] Handle Java to managed type maps properly (dotnet#4656)

Fixes: dotnet#4596
Fixes: dotnet#4660
Context: xamarin/monodroid@192b1f1
Context: https://github.com/xamarin/java.interop/blob/fc18c54b2ccf14f444fadac0a1e7e81f837cc470/src/Java.Interop.Tools.JavaCallableWrappers/Java.Interop.Tools.JavaCallableWrappers/TypeNameMapGenerator.cs#L149-L186
Context: https://github.com/xamarin/java.interop/blob/010161d1ef6625b762429334f02e7b15e1f7caae/src/Java.Interop.Tools.JavaCallableWrappers/Java.Interop.Tools.JavaCallableWrappers/TypeNameMapGenerator.cs#L155

~~ Debug Mode ~~

In some environments, `BundleTest.TestBundleIntegerArrayList2()` fails
when using the commercial shared runtime:

	Test 'Xamarin.Android.RuntimeTests.BundleTest.TestBundleIntegerArrayList2' failed: System.MemberAccessException : Cannot create an instance of Android.Runtime.JavaList`1[T] because Type.ContainsGenericParameters is true.
	  at System.Reflection.RuntimeConstructorInfo.DoInvoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture)
	  at System.Reflection.RuntimeConstructorInfo.Invoke (System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture)
	  at System.Reflection.ConstructorInfo.Invoke (System.Object[] parameters)
	  at Java.Interop.TypeManager.CreateProxy (System.Type type, System.IntPtr handle, Android.Runtime.JniHandleOwnership transfer)
	  at Java.Interop.TypeManager.CreateInstance (System.IntPtr handle, Android.Runtime.JniHandleOwnership transfer, System.Type targetType)
	  at Java.Lang.Object.GetObject (System.IntPtr handle, Android.Runtime.JniHandleOwnership transfer, System.Type type)
	  at Java.Lang.Object._GetObject[T] (System.IntPtr handle, Android.Runtime.JniHandleOwnership transfer)
	  at Java.Lang.Object.GetObject[T] (System.IntPtr handle, Android.Runtime.JniHandleOwnership transfer)
	  at Android.OS.Bundle.Get (System.String key)
	  at Xamarin.Android.RuntimeTests.BundleTest.TestBundleIntegerArrayList2 ()
	  at (wrapper managed-to-native) System.Reflection.RuntimeMethodInfo.InternalInvoke(System.Reflection.RuntimeMethodInfo,object,object[],System.Exception&)
	  at System.Reflection.RuntimeMethodInfo.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture)

The problem appears to be rooted in ce2bc68.  In a pre-ce2bc689 world,
there were two separate sets of "typemap" files that the application
could use:

  * Java-to-managed mappings, in `typemap.jm`
  * Managed-to-Java mappings, in `typemap.mj`.

These files did *not* need to have the same number of entries!

	% unzip /Library/Frameworks/Xamarin.Android.framework/Versions/10.2.0.100/lib/xamarin.android/xbuild/Xamarin/Android/Mono.Android.DebugRuntime-debug.apk typemap.jm
	% unzip /Library/Frameworks/Xamarin.Android.framework/Versions/10.2.0.100/lib/xamarin.android/xbuild/Xamarin/Android/Mono.Android.DebugRuntime-debug.apk typemap.mj
	% strings typemap.jm | wc -l
	   10318
	% strings typemap.mj | wc -l
	   11956

The reason why they have a different number of entries is *aliasing*:
there can be *multiple* bindings for a given Java type.  The
managed-to-Java mappings can contain *all* of them; the
Java-to-managed mapping *must* pick Only One.

For example, [`java.util.ArrayList`][0] contains (at least?) *three*
bindings:

  * [`Android.Runtime.JavaList`][1]
  * [`Android.Runtime.JavaList<T>`][2]
  * `Java.Util.ArrayList` (generated at build time)

In a pre-ce2bc689 world, this was "fine": we'd binary search each file
*separately* to find type mappings.

This changed in ce2bc68, as the typemap information was merged into a
*single* array in order to de-duplicate information.  This reduced
`.apk` size.

