List to IEnumerable problem in interface

Question

namespace WpfApplication3
{
public class Hex
{
    public String terr;
}    

public class HexC : Hex
{
    int Cfield;
}

public