The C# spec states that an argument type cannot be both covariant and contravariant at the same time.

This is apparent when creating a covariant or contravariant interface you decorate your type parameters with "out" or "in" respectively. There is not option that allows both at the same time ("outin").

Is this limitation simply a language specific constraint or are there deeper, more fundamental reasons based in category theory that would make you not want your type to be both covariant and contravariant?

Edit:

My understanding was that arrays were actually both covariant and contravariant.

public class Pet{}
public class Cat : Pet{}
public class Siamese : Cat{}
Cat[] cats = new Cat[10];
Pet[] pets = new Pet[10];
Siamese[] siameseCats = new Siamese[10];

//Cat array is covariant
pets = cats; 
//Cat array is also contravariant since it accepts conversions from wider types
cats = siameseCats;