From 07690c841f2f9845d39686e9c9d86306e29edd29 Mon Sep 17 00:00:00 2001
From: Aaron Oneal <aaron.oneal@spicypixel.com>
Date: Fri, 23 Nov 2012 16:57:34 -0800
Subject: [PATCH] Improve exception message when a type cannot be converted
 during marshaling

Now when a type cannot be converted a more detailed exception message is generated stating the types converted from and to. This makes it easier to debug applications.
---
 src/ObjCRuntime/NSObjectMarshaler.cs | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/src/ObjCRuntime/NSObjectMarshaler.cs b/src/ObjCRuntime/NSObjectMarshaler.cs
index 2734cc346..056c9b30c 100644
--- a/src/ObjCRuntime/NSObjectMarshaler.cs
+++ b/src/ObjCRuntime/NSObjectMarshaler.cs
@@ -29,8 +29,19 @@ namespace MonoMac.ObjCRuntime {
 	public class NSObjectMarshaler<T> : ICustomMarshaler where T : NSObject {
 		static NSObjectMarshaler<T> marshaler;
 
-		public object MarshalNativeToManaged (IntPtr handle) {
-			return (T) Runtime.GetNSObject (handle);
+		public object MarshalNativeToManaged (IntPtr handle)
+		{
+			if (handle == IntPtr.Zero)
+				return null;
+
+			var obj = Runtime.GetNSObject (handle);
+			if (!(obj is T)) {
+				throw new MarshalDirectiveException (
+					string.Format ("NSObjectMarshaler<{0}>: Could not cast from type {1}", typeof(T), obj.GetType ()));
+			}
+			else {
+				return (T) obj;
+			}
 		}
 
 		public IntPtr MarshalManagedToNative (object obj) {