Commits on Oct 27, 2022

1. [ObjCRuntime] Change Runtime.GetNSObject<T> to create a new instance … …

   …if the existing one is of the wrong type. Fixes xamarin#13704.
   
   Objective-C has an optimization where creating an empty dictionary would
   return the same instance every time (probably to a constant empty dictionary).
   
   This causes a problem with how we've bound generic dictionaries, because all
   empty dictionaries would have the same native handle, even if we'd bound them
   with different managed types.
   
   This surfaces in unfortunate ways when we try to look up a managed instance
   given a native handle, we find that we already have a managed instance, but
   the managed instance is of the wrong type.
   
   Example code:
   
       var a = new NSDictionary ();
       var b = new NSDictionary<NSString, NSString> ();
       var c = new NSDictionary<NSString, NSObject> ();
       Console.WriteLine ($"a: 0x{a.Handle:X}");
       Console.WriteLine ($"b: 0x{b.Handle:X}");
       Console.WriteLine ($"c: 0x{c.Handle:X}");
   
   would produce something like:
   
       a: 0x0x7fff80821080
       b: 0x0x7fff80821080
       c: 0x0x7fff80821080
   
   now for this code:
   
       Runtime.GetNSObject<NSDictionary<NSString, NSString>> (b.Handle)
   
   it would throw an exception like this:
   
       Unable to cast object of type 'Foundation.NSDictionary`2[[Foundation.NSString],[Foundation.NSObject]]' to type 'Foundation.NSDictionary`2[[Foundation.NSString],[Foundation.NSString]]'
   
   because we'll have the 'c' object (with type `NSDictionary<NSString, Object>`)
   in our dictionary of native handles -> managed instances, and that's not
   compatible with the desired return type from GetNSObject
   (`NSDictionary<NSString, NSString>`)
   
   This likely happens with all the non-mutable collection types we have a
   generic version of (NSArray, NSDictionary, NSOrderedSet, NSSet).
   
   Fixes xamarin#16378.
   Fixes xamarin#13704.

   

   rolfbjarne committed Oct 27, 2022
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