[generator] If forcing a managed type, we must do so even if there's another managed instance of an incompatible type. Fixes #7441. (#7460)

If forcing a managed type for a particular native handle, we must ensure we're
always successful.

This means creating another managed instance even if there already is an
existing instance for the native handle. This is not optimal, but the
alternative is worse: some functionality can be completely broken otherwise
(such as NSSavePanel⁄NSOpenPanel may stop working).

If creating multiple managed instances for the same native handle ends up
being too problematic, we'll probably have to add full support for this
scenario (see #7442).

Fixes https://github.com/xamarin/xamarin-macios/issues/7441.

src/ObjCRuntime/Runtime.cs

@@ -1422,6 +1422,11 @@ namespace ObjCRuntime {
 
 		// this method is identical in behavior to the non-generic one.
 		public static T GetINativeObject<T> (IntPtr ptr, bool owns) where T : class, INativeObject
+		{
+			return GetINativeObject<T> (ptr, false, owns);
+		}
+
+		public static T GetINativeObject<T> (IntPtr ptr, bool forced_type, bool owns) where T : class, INativeObject
 		{
 			if (ptr == IntPtr.Zero)
 				return null;
@@ -1433,7 +1438,10 @@ namespace ObjCRuntime {
 				return t;
 			}
 
-			if (o != null) {
+			// If forced type is true, we ignore any existing instances if the managed type of the existing instance isn't compatible with T.
+			// This may end up creating multiple managed wrapper instances for the same native handle,
+			// which is not optimal, but sometimes the alternative can be worse :/
+			if (o != null && !forced_type) {
 				// found an existing object, but with an incompatible type.
 				if (!typeof (T).IsInterface && typeof(NSObject).IsAssignableFrom (typeof (T))) {
 					// if the target type is another NSObject subclass, there's nothing we can do.
@@ -1445,7 +1453,7 @@ namespace ObjCRuntime {
 			var implementation = LookupINativeObjectImplementation (ptr, typeof (T));
 
 			if (implementation.IsSubclassOf (typeof (NSObject))) {
-				if (o != null) {
+				if (o != null && !forced_type) {
 					// We already have an instance of an NSObject-subclass for this ptr.
 					// Creating another will break the one-to-one assumption we have between
 					// native objects and NSObject instances.

src/generator.cs

@@ -1649,7 +1649,7 @@ public partial class Generator : IMemberGatherer {
 				if (IsProtocolInterface (pi.ParameterType)) {
 					invoke.AppendFormat (" Runtime.GetINativeObject<{1}> ({0}, false)", pi.Name.GetSafeParamName (), pi.ParameterType);
 				} else if (isForced) {
-					invoke.AppendFormat (" Runtime.GetINativeObject<{1}> ({0}, {2})", pi.Name.GetSafeParamName (), RenderType (pi.ParameterType), isForcedOwns);
+					invoke.AppendFormat (" Runtime.GetINativeObject<{1}> ({0}, true, {2})", pi.Name.GetSafeParamName (), RenderType (pi.ParameterType), isForcedOwns);
 				} else {
 					invoke.AppendFormat (" Runtime.GetNSObject<{1}> ({0})", pi.Name.GetSafeParamName (), RenderType (pi.ParameterType));
 				}
@@ -3611,7 +3611,7 @@ public partial class Generator : IMemberGatherer {
 				cast_b = ", false)";
 			} else if (minfo != null && minfo.is_forced) {
 				cast_a = " Runtime.GetINativeObject<" + FormatType (declaringType, GetCorrectGenericType (mi.ReturnType)) + "> (";
-				cast_b = $", {minfo.is_forced_owns})";
+				cast_b = $", true, {minfo.is_forced_owns})";
 			} else if (minfo != null && minfo.is_bindAs) {
 				var bindAs = GetBindAsAttribute (minfo.mi);
 				var nullableBindAsType = TypeManager.GetUnderlyingNullableType (bindAs.Type);
@@ -4138,7 +4138,7 @@ public partial class Generator : IMemberGatherer {
 				} else if (isINativeObjectSubclass) {
 					if (!pi.IsOut)
 						by_ref_processing.AppendFormat ("if ({0}Value != ({0} == null ? IntPtr.Zero : {0}.Handle))\n\t", pi.Name.GetSafeParamName ());
-					by_ref_processing.AppendFormat ("{0} = Runtime.GetINativeObject<{1}> ({0}Value, {2});\n", pi.Name.GetSafeParamName (), RenderType (elementType), isForcedType ? isForcedOwns : "false");
+					by_ref_processing.AppendFormat ("{0} = Runtime.GetINativeObject<{1}> ({0}Value, {2}, {3});\n", pi.Name.GetSafeParamName (), RenderType (elementType), isForcedType ? "true" : "false", isForcedType ? isForcedOwns : "false");
 				} else {
 					throw ErrorHelper.CreateError (99, $"Internal error: don't know how to create ref/out (output) code for {mai.Type} in {mi}. Please file a bug report with a test case (https://github.com/xamarin/xamarin-macios/issues/new).");
 				}

tests/monotouch-test/ObjCRuntime/RuntimeTest.cs

@@ -699,5 +699,24 @@ Additional information:
 			}
 #endif
 		} 
+
+		[Test]
+		public void GetINativeObject_ForcedType ()
+		{
+			using (var str = new NSString ("hello world")) {
+				NSDate date;
+
+				Assert.Throws<InvalidCastException> (() => Runtime.GetINativeObject<NSDate> (str.Handle, false), "EX1"); //
+				Assert.Throws<InvalidCastException> (() => Runtime.GetINativeObject<NSDate> (str.Handle, false, false), "EX2"); //
+
+				// Making a string quack like a date.
+				// This is a big hack, but hopefully it works well enough for this particular test case.
+				// Just don't inspect the date variable in a debugger (it will most likely crash the app).
+				date = Runtime.GetINativeObject<NSDate> (str.Handle, true, false);
+				Assert.AreEqual (date.Handle, str.Handle, "Same native pointer");
+				date.Dispose ();
+				date = null;
+			}
+		}
 	}
 }