test.cs

using ObjCRuntime;

class Helper {
	IntPtr GetObjCIvar (string name)
	{
		IntPtr native;
		object_getInstanceVariable (handle, name, out native);
		return native;
	}

	void SetObjCIvar (string name, NSObject value)
	{
		// get existing
		var existing = GetObjCIvar (name);
		if (existing != IntPtr.Zero) {
			if (value != null && existing == value.Handle)
				return; // setting the same value, just ignore
		} else if (value == null) {
			return; // both existing and new values are null, just ignore
		}

		if (value != null)
			value.DangerousRetain ();
		object_setInstanceVariable (handle, "myProperty", value != null ? value.Handle : IntPtr.Zero);
		if (existing != null)
			existing.DangerousRelease ();
	}

	[DllImport ("/usr/lib/libobjc.dylib")]
	extern static void object_getInstanceVariable (IntPtr obj, string name, out IntPtr val);

	[DllImport ("/usr/lib/libobjc.dylib")]
	extern static void object_setInstanceVariable (IntPtr obj, string name, IntPtr val);
}

class Test : NSObject
{
	[Export ("myProperty")]
	public NSObject MyProperty {
		get {
			return Runtime.GetNSObject (Helper.GetObjCIvar ("myProperty"));
		}
		set {
			Helper.SetObjCIvar ("myProperty", value);
		}
	}

	public override void Dispose ()
	{
		// This must be done so that the value's reference count is released
		MyObject = null;
	}
}