Question

This compiles:

    class ReplicatedBaseType
    {
    }

    class NewType: ReplicatedBaseType
    {
    }

    class Document
    {
    ReplicatedBaseType BaseObject;

    Document()
    {
     BaseObject = new NewType();
    }
}

But this does not:

    class DalBase<T> : where T: ReplicatedBaseType
    {
    }

    class DocumentTemplate
    {
    DalBase<ReplicatedBaseType> BaseCollection;
    DocumentTemplate ()
    {
    BaseCollection= new DalBase<NewType>(); // Error in this line. It seems this is not possible
    }
    }

What’s the reason?

Answer 1

Variance exists in C# 4.0 targetting .NET 4), but is limited to interfaces and usage of in/out (oh, and arrays of reference-types). For example, to make a covariant sequence:

class DalBase<T> : IEnumerable<T> where T: ReplicatedBaseType
{
    public IEnumerator<T> GetEnumerator() {throw new NotImplementedException();}
    IEnumerator IEnumerable.GetEnumerator() { throw new NotImplementedException(); }
}

class DocumentTemplate
{
    IEnumerable<ReplicatedBaseType> BaseCollection;
    DocumentTemplate()
    {
        BaseCollection = new DalBase<NewType>(); // Error in this line. It seems this is not possible
    }
}

But other than that… no. Stick to either non-generic lists (IList), or use the expected list type.