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
    };
}

Answer 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 :-)