Why does the following doesn't compile? (involves generics and inheritance in c#)

Question

This compiles:

    class ReplicatedBaseType
    {
    }

    class NewType: ReplicatedBaseType
    {
    }

    class Document
    {
    ReplicatedBas         


        

Answer 1

This code compiles in VS2010, net framework 4

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication2
{
    class ReplicatedBaseType
    {
    }

    class NewType : ReplicatedBaseType
    {
    }

    class Document
    {
        ReplicatedBaseType BaseObject;

        Document()
        {
            BaseObject = new NewType();
        }
    }
    interface DalBase<out T>  where T: ReplicatedBaseType
    {
    }

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

    }
    class DocumentTemplate
    {
        DalBase<ReplicatedBaseType> BaseType;
        DocumentTemplate ()
        {
            BaseType = new DalBaseExample<NewType>(); // no error here
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
        }
    }
}

Answer 2

the feature that you want is called "covariance" and was introduced only in C# 4.0 http://blog.t-l-k.com/dot-net/2009/c-sharp-4-covariance-and-contravariance

you code fails because there is no such cast (both explicit and implicit). Current rules of C# (<= 3.0) forbid it.

to check possible casts see §6.1.6 C# Specification

Answer 3

As Andrey says, you want (generic) covariance. However:

• Generic variance is only supported in C# 4
• Generic variance isn't supported on classes
• In your real life case, this may be unsafe.

To go into the final point, suppose DalBase<T> has this method:

void AddEntity(T entity)

Now you've got something like this which you want to be able to compile - but would obviously be dangerous:

DalBase<Fruit> fruitDal = new DalBase<Banana>();
fruitDal.AddEntity(new Apple());

The second line would have to compile - so for this to fail at compile time, it has to be the first line which would fail.

I gave an hour long talk on generic variance recently which you may find useful if you want to know more - see the NDC 2010 video page and search for "variance". Alternatively you could read Eric Lippert's blog posts on the topic - but be aware that that will probably take longer than an hour ;)

Answer 4

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.

Answer 5

It doesn't work because a DalBase<NewType> is not a DalBase<ReplicatedBaseType> - generics do not have co/contra-variance. In C# 4 you can get generic variance, but only on interfaces and delegates.