ArrayTypeMismatchException - Covariance Mono bug

Hi,

We've got some code that looks good to me but is causing intermittent ArrayTypeMismatchExceptions at runtime -- appears related to (likely the same) as this specific Mono bug (#6266) -- and after checking the code, looks like the fix isn't contained in Unity's Mono fork. Another repro of the ~ same thing can be found on the Mono bug report (see commit message on linked GH commit that fixes #6266 above).

Repro Specs
Unity Editor 2019.2.21f1 on macOS
Scripting Backend: Mono
Scripting API: .NET 4.x

Code to Reproduce
Note, it's intermittent; run RunTest() 20 times or so and you should see at least 1 exception, and possibly 15-20 even.

Code (CSharp):

1.  
2. public interface ITypical {}
3. public class StdClass : ITypical {}
4. public class MonoClass : MonoBehaviour, ITypical { }
5. public class ContainerClass {
6.     public GameObject go;
7.     public StdClass nonMono;
8. }
9.  
10. public class MiscClass {
11.     static List<ITypical> list = new List<ITypical>();
12.  
13.     public void RunTest() {
14.         var test = new ContainerClass() {
15.             go = new GameObject().AddComponent<MonoClass>().gameObject,
16.             nonMono = new StdClass()
17.         };
18.  
19.         Test(test);
20.     }
21.  
22.     private List<ITypical> Test(ContainerClass container) {
23.         list.Clear();
24.  
25.         if (container.go != null)
26.             container.go.GetComponents<ITypical>(list);
27.  
28.         if (container.nonMono != null && container.nonMono is ITypical itypInstance) {
29.             // All 3 variants will intermittently result in runtime errors;  it always compiles, and is (AFAICT) impossible for a TypeMismatch to *actually* occur.  Stack will point to these lines.
30.             list.Add(container.nonMono);                // result = intermittent runtime error
31.             //list.Add(itypInstance);                    // result = intermittent runtime error
32.             //list.Add(container.nonMono as ITypical);    // result = intermittent runtime error
33.         }
34.  
35.         return list;
36.     }
37. }
38.  

 

Did you report a bug?

 

I did indeed just now report a bug, yes.

Case # is: 1235903