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

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?

You don't have () after new DalBase ? //edit I've created a project, this is not the case — kubal5003, Jun 25 '10 at 10:58
No, no.. that's because I just typed this from memory. Actual code is very long, so didn't duplicate here. Will correct it. Sorry. :( — virtualmic, Jun 25 '10 at 11:00
possible duplicate of [Explicit Casting Problem](http://stackoverflow.com/questions/1443341/explicit-casting-problem) — thecoop, Jun 25 '10 at 11:13

score 8 · Answer 1 · edited Oct 23 '22 at 08:10

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 ;)

lol I think you've made more rep explaining covariance/contravariance than for any other subject on SO. — , Jun 25 '10 at 11:39

score 6 · Accepted Answer · answered Jun 25 '10 at 11:05

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.

score 2 · Answer 3 · answered Jun 25 '10 at 11:06

2

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.

answered Jun 25 '10 at 11:06

thecoop

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Andrey · Answer 4 · 2010-06-25T11:03:53.623

1

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

edited Jun 25 '10 at 11:03

answered Jun 25 '10 at 10:57

Andrey

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@Marc Gravell♦ i didn't say that. i meant that in C# < 4.0 there is no way to do this casts (well, except for overriding cast operators) – Andrey Jun 25 '10 at 11:04

score 0 · Answer 5 · answered Jun 25 '10 at 11:11

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)
        {
        }
    }
}

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

5 Answers5

Linked