An unanticipated result of this is that the Java-to-managed mappings
can now contain *duplicate* keys, "as if" the original `typemap.jm`
had the contents:

	java/util/ArrayList
	Android.Runtime.JavaList, Mono.Android
	java/util/ArrayList
	Android.Runtime.JavaList`1, Mono.Android
	java/util/ArrayList
	Java.Util.ArrayLlist, Mono.Android

Whereas pre-ce2bc689 there would have only been the first entry.

"Sometimes" this duplication is "fine": the typemap search
"Just happens" to return the first entry, or the 3rd entry, and apps
continue to work as intended.

"Sometimes" it isn't: the typemap binary search finds the 2nd entry,
which is a generic type.  This results in attempting to instantiate a
generic type *without appropriate type parameters*, which results in
the aforementioned `MemberAccessException`.

Update typemap generation code in `Xamarin.Android.Build.Tasks.dll` so
that all the duplicate Java type names will point to the same managed
type name.  Additionally, make sure we select the managed type in the
same fashion the old typemap generator in `Java.Interop` worked: prefer
base types to the derived ones if the type is an interface or an
abstract class.  The effect of this change is that no matter which
entry `EmbeddedAssemblies::binary_search()` ends up selecting it will
always return the same managed type name for all aliased Java types.


~~ Release config apps ~~

Another issue introduced in ce2bc68 is that we no longer ignore
interface and generic types when generating the Java-to-managed maps.
This adversely affects the `Release` mode in which it is possible that
an attempt to instantiate an open generic type (or an interface) will
happen, leading to error similar to:

	FATAL EXCEPTION: main
	  Process: com.companyname.androidsingleviewapp1, PID: 24068
	  android.runtime.JavaProxyThrowable: System.MemberAccessException: Cannot create an instance of Com.Contoso.Javalibrary1.Class0`1[T] because Type.ContainsGenericParameters is true.
	    at System.Reflection.RuntimeConstructorInfo.DoInvoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture)
	    at System.Reflection.RuntimeConstructorInfo.Invoke (System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture)
	    at System.Reflection.ConstructorInfo.Invoke (System.Object[] parameters)
	    at Java.Interop.TypeManager.CreateProxy (System.Type type, System.IntPtr handle, Android.Runtime.JniHandleOwnership transfer)
	    at Java.Interop.TypeManager.CreateInstance (System.IntPtr handle, Android.Runtime.JniHandleOwnership transfer, System.Type targetType)
	    at Java.Lang.Object.GetObject (System.IntPtr handle, Android.Runtime.JniHandleOwnership transfer, System.Type type)
	    at Java.Lang.Object._GetObject[T] (System.IntPtr handle, Android.Runtime.JniHandleOwnership transfer)
	    at Java.Lang.Object.GetObject[T] (System.IntPtr handle, Android.Runtime.JniHandleOwnership transfer)
	    at Com.Contoso.Javalibrary1.Class1.Create ()
	    at AndroidSingleViewApp1.MainActivity.OnCreate (Android.OS.Bundle savedInstanceState)
	    at Android.App.Activity.n_OnCreate_Landroid_os_Bundle_ (System.IntPtr jnienv, System.IntPtr native__this, System.IntPtr native_savedInstanceState)
	    at (wrapper dynamic-method) Android.Runtime.DynamicMethodNameCounter.1(intptr,intptr,intptr)
	         at crc647d7dcab8da8ee782.MainActivity.n_onCreate(Native Method)
	         at crc647d7dcab8da8ee782.MainActivity.onCreate(MainActivity.java:29)
	         at android.app.Activity.performCreate(Activity.java:7144)
	         at android.app.Activity.performCreate(Activity.java:7135)
	         at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)
	         at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2931)
	         at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3086)
	         at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
	         at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
	         at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
	         at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1816)
	         at android.os.Handler.dispatchMessage(Handler.java:106)
	         at android.os.Looper.loop(Looper.java:193)
	         at android.app.ActivityThread.main(ActivityThread.java:6718)
	         at java.lang.reflect.Method.invoke(Native Method)
	         at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
	         at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

Update typemap generation code to also ignore generic types and
interfaces when generating the Java to Managed map in `Release` mode.
We must not simply skip outputting entries for such types because we
need to maintain element count parity between java-to-managed and
managed-to-java lookup tables.  Therefore, the way we skip such types
is to output a type token ID of `0` instead of the actual one.  This
will cause the runtime code to fail to find a mapping, thus allowing
the managed code to deal with such a type properly.


