实现IEnumerable <T> 在列表中 <T> 派生集合

Question

I'm getting an error. Here's the code copied across to a Console project and stripped down:

namespace ConsoleApplication1
{
public interface IHexGrid
{ 
    IEnumerable<Hex> hexs { get; } //error related location
}

public class HexC : Hex
{ public int var1;}

public abstract class Hex
{ public int var2; }    

public class HexGridC : IHexGrid //error CS0738
{        
    public List<HexC> hexs { get; set; } // error related location
}    

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

I'm getting the following: error CS0738:

'ConsoleApplication1.HexGridC' does not implement interface
member 'ConsoleApplication1.IHexGrid.hexs'. 'ConsoleApplication1.HexGridC.hexs' cannot 
implement 'ConsoleApplication1.IHexGrid.hexs' because it does not have the matching 
return type of '`System.Collections.Generic.IEnumerable<ConsoleApplication1.Hex>`'.

Not sure why as IENumerable is Covariant. Any help much appreciated.

Edit: the code has been simplified

Answer 1

The problem is that your property is of the wrong type. C# doesn't support covariant return types for properties or methods specified in interfaces or for virtual method overriding. You can use explicit interface implementation though:

public class HexGridC : IHexGrid //error CS0738: etc
{        
    public GridElList<HexC> hexs { get; set; } // error related location

    IEnumerable<Hex> IHexGrid.hexs { get { return hexs; } }
}

As an aside, this all seems terribly complicated code - and it's generally not a good idea to derive from List<T> in the first place. (Favour composition, or derive from Collection<T> which is designed for inheritance.) Does it really need to be so complicated? If it does, it would still have been worth cutting down the complexity of the example for the sake of the question.