Covariance and Contravariance in C#, Part 2: Array Covariance

C# implements variance in two ways. Today, the broken way.

Ever since C# 1.0, arrays where the element type is a reference type are covariant. This is perfectly legal:

Animal[] animals = new Giraffe[10];

Since Giraffe is smaller than Animal, and “make an array of” is a covariant operation on types, Giraffe[] is smaller than Animal[], so an instance fits into that variable.

Unfortunately, this particular kind of covariance is broken. It was added to the CLR because Java requires it and the CLR designers wanted to be able to support Java-like languages. We then up and added it to C# because it was in the CLR. This decision was quite controversial at the time and I am not very happy about it, but there’s nothing we can do about it now.

Why is this broken? Because it should always be legal to put a Turtle into an array of Animals. With array covariance in the language and runtime you cannot guarantee that an array of Animals can accept a Turtle because the backing store might actually be an array of Giraffes.

This means that we have turned a bug which could be caught by the compiler into one that can only be caught at runtime. This also means that every time you put an object into an array we have to do a run-time check to ensure that the type works out and throw an exception if it doesn’t. That’s potentially expensive if you’re putting a zillion of these things into the array.


Unfortunately, we’re stuck with this now. Giraffe[] is smaller than Animal[], and that’s just the way it goes.

I would like to take this opportunity to clarify some points brought up in comments to Part One.

First, by “subtype” and “supertype” I mean “is on the chain of base classes” for classes and “is on the tree of base interfaces” for interfaces. I do not mean the more general notion of “is substitutable for”. 

By “bigger than” and “smaller than” I explicitly do not mean “is a base class of” and “is a derived type of”. It is the case that every subclass is smaller than its superclass, yes. But it is not the case that every smaller type is derived from its larger type. 

Giraffe[] is smaller than both Animal[] and System.Array. Clearly Giraffe[] is derived from System.Array, but it is not derived from Animal[].

The “is smaller than” relationship I am defining is more general than the “is a kind of” relationship. I want to draw a distinction between assignment compatibility (smaller than) and inheritance (subtype of).

Next time on FAIC: we’ll discuss a kind of variance that we added to C# 2.0 which is not broken.

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