~~ Test Updates ~~

Update `BundleTest.TestBundleIntegerArrayList2()` so that it asserts
the runtime *type* of `obj` returned, asserting that it is in fact a
`JavaList` and not e.g. `Java.Util.ArrayList` or something.
(The linker could otherwise "change" things, and this assertion will
ensure that `JavaList` won't be linked away…)

Add a unit test for the Release config app bug in Issue dotnet#4660; thanks
to @brendanzagaeski.  It works by provoking the binary search bug by
"ensuring" that a *generic* type comes *before* the "normally
preferred" *non*-generic type.  Thus, even if/when binary search
returns the first entry, that would still be the *wrong entry*,
because that'll be a generic type!

[0]: https://developer.android.com/reference/java/util/ArrayList
[1]: https://github.com/xamarin/xamarin-android/blob/61f09bf2ef0c8f09550e2d77eb97f417432b315b/src/Mono.Android/Android.Runtime/JavaList.cs#L11-L13
[2]: https://github.com/xamarin/xamarin-android/blob/61f09bf2ef0c8f09550e2d77eb97f417432b315b/src/Mono.Android/Android.Runtime/JavaList.cs#L667-L668

src/Mono.Android/Test/Android.OS/BundleTest.cs

@@ -19,6 +19,7 @@ public void TestBundleIntegerArrayList()
 			Assert.NotNull (list, "'key' doesn't refer to a list of integers");
 			var obj = b.Get ("key");
 			Assert.NotNull (obj, "Missing 'key' in bundle");
+			Assert.IsTrue (obj is global::Android.Runtime.JavaList, "`obj` should be a JavaList!");
 			try {
 				list = b.GetIntegerArrayList ("key");
 				Assert.NotNull (list, "'key' doesn't refer to a list of integers after non-generic call");

src/Xamarin.Android.Build.Tasks/Tasks/GenerateJavaStubs.cs

@@ -174,7 +174,7 @@ void Run (DirectoryAssemblyResolver res)
 
 			// Step 2 - Generate type maps
 			//   Type mappings need to use all the assemblies, always.
-			WriteTypeMappings (allJavaTypes);
+			WriteTypeMappings (allJavaTypes, cache);
 
 			var javaTypes = new List<TypeDefinition> ();
 			foreach (TypeDefinition td in allJavaTypes) {
@@ -399,10 +399,10 @@ void SaveResource (string resource, string filename, string destDir, Func<string
 			MonoAndroidHelper.CopyIfStringChanged (template, Path.Combine (destDir, filename));
 		}
 
-		void WriteTypeMappings (List<TypeDefinition> types)
+		void WriteTypeMappings (List<TypeDefinition> types, TypeDefinitionCache cache)
 		{
 			var tmg = new TypeMapGenerator ((string message) => Log.LogDebugMessage (message), SupportedAbis);
-			if (!tmg.Generate (Debug, SkipJniAddNativeMethodRegistrationAttributeScan, types, TypemapOutputDirectory, GenerateNativeAssembly, out ApplicationConfigTaskState appConfState))
+			if (!tmg.Generate (Debug, SkipJniAddNativeMethodRegistrationAttributeScan, types, cache, TypemapOutputDirectory, GenerateNativeAssembly, out ApplicationConfigTaskState appConfState))
 				throw new XamarinAndroidException (4308, Properties.Resources.XA4308);
 			GeneratedBinaryTypeMaps = tmg.GeneratedBinaryTypeMaps.ToArray ();
 			BuildEngine4.RegisterTaskObject (ApplicationConfigTaskState.RegisterTaskObjectKey, appConfState, RegisteredTaskObjectLifetime.Build, allowEarlyCollection: false);

src/Xamarin.Android.Build.Tasks/Utilities/TypeMapGenerator.cs

@@ -4,6 +4,7 @@
 using System.Linq;
 using System.Text;
 
