Question

I recently started to think of this problem and I can’t find the answer.
The following code compiles and executes as expected

object[] test = new string[12];

However, I don’t know why.

I mean, should we consider string[] as the derived class of object[]?
I think in C#, every array is an instance of Array class. If Array is generic, it should be Array<T>, and Array<string> can be assigned to Array<object>, it doesn’t make sense. I remember only interface can use in/out keyword.

And in Java, I’m not sure, but still feel weird. Why different types of references can be possibly assigned to each other when they don’t have super-sub class relationship?

Can somebody explain a little?

Thanks a lot!

Answer 1

It’s because reference type arrays support covariance in both Java and C#. It also means that every write into a reference type array has to be checked at execution time, to make sure you don’t write the wrong type of element into it 🙁

Don’t forget that both Java and C# (and .NET in general) started off without generics. If they had had generics to start with, life could have been somewhat different.

Note that both Java and C# support generic variance now, but in rather different ways. So for example in C# 4 you can write:

IEnumerable<string> strings = // Get some string sequence here
IEnumerable<object> objects = strings;

but you can’t write

IList<string> strings = // Get some string list here
// Compile-time error: IList<T> isn't covariant in T
IList<object> objects = strings;

This wouldn’t be safe, because you can add to an IList<T> as well as taking items from it.

This is a big topic – for more details, see Eric Lippert’s blog series.