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Question by je1337 · May 14, 2013 at 01:11 PM · c#

C# Covariance+Arrays raises ArrayTypeMismatchException

Consider following generic interface ITest with a covariant type parameter T, the generic class Test implementing the interface, and a class A and with a subclass B:

 interface ITest<out T> 
 {    
   T prop{ get;}
 }
 class Test<T> : ITest<T>
 {    
     public T prop{ get {
        return default(T);    
     }}
 }
 class A {    
 }
 class B: A {    
 }


The following code compiles with errors but throws the runtime exception System.ArrayTypeMismatchException

 ITest<A>[] a = new ITest<A>[1];
 a[0] = new Test<B>(); //<-- throws exception

but this code works just fine:

 ITest<A> r = new Test<B>();
 

This has be tested on Unity3d 4.1. I already asked on StackOverflow this question. The comments suggests that this looks like a issue that is unity specific. Since the standard Mono implementation and the .Net work fine.

Comment
puglic

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avatar image Bunny83 · May 14, 2013 at 01:27 PM 0
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To me that sounds like it's the issue mentioned here( or basically here) and that it has been solved in C# 4.0 which we don't have in Unity.

Haven't come across this situation myself, so can't say much about it.

avatar image je1337 · May 14, 2013 at 01:39 PM 0
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Unity supports C# 4.0 covariance / contravariance at least in the basic form. ( see third code piece ) only when an array is used an error is thrown at runtime.

avatar image puglic · Jan 28, 2018 at 12:04 AM 0
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In 2018, this issue still exists in Unity 2017.3.0f3.

avatar image Bunny83 puglic · Jan 28, 2018 at 12:43 AM 0
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