+using Java.Interop.Tools.Cecil;
 using Mono.Cecil;
 
 namespace Xamarin.Android.Tasks
@@ -14,6 +15,7 @@ class TypeMapGenerator
 		const string TypeMapIndexMagicString = "XATI"; // Xamarin Android Typemap Index
 		const uint TypeMapFormatVersion = 2; // Keep in sync with the value in src/monodroid/jni/xamarin-app.hh
 		const string TypemapExtension = ".typemap";
+		const uint InvalidJavaToManagedMappingIndex = UInt32.MaxValue;
 
 		internal sealed class ModuleUUIDArrayComparer : IComparer<ModuleReleaseData>
 		{
@@ -52,6 +54,7 @@ internal sealed class TypeMapReleaseEntry
 			public uint Token;
 			public int AssemblyNameIndex = -1;
 			public int ModuleIndex = -1;
+			public bool SkipInJavaToManaged;
 		}
 
 		internal sealed class ModuleReleaseData
@@ -76,6 +79,8 @@ internal sealed class TypeMapDebugEntry
 			public string ManagedLabel;
 			public int JavaIndex;
 			public int ManagedIndex;
+			public TypeDefinition TypeDefinition;
+			public bool SkipInJavaToManaged;
 		}
 
 		// Widths include the terminating nul character but not the padding!
@@ -87,6 +92,7 @@ internal sealed class ModuleDebugData
 			public List<TypeMapDebugEntry> JavaToManagedMap;
 			public List<TypeMapDebugEntry> ManagedToJavaMap;
 			public string OutputFilePath;
+			public string ModuleName;
 			public byte[] ModuleNameBytes;
 		}
 
@@ -125,7 +131,7 @@ void UpdateApplicationConfig (TypeDefinition javaType, ApplicationConfigTaskStat
 			}
 		}
 
-		public bool Generate (bool debugBuild, bool skipJniAddNativeMethodRegistrationAttributeScan, List<TypeDefinition> javaTypes, string outputDirectory, bool generateNativeAssembly, out ApplicationConfigTaskState appConfState)
+		public bool Generate (bool debugBuild, bool skipJniAddNativeMethodRegistrationAttributeScan, List<TypeDefinition> javaTypes, TypeDefinitionCache cache, string outputDirectory, bool generateNativeAssembly, out ApplicationConfigTaskState appConfState)
 		{
 			if (String.IsNullOrEmpty (outputDirectory))
 				throw new ArgumentException ("must not be null or empty", nameof (outputDirectory));
@@ -140,25 +146,26 @@ public bool Generate (bool debugBuild, bool skipJniAddNativeMethodRegistrationAt
 			string typemapsOutputDirectory = Path.Combine (outputDirectory, "typemaps");
 
 			if (debugBuild) {
-				return GenerateDebug (skipJniAddNativeMethodRegistrationAttributeScan, javaTypes, typemapsOutputDirectory, generateNativeAssembly, appConfState);
+				return GenerateDebug (skipJniAddNativeMethodRegistrationAttributeScan, javaTypes, cache, typemapsOutputDirectory, generateNativeAssembly, appConfState);
 			}
 
 			return GenerateRelease (skipJniAddNativeMethodRegistrationAttributeScan, javaTypes, typemapsOutputDirectory, appConfState);
 		}
 
-		bool GenerateDebug (bool skipJniAddNativeMethodRegistrationAttributeScan, List<TypeDefinition> javaTypes, string outputDirectory, bool generateNativeAssembly, ApplicationConfigTaskState appConfState)
+		bool GenerateDebug (bool skipJniAddNativeMethodRegistrationAttributeScan, List<TypeDefinition> javaTypes, TypeDefinitionCache cache, string outputDirectory, bool generateNativeAssembly, ApplicationConfigTaskState appConfState)
 		{
 			if (generateNativeAssembly)
-				return GenerateDebugNativeAssembly (skipJniAddNativeMethodRegistrationAttributeScan, javaTypes, outputDirectory, appConfState);
-			return GenerateDebugFiles (skipJniAddNativeMethodRegistrationAttributeScan, javaTypes, outputDirectory, appConfState);
+				return GenerateDebugNativeAssembly (skipJniAddNativeMethodRegistrationAttributeScan, javaTypes, cache, outputDirectory, appConfState);
+			return GenerateDebugFiles (skipJniAddNativeMethodRegistrationAttributeScan, javaTypes, cache, outputDirectory, appConfState);
 		}
 
