[ObjCRuntime] Change Runtime.GetNSObject<T> to create a new instance if the existing one is of the wrong type. Fixes #13704.

src/ObjCRuntime/Runtime.cs

@@ -1437,9 +1437,13 @@ static IntPtr CreateNSObject (IntPtr type_gchandle, IntPtr handle, NSObject.Flag
 				return null;
 
 			var obj = TryGetNSObject (ptr, evenInFinalizerQueue: false);
-			T? o;
 
-			if (obj is null) {
+			// First check if we got an object of the expected type
+			if (obj is T o)
+				return o;
+
+			// We either didn't find an object, or it was of the wrong type, so we need to create a new instance.
+
 			// Try to get the managed type that correspond to this exact native type
 			IntPtr p = Class.GetClassForObject (ptr);
 			// If unknown then we'll get the Class that Lookup to NSObject even if this is not NSObject.
@@ -1461,14 +1465,8 @@ static IntPtr CreateNSObject (IntPtr type_gchandle, IntPtr handle, NSObject.Flag
 			} else {
 				target_type = typeof(NSObject);
 			}
-				o = ConstructNSObject<T> (ptr, target_type, MissingCtorResolution.ThrowConstructor1NotFound);
-			} else {
-				o = obj as T;
-				if (o is null)
-					throw new InvalidCastException ($"Unable to cast object of type '{obj.GetType ().FullName}' to type '{typeof(T).FullName}'.");
-			}
 
-			return o;
+			return ConstructNSObject<T> (ptr, target_type, MissingCtorResolution.ThrowConstructor1NotFound);
 		}
 
 		static public T? GetNSObject<T> (IntPtr ptr, bool owns) where T : NSObject

tests/monotouch-test/ObjCRuntime/RegistrarTest.cs

@@ -2172,8 +2172,9 @@ public void TestCtors ()
 				// sure this is by design or by accident, but that's how we're behaving right now at least
 				ptr = Messaging.IntPtr_objc_msgSend (Class.GetHandle (typeof (D2)), Selector.GetHandle ("alloc"));
 				ptr = Messaging.IntPtr_objc_msgSend_long (ptr, Selector.GetHandle ("initWithBar:"), 2);
-				// Unable to cast object of type 'AppDelegate+D1' to type 'AppDelegate+D2'
-				Assert.Throws<InvalidCastException> (() => Runtime.GetNSObject<D2> (ptr), "b ex");
+				// Failed to marshal the Objective-C object 0x60000230a410 (type: MonoTouchFixtures_ObjCRuntime_RegistrarTest_D2). Could not find an existing managed instance for this object, nor was it possible to create a new managed instance (because the type 'MonoTouchFixtures.ObjCRuntime.RegistrarTest+D2' does not have a constructor that takes one NativeHandle argument
+				var ex = Assert.Throws<RuntimeException> (() => Runtime.GetNSObject<D2> (ptr), "b ex");
+				Assert.That (ex.Message, Does.Contain ("Could not find an existing managed instance for this object, nor was it possible to create a new managed instance (because the type 'MonoTouchFixtures.ObjCRuntime.RegistrarTest+D2' does not have a constructor that takes one"), "Exception message");
 				var obj = Runtime.GetNSObject<D1> (ptr);
 				Assert.AreEqual ("bar", obj.ctor1, "b ctor1");
 			} finally {

tests/monotouch-test/ObjCRuntime/RuntimeTest.cs

@@ -141,6 +141,23 @@ public void GetNSObject_Posing_Class ()
 			}
 		}
 
+		[Test]
+		public void GetNSObject_T_SameHandleDifferentInstances ()
+		{
+			using var a = new NSDictionary<NSString, NSObject> ();
+			using var b = new NSDictionary<NSString, NSString> ();
+			using var c = new NSDictionary ();
+
+			if (a.Handle != b.Handle || a.Handle != c.Handle)
+				Assert.Ignore ("The dictionaries are actually different");
+
+			var handle = a.Handle;
+
+			Assert.DoesNotThrow (() => Runtime.GetNSObject<NSDictionary> (handle), "A");
+			Assert.DoesNotThrow (() => Runtime.GetNSObject<NSDictionary<NSString, NSObject>> (handle), "B");
+			Assert.DoesNotThrow (() => Runtime.GetNSObject<NSDictionary<NSString, NSString>> (handle), "C");
+		}
+
 		[Test]
 		public void UsableUntilDead ()
 		{