Using inherited types with C# collection generic type parameters

Question

I have an abstract class with a few inheriting classes like so:

internal abstract class SomeBaseType {}
internal class FirstSpecificType : SomeBaseType {}
internal class SecondSpecificType : SomeBaseType {}
internal class ThirdSpecificType : SomeBaseType {}

and a class with a constrained generic type parameter:

internal class SomeCollection<T> : ICollection<SomeDataStructure>
    where T : SomeBaseType {}

I created a List<SomeCollection<SomeBaseType>> and tried to add elements of various inheriting classes of SomeBaseType but I get the following errors (CS1503 and CS1950):

Argument 1: cannot convert from 'Namespace.SomeCollection<FirstSpecificType>' to 'Namespace.SomeCollection<SomeBaseType>'
The best overloaded Add method 'List<SomeCollection<SomeBaseType>>.Add(SomeCollection<SomeBaseType>)' for the collection initializer has some invalid arguments

Since FirstSpecificType is a SomeBaseType, I should be able to do this, right? It's basically the same as adding some arbitrary set of object to an IList<object>.

For additional context, the code where this error manifests looks like this:

protected Constructor()
{
    collectionOne = new SomeCollection<FirstSpecificType>();
    collectionTwo = new SomeCollection<SecondSpecificType>();
    collectionThree = new SomeCollection<ThirdSpecificType>();

    allCollections = new List<SomeCollection<SomeBaseType>>
    {
        // Each of these three collections has the error
        collectionOne,
        collectionTwo,
        collectionThree
    };
}

That's covariance and covariance on generic classes doesn't work. — Rahul
– Rahul, Commented Mar 1, 2016 at 21:55
This question is asked every day. One more time: a list of giraffes is not a list of animals. Why? Because you can put a tiger into a list of animals but not into a list of giraffes. A list of animals is not a list of giraffes. Why? Because there might already be a tiger inside the list of animals. Therefore a list of giraffes and a list of animals are never compatible, because the operations you can do on those two types are different. The fact that some animals are giraffes is irrelevant. — Eric Lippert
– Eric Lippert, Commented Mar 2, 2016 at 0:29

Amit · Accepted Answer · 2016-03-01 21:58:41Z

1

This is an issue of covariance and contravariance, that basically says that while FirstSpecificType is indeed SomeBaseType, SomeCollection<FirstSpecificType> is strictly not SomeCollection<SomeBaseType>.

Eric Lippert wrote some fabulous articles about this and giraffes that I'd recommend everybody to read at least once :-)

edited Mar 1, 2016 at 21:58

answered Mar 1, 2016 at 21:52

Amit

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3 Comments

Rahul Over a year ago

John Skeet wrote some fabulous articles... ... Why not link that article as well?

Amit Over a year ago

@Rahul - when mixing up .net guru's, it's hard to find the right link ;-)

Jason Over a year ago

Thanks for the explanation and links, @Amit. That helped me figure out a new way to my goal.

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