-		bool GenerateDebugFiles (bool skipJniAddNativeMethodRegistrationAttributeScan, List<TypeDefinition> javaTypes, string outputDirectory, ApplicationConfigTaskState appConfState)
+		bool GenerateDebugFiles (bool skipJniAddNativeMethodRegistrationAttributeScan, List<TypeDefinition> javaTypes, TypeDefinitionCache cache, string outputDirectory, ApplicationConfigTaskState appConfState)
 		{
 			var modules = new Dictionary<string, ModuleDebugData> (StringComparer.Ordinal);
 			int maxModuleFileNameWidth = 0;
 			int maxModuleNameWidth = 0;
 
+			var javaDuplicates = new Dictionary<string, List<TypeMapDebugEntry>> (StringComparer.Ordinal);
 			foreach (TypeDefinition td in javaTypes) {
 				UpdateApplicationConfig (td, appConfState);
 				string moduleName = td.Module.Assembly.Name.Name;
@@ -173,7 +180,8 @@ bool GenerateDebugFiles (bool skipJniAddNativeMethodRegistrationAttributeScan, L
 						JavaToManagedMap = new List<TypeMapDebugEntry> (),
 						ManagedToJavaMap = new List<TypeMapDebugEntry> (),
 						OutputFilePath = Path.Combine (outputDirectory, outputFileName),
-						ModuleNameBytes = outputEncoding.GetBytes (moduleName)
+						ModuleName = moduleName,
+						ModuleNameBytes = outputEncoding.GetBytes (moduleName),
 					};
 
 					if (module.ModuleNameBytes.Length > maxModuleNameWidth)
@@ -186,6 +194,7 @@ bool GenerateDebugFiles (bool skipJniAddNativeMethodRegistrationAttributeScan, L
 				}
 
 				TypeMapDebugEntry entry = GetDebugEntry (td);
+				HandleDebugDuplicates (javaDuplicates, entry, td, cache);
 				if (entry.JavaName.Length > module.JavaNameWidth)
 					module.JavaNameWidth = (uint)entry.JavaName.Length + 1;
 
@@ -195,6 +204,7 @@ bool GenerateDebugFiles (bool skipJniAddNativeMethodRegistrationAttributeScan, L
 				module.JavaToManagedMap.Add (entry);
 				module.ManagedToJavaMap.Add (entry);
 			}
+			SyncDebugDuplicates (javaDuplicates);
 
 			foreach (ModuleDebugData module in modules.Values) {
 				PrepareDebugMaps (module);
@@ -217,18 +227,22 @@ bool GenerateDebugFiles (bool skipJniAddNativeMethodRegistrationAttributeScan, L
 			return true;
 		}
 
-		bool GenerateDebugNativeAssembly (bool skipJniAddNativeMethodRegistrationAttributeScan, List<TypeDefinition> javaTypes, string outputDirectory, ApplicationConfigTaskState appConfState)
+		bool GenerateDebugNativeAssembly (bool skipJniAddNativeMethodRegistrationAttributeScan, List<TypeDefinition> javaTypes, TypeDefinitionCache cache, string outputDirectory, ApplicationConfigTaskState appConfState)
 		{
 			var javaToManaged = new List<TypeMapDebugEntry> ();
 			var managedToJava = new List<TypeMapDebugEntry> ();
 
+			var javaDuplicates = new Dictionary<string, List<TypeMapDebugEntry>> (StringComparer.Ordinal);
 			foreach (TypeDefinition td in javaTypes) {
 				UpdateApplicationConfig (td, appConfState);
 
