Why can't I use a compatible concrete type when implementing an interface

Question

I would like to be able to do something like this :

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

namespace Test
{
          


        

Answer 1

This is a Type Covariance/Contravariance issue (see http://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_science)#C.23 ).

There's a workaround: use explicit interfaces, like so:

public class Bar : IFoo {

    private IList _integers;

    IEnumerable IFoo.integers {
        get { return _integers };
        set { _integers = value as IList; }
    }

    public IList integers {
        get { return _integers; }
        set { _integers = vale; }
    }
}

Note that integers should be TitleCased to conform to .NET's guidelines.

Hopefully you can see the problem in the code above: IList is compatible with IEnumerable only for the accessor, but not for setting. What happens if someone calls IFoo.integers = new Qux() (where Qux : IEnumerable but not Qux : IList).