Question

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Upcasting and generic lists

Ok, I want to send a List<CardHolder> as an IEnumerable<ICardHolder> where CardHolder : ICardHolder. However, the compiler errors:

Error 4 Argument ‘1’: cannot convert from ‘System.Collections.Generic.List’ to ‘System.Collections.Generic.IEnumerable’

This seems strange to me, considering that an List<T> : IEnumerable<T>. What’s going wrong?

public interface ICardHolder
{
    List<Card> Cards { get; set; } 
}

public class CardHolder : ICardHolder
{
    private List<Card> cards = new List<Card>();
    public List<Card> Cards
    {
        get { return cards; }
        set { cards = value; }
    }

    // ........
}

public class Deck : ICardHolder
{
    // .........

    public void Deal(IEnumerable<ICardHolder> cardHolders)
    {
         // ........
    }

    // .........
}

public class Game
{
    Deck deck = new Deck();
    List<CardHolder> players = new List<CardHolder>();

    // .........

    deck.Deal(players); // Problem is here!

    // .........
}

Answer 1

The problem is that List<T> is not a subtype of IEnumerable<T1> even if T : T1.

Generics in C# (before C# 4.0) are ‘invariant’ (ie. don’t have this sub-typing relationship). In .Net 4, IEnumerable<T> will have its type parameter annotated as being ‘covariant’. This means that List<T> will be a subtype of IEnumerable<T1> if T : T1.

See this page on MSDN for more details of this feature.

Edit – You can work around this in your case by making the Deal method generic:

public void Deal<T>(IEnumerable<T> cardHolders) where T : ICardHolder
{
     // ........
}