 				TypeMapDebugEntry entry = GetDebugEntry (td);
+				HandleDebugDuplicates (javaDuplicates, entry, td, cache);
+
 				javaToManaged.Add (entry);
 				managedToJava.Add (entry);
 			}
+			SyncDebugDuplicates (javaDuplicates);
 
 			var data = new ModuleDebugData {
 				EntryCount = (uint)javaToManaged.Count,
@@ -246,6 +260,39 @@ bool GenerateDebugNativeAssembly (bool skipJniAddNativeMethodRegistrationAttribu
 			return true;
 		}
 
+		void SyncDebugDuplicates (Dictionary<string, List<TypeMapDebugEntry>> javaDuplicates)
+		{
+			foreach (List<TypeMapDebugEntry> duplicates in javaDuplicates.Values) {
+				if (duplicates.Count < 2) {
+					continue;
+				}
+
+				TypeMapDebugEntry template = duplicates [0];
+				for (int i = 1; i < duplicates.Count; i++) {
+					duplicates[i].TypeDefinition = template.TypeDefinition;
+					duplicates[i].ManagedName = template.ManagedName;
+				}
+			}
+		}
+
+		void HandleDebugDuplicates (Dictionary<string, List<TypeMapDebugEntry>> javaDuplicates, TypeMapDebugEntry entry, TypeDefinition td, TypeDefinitionCache cache)
+		{
+			List<TypeMapDebugEntry> duplicates;
+
+			if (!javaDuplicates.TryGetValue (entry.JavaName, out duplicates)) {
+				javaDuplicates.Add (entry.JavaName, new List<TypeMapDebugEntry> { entry });
+			} else {
+				duplicates.Add (entry);
+				TypeMapDebugEntry oldEntry = duplicates[0];
+				if (td.IsAbstract || td.IsInterface || oldEntry.TypeDefinition.IsAbstract || oldEntry.TypeDefinition.IsInterface) {
+					if (td.IsAssignableFrom (oldEntry.TypeDefinition, cache)) {
+						oldEntry.TypeDefinition = td;
+						oldEntry.ManagedName = GetManagedTypeName (td);
+					}
+				}
+			}
+		}
+
 		void PrepareDebugMaps (ModuleDebugData module)
 		{
 			module.JavaToManagedMap.Sort ((TypeMapDebugEntry a, TypeMapDebugEntry b) => String.Compare (a.JavaName, b.JavaName, StringComparison.Ordinal));
@@ -261,6 +308,16 @@ void PrepareDebugMaps (ModuleDebugData module)
 		}
 
 		TypeMapDebugEntry GetDebugEntry (TypeDefinition td)
+		{
+			return new TypeMapDebugEntry {
+				JavaName = Java.Interop.Tools.TypeNameMappings.JavaNativeTypeManager.ToJniName (td),
+				ManagedName = GetManagedTypeName (td),
+				TypeDefinition = td,
+				SkipInJavaToManaged = ShouldSkipInJavaToManaged (td),
+			};
+		}
+
+		string GetManagedTypeName (TypeDefinition td)
 		{
 			// This is necessary because Mono runtime will return to us type name with a `.` for nested types (not a
 			// `/` or a `+`. So, for instance, a type named `DefaultRenderer` found in the
@@ -277,17 +334,13 @@ TypeMapDebugEntry GetDebugEntry (TypeDefinition td)
 			//
 			string managedTypeName = td.FullName.Replace ('/', '+');
 
-			return new TypeMapDebugEntry {
-				JavaName = Java.Interop.Tools.TypeNameMappings.JavaNativeTypeManager.ToJniName (td),
-				ManagedName = $"{managedTypeName}, {td.Module.Assembly.Name.Name}",
-			};
+			return $"{managedTypeName}, {td.Module.Assembly.Name.Name}";
 		}
 
