[Java.Interop] Generic Type Definitions are allowed

src/Java.Interop/Java.Interop/JavaObject.cs

@@ -64,6 +64,7 @@ public JavaObject (ref JniObjectReference reference, JniObjectReferenceOptions o
 			Construct (ref reference, options);
 		}
 
+		[global::Java.Interop.JniConstructorSignature ("()V")]
 		public unsafe JavaObject ()
 			: this (ref *InvalidJniObjectReference, JniObjectReferenceOptions.None)
 		{

src/Java.Interop/Java.Interop/JniRuntime.JniTypeManager.cs

@@ -145,8 +145,7 @@ public JniTypeSignature GetTypeSignature (Type type)
 
 				if (type == null)
  					throw new ArgumentNullException (nameof (type));
-				if (type.ContainsGenericParameters)
-					throw new NotSupportedException ($"'{type}' contains a generic type definition. This is not supported.");
+
 
 				type = GetUnderlyingType (type, out int rank);
 
@@ -183,8 +182,6 @@ public IEnumerable<JniTypeSignature> GetTypeSignatures (Type type)
 
 				if (type == null)
 					yield break;
-				if (type.ContainsGenericParameters)
-					throw new NotSupportedException ($"'{type}' contains a generic type definition. This is not supported.");
 
 				type = GetUnderlyingType (type, out int rank);
 

tests/Java.Interop-Tests/Java.Interop/JniTypeManagerTests.cs

@@ -18,7 +18,6 @@ public void GetTypeSignature_Type ()
 			Assert.Throws<ArgumentException>(() => JniRuntime.CurrentRuntime.TypeManager.GetTypeSignature (typeof (int[,])));
 			Assert.Throws<ArgumentException>(() => JniRuntime.CurrentRuntime.TypeManager.GetTypeSignature (typeof (int[,][])));
 			Assert.Throws<ArgumentException>(() => JniRuntime.CurrentRuntime.TypeManager.GetTypeSignature (typeof (int[][,])));
-			Assert.Throws<NotSupportedException>(() => JniRuntime.CurrentRuntime.TypeManager.GetTypeSignature (typeof (Action<>)));
 			Assert.AreEqual (null, JniRuntime.CurrentRuntime.TypeManager.GetTypeSignature (typeof (JniRuntimeTest)).SimpleReference);
 
 			AssertGetJniTypeInfoForType (typeof (string),   "java/lang/String",   false,  0);
@@ -91,6 +90,11 @@ public void GetTypeSignature_Type ()
 			// Note: dotnet/android@5c23bcda updates Java.Lang.Object to inherit JavaObject; this is not enough,
 			// as `<GenerateJavaStubs/>` only processes assemblies if they reference Mono.Android.dll.
 			AssertGetJniTypeInfoForType (typeof (GenericHolder<int>),       GenericHolder<int>.JniTypeName,    false,   0);
+
+			// XAJavaInterop1 Java Callable Wrappers, as used in dotnet/android, may contain
+			// `Runtime.register("Java.InteropTests.GenericHolder`1, Mono.Android-Tests", …)`,
+			// which results in a generic type definition.  Permit this.
+			AssertGetJniTypeInfoForType (typeof (GenericHolder<>),          GenericHolder<int>.JniTypeName,    false,   0);
 #endif  // !__ANDROID__
 		}
 
@@ -215,6 +219,24 @@ public void GetTypeSignature ()
 			Assert.AreEqual (typeof (string),   GetTypeForSimpleReference ("java/lang/String"));
 			Assert.AreEqual (null,              GetTypeForSimpleReference ("com/example/does/not/exist"));
 		}
+
+		[Test]
+		public void CanCreateGenericHolder ()
+		{
+			using var holder = new GenericHolder<int> ();
+		}
+
+		[Test]
+		public unsafe void CannotCreateGenericHolderFromJava ()
+		{
+			var signature   = JniRuntime.CurrentRuntime.TypeManager.GetTypeSignature (typeof (GenericHolder<>));
+			using var type  = new JniType (signature.Name);
+			var ctor        = type.GetConstructor ("()V");
+			var instance    = type.AllocObject ();
+
+			Assert.Throws<NotSupportedException>(() =>
+				JniEnvironment.InstanceMethods.CallNonvirtualVoidMethod (instance, type.PeerReference, ctor));
+		}
 	}
 
 	[JniTypeSignature (JniTypeName)]