How to convert nested generic to another nested generic that inherits from the first one?

Question

This code:

public interface IInter { }
public class Concrete : IInter { /*... body ...*/ }
var t = (List<IInter>)new List<Concrete>();

Yields this error:

Cannot convert type 'System.Collections.Generic.List<Concrete>' to 'System.Collections.Generic.List<IInter>'

Why is it ? how do I overcome it ? my goal is this:

var t = new List<List<IInter>>()
{
    new List<ConcreteA>(){/* ... data ... */},
    new List<ConcreteB>(){/* ... data ... */},
    // ...
    new List<ConcreteX>(){/* ... data ... */},
};

Edit:

Thanks for all your help. Ahh, I kinda abstracted things, to make it easier to read... but my real problem is this:

public class SingletonFactory<T> where T : IToken
{
    private SingletonFactory() { }
    private static SingletonFactory<T> _instance = new SingletonFactory<T>();
    public static SingletonFactory<T> Instance { get { return _instance; } }

    public T Produce(int position) { return (T)Activator.CreateInstance(typeof(T), position); }
    public T Produce(int position, string token) { return (T)Activator.CreateInstance(typeof(T), position, token); }
}

And then:

var keywords = new Dictionary<string, SingletonFactory<IToken>>()
{
    { "abc", SingletonFactory<Abc>.Instance },
    { "xyz", SingletonFactory<Xyz>.Instance },
    { "123", SingletonFactory<Num>.Instance }
};

So I guess it's much more complicated...

I'm using c# 4.0

Mehmet Ataş · Accepted Answer · 2013-01-17 15:22:53Z

2

var res = new List<Concrete>().Cast<IInter>().ToList();

answered Jan 17, 2013 at 15:22

Mehmet Ataş

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Comments

Tim Schmelter · Accepted Answer · 2013-01-17 15:23:19Z

2

A List<Concrete> is not a List<IInter> although Concrete implements IInter.

You can use:

List<IInter> t = ConcreteList.Cast<IInter>().ToList();

Covariance and Contravariance FAQ

answered Jan 17, 2013 at 15:23

Tim Schmelter

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Comments

D Stanley · Accepted Answer · 2013-01-17 15:27:57Z

2

Because you cannot assign a List<ConcreteA> to a List<IInter>, otherwise you could do this:

concreteAList = newList<ConcreteA>();
List<Inter> interList = concreteAList as List<Inter>;  // seems harmless
interList.Add(new ConcreteB());                        // not allowable

You could do:

var t = new List<List<IInter>>()
{
    new List<IInter>(){/* ... fill with ConcreteAs ... */},
    new List<IInter>(){/* ... fill with ConcreteBs ... */},
    // ...
    new List<IInter>(){/* ... fill with ConcreteXs ... */},
};

But I don't know if that accomplishes what you want.

answered Jan 17, 2013 at 15:27

D Stanley

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Comments

Paul Ruane · Accepted Answer · 2013-01-17 15:46:18Z

2

This is called covariance and C# supports covariance (and contravariance) on generic interfaces and not generic classes. This is the following works but your example does not:

IEnumerable<IInter> e = new List<Concrete>();
ICollection<IInter> c = new List<Concrete>();

Covariance is also supported on arrays:

IInter[] a = new Concrete[3];

edited Jan 17, 2013 at 15:46

answered Jan 17, 2013 at 15:26

Paul Ruane

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6 Comments

Mehmet Ataş Over a year ago

Are you sure that we can write the first line? I did not know that and could not make it work. May it be new List<IInter> instead of new List<Concrete>

Paul Ruane Over a year ago

@MehmetAtaş: actually I was right the first time (damn you self doubt). Covariance for generic interfaces was added in .NET 4, so perhaps you are using an earlier version?

Mehmet Ataş Over a year ago

I did a little bit of tests and now I am clear. It works for FW >= 4.0 and does not work for FW < 4.0 (:

Tar Over a year ago

I use c# 4.0, is there a way to do it with Dictionary<> too (see my edit) ?

Paul Ruane Over a year ago

@Tal, IDictionary<IFruit> fruits = new Dictionary<Banana>();

|

jam40jeff · Accepted Answer · 2013-01-25 05:06:52Z

Make a covrariant interface for the methods you need to access from SingletonFactory<T>. This will ensure that your type T is output type safe.

public interface IToken { }
public class Abc : IToken { }
public class Xyz : IToken { }
public class Num : IToken { }

public interface ISingletonFactory<out T> where T : IToken
{
    T Produce(int position);
    T Produce(int position, string token);
}

public class SingletonFactory<T> : ISingletonFactory<T> where T : IToken
{
    private SingletonFactory() { }
    private static SingletonFactory<T> _instance = new SingletonFactory<T>();
    public static SingletonFactory<T> Instance { get { return _instance; } }

    public T Produce(int position) { return (T)Activator.CreateInstance(typeof(T), position); }
    public T Produce(int position, string token) { return (T)Activator.CreateInstance(typeof(T), position, token); }
}

var keywords = new Dictionary<string, ISingletonFactory<IToken>>()
{
    { "abc", SingletonFactory<Abc>.Instance },
    { "xyz", SingletonFactory<Xyz>.Instance },
    { "123", SingletonFactory<Num>.Instance }
};

Johan Larsson · Accepted Answer · 2013-01-17 15:38:58Z

1

One way is:

IEnumerable<IInter> t = new List<Concrete>();

Another is:

List<IInter> t = new List<Concrete>().ConvertAll(x => (IInter) x);

Edit, adding better sample to show that it works (the test passes):

[Test]
public void CovarianceTest()
{
    var concrete = new Concrete();
    IEnumerable<IInter> t = new List<Concrete> { concrete };
    Assert.IsTrue(new[] { concrete }.SequenceEqual(t));

    List<IInter> t2 = new List<Concrete> { concrete }.ConvertAll(x => (IInter)x);
    Assert.IsTrue(new[] { concrete }.SequenceEqual(t2));
}

edited Jan 17, 2013 at 15:38

answered Jan 17, 2013 at 15:23

Johan Larsson

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4 Comments

Mehmet Ataş Over a year ago

Are you sure that we can write the first line? I did not know that and could not make it work. May it be new List<IInter> instead of new List<Concrete>

D Stanley Over a year ago

@MehmetAtaş yes because IEnumerable<T> is covariant (you couldn't add a different type to the underlying list)

Mehmet Ataş Over a year ago

Ok, I tested too and now I am clear. It works for FW >= 4.0 and does not work for FW < 4.0 (:

Tar Over a year ago

The first method is great, but my real problem (sorry for not mentioning it first place - see my edit) is with Dictionary<>. The second would change my object's behavior.

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