 		bool GenerateRelease (bool skipJniAddNativeMethodRegistrationAttributeScan, List<TypeDefinition> javaTypes, string outputDirectory, ApplicationConfigTaskState appConfState)
 		{
 			int assemblyId = 0;
 			int maxJavaNameLength = 0;
-			int maxModuleFileNameLength = 0;
 			var knownAssemblies = new Dictionary<string, int> (StringComparer.Ordinal);
 			var tempModules = new Dictionary<byte[], ModuleReleaseData> ();
 			Dictionary <AssemblyDefinition, int> moduleCounter = null;
@@ -329,12 +382,18 @@ bool GenerateRelease (bool skipJniAddNativeMethodRegistrationAttributeScan, List
 				}
 
 				string javaName = Java.Interop.Tools.TypeNameMappings.JavaNativeTypeManager.ToJniName (td);
+				// We will ignore generic types and interfaces when generating the Java to Managed map, but we must not
+				// omit them from the table we output - we need the same number of entries in both java-to-managed and
+				// managed-to-java tables.  `SkipInJavaToManaged` set to `true` will cause the native assembly generator
+				// to output `0` as the token id for the type, thus effectively causing the runtime unable to match such
+				// a Java type name to a managed type. This fixes https://github.com/xamarin/xamarin-android/issues/4660
 				var entry = new TypeMapReleaseEntry {
 					JavaName = javaName,
 					JavaNameLength = outputEncoding.GetByteCount (javaName),
 					ManagedTypeName = td.FullName,
 					Token = td.MetadataToken.ToUInt32 (),
-					AssemblyNameIndex = knownAssemblies [assemblyName]
+					AssemblyNameIndex = knownAssemblies [assemblyName],
+					SkipInJavaToManaged = ShouldSkipInJavaToManaged (td),
 				};
 
 				if (entry.JavaNameLength > maxJavaNameLength)
@@ -376,6 +435,11 @@ bool GenerateRelease (bool skipJniAddNativeMethodRegistrationAttributeScan, List
 			return true;
 		}
 
+		bool ShouldSkipInJavaToManaged (TypeDefinition td)
+		{
+			return td.IsInterface || td.HasGenericParameters;
+		}
+
 		void GenerateNativeAssembly (Func<NativeAssemblerTargetProvider, bool, bool, NativeAssemblyGenerator> getGenerator)
 		{
 			NativeAssemblerTargetProvider asmTargetProvider;
@@ -491,6 +555,10 @@ void OutputModule (ModuleDebugData moduleData)
 		//
 		void OutputModule (BinaryWriter bw, ModuleDebugData moduleData)
 		{
+			if ((uint)moduleData.JavaToManagedMap.Count == InvalidJavaToManagedMappingIndex) {
+				throw new InvalidOperationException ($"Too many types in module {moduleData.ModuleName}");
+			}
+
 			bw.Write (moduleMagicString);
 			bw.Write (TypeMapFormatVersion);
 			bw.Write (moduleData.JavaToManagedMap.Count);
@@ -502,7 +570,7 @@ void OutputModule (BinaryWriter bw, ModuleDebugData moduleData)
 			foreach (TypeMapDebugEntry entry in moduleData.JavaToManagedMap) {
 				bw.Write (outputEncoding.GetBytes (entry.JavaName));
 				PadField (bw, entry.JavaName.Length, (int)moduleData.JavaNameWidth);
-				bw.Write (entry.ManagedIndex);
+				bw.Write (entry.SkipInJavaToManaged ? InvalidJavaToManagedMappingIndex : (uint)entry.ManagedIndex);
 			}
 
 			foreach (TypeMapDebugEntry entry in moduleData.ManagedToJavaMap) {

src/Xamarin.Android.Build.Tasks/Utilities/TypeMappingDebugNativeAssemblyGenerator.cs

@@ -70,7 +70,7 @@ protected override void WriteSymbols (StreamWriter output)
 			if (haveJavaToManaged) {
 				foreach (TypeMapGenerator.TypeMapDebugEntry entry in data.JavaToManagedMap) {
 					size += WritePointer (output, entry.JavaLabel);
-					size += WritePointer (output, entry.ManagedLabel);
+					size += WritePointer (output, entry.SkipInJavaToManaged ? null : entry.ManagedLabel);
 				}
 			}
 			WriteStructureSize (output, JavaToManagedSymbol, size, alwaysWriteSize: true);

src/Xamarin.Android.Build.Tasks/Utilities/TypeMappingReleaseNativeAssemblyGenerator.cs

