Generic Variance Issues

Hello there,
I've noticed that the mono runtime in unity doesn't support a feature that's available since .Net 2.0: generic variance.

Long story short, below code works fine in .Net 2.0, 3.5 and 4.5 (which I've tested). But throws an ArrayTypeMismatchException at `consumers.Add(new BaseConsumer());`.

Code (CSharp):

1. using System;
2. using System.Collections.Generic;
3.  
4. interface IBase
5. {
6.     int x {get;}
7. }
8.  
9. class BaseClass : IBase
10. {
11.     public int x {get; set;}
12. }
13.  
14. interface IDerived : IBase
15. {
16.     int y {get;}
17. }
18.  
19. class DerivedClass : BaseClass, IDerived
20. {
21.     public int y {get; set;}
22. }
23.  
24. interface IConsumer<in T> where T: class, IBase
25. {
26.     void Feed(T arg);
27. }
28.  
29. class BaseConsumer : IConsumer<IBase>
30. {
31.     public void Feed(IBase arg)
32.     {
33.         Console.WriteLine(arg.x);
34.     }
35. }
36.  
37. class DerivedConsumer : IConsumer<IDerived>
38. {
39.     public void Feed(IDerived arg)
40.     {
41.         Console.WriteLine(arg.y);    
42.     }
43. }
44.  
45. public class Program
46. {
47.     public static void Main()
48.     {
49.         List<IConsumer<IDerived>> consumers = new List<IConsumer<IDerived>>();
50.         consumers.Add(new BaseConsumer());
51.         consumers.Add(new DerivedConsumer());
52.  
53.         DerivedClass d = new DerivedClass() { x = 3, y = 5};
54.  
55.         foreach (IConsumer<IDerived> consumer in consumers)
56.             consumer.Feed(d);
57.     }
58. }

Is there a fix or workaround available? Or is it going to be fixed, ever? I've seen questions about this in SO and Unity Answers from 2013, but I haven't been able to find any answer.

Thank you very much.

 

Have you tried compiling the code using a different compiler (e.g. cl.exe) and then importing the DLL into your project?

If its a problem with the mono compiler generating wrong checks, then this might help.

 

Thank you for your reply, yes I've tried using it from a DLL. I guess since the runtime is problematic, compilation makes no difference