@@ -219,7 +219,7 @@ uint WriteJavaMapEntry (StreamWriter output, TypeMapGenerator.TypeMapReleaseEntr
 			size += WriteData (output, entry.ModuleIndex);
 
 			WriteCommentLine (output, "type_token_id");
-			size += WriteData (output, entry.Token);
+			size += WriteData (output, entry.SkipInJavaToManaged ? 0 : entry.Token);
 
 			WriteCommentLine (output, "java_name");
 			size += WriteAsciiData (output, entry.JavaName, mappingData.JavaNameWidth);

src/monodroid/jni/embedded-assemblies.cc

@@ -11,6 +11,7 @@
 #include <libgen.h>
 #include <errno.h>
 #include <unistd.h>
+#include <climits>
 
 #include <mono/metadata/assembly.h>
 #include <mono/metadata/image.h>
@@ -208,6 +209,10 @@ EmbeddedAssemblies::typemap_java_to_managed (const char *java_type_name)
 	}
 
 	const char *managed_type_name = entry->to;
+	if (managed_type_name == nullptr) {
+		log_debug (LOG_ASSEMBLY, "typemap: Java type '%s' maps either to an open generic type or an interface type.");
+		return nullptr;
+	}
 	log_debug (LOG_DEFAULT, "typemap: Java type '%s' corresponds to managed type '%s'", java_type_name, managed_type_name);
 
 	MonoType *type = mono_reflection_type_from_name (const_cast<char*>(managed_type_name), nullptr);
@@ -731,12 +736,18 @@ EmbeddedAssemblies::typemap_load_file (BinaryTypeMapHeader &header, const char *
 	uint8_t *managed_pos = managed_start;
 	TypeMapEntry *cur;
 
+	constexpr uint32_t INVALID_TYPE_INDEX = UINT32_MAX;
 	for (size_t i = 0; i < module.entry_count; i++) {
 		cur = &module.java_to_managed[i];
 		cur->from = reinterpret_cast<char*>(java_pos);
 
 		uint32_t idx = *(reinterpret_cast<uint32_t*>(java_pos + header.java_name_width));
-		cur->to = reinterpret_cast<char*>(managed_start + (managed_entry_size * idx));
+		if (idx < INVALID_TYPE_INDEX) {
+			cur->to = reinterpret_cast<char*>(managed_start + (managed_entry_size * idx));
+		} else {
+			// Ignore the type mapping
+			cur->to = nullptr;
+		}
 		java_pos += java_entry_size;
 
 		cur = &module.managed_to_java[i];

tests/CodeGen-Binding/Xamarin.Android.JcwGen-Tests/BindingTests.cs

@@ -12,6 +12,13 @@ namespace Xamarin.Android.JcwGenTests {
 	[TestFixture]
 	public class BindingTests {
 
+		[Test]
+		public void TestTimingCreateTimingIsCorrectType ()
+		{
+			var t = Com.Xamarin.Android.Timing.CreateTiming ();
+			Assert.IsTrue (t is Com.Xamarin.Android.Timing);
+		}
+
 		[Test]
 		public void TestResourceId ()
 		{

tests/CodeGen-Binding/Xamarin.Android.McwGen-Tests/Timing.cs

@@ -8,6 +8,12 @@
 
 namespace Com.Xamarin.Android {
 
+	// Provoke https://github.com/xamarin/xamarin-android/issues/4660
+	// We need this type to appear *before* `Timing` in `monodis --typedef` output
+	[Register ("com/xamarin/android/Timing", DoNotGenerateAcw = true)]
+	public partial class Timinf<T> : Timing {
+	}
+
 	public partial class Timing {
 
 		static IntPtr jonp_id_VirtualVoidMethod;

tests/CodeGen-Binding/Xamarin.Android.McwGen-Tests/java/com/xamarin/android/Timing.java

@@ -4,6 +4,10 @@
 
 public class Timing {
 
+	public static Timing createTiming () {
+		return new Timing ();
+	}
+
 	public static void StaticVoidMethod ()
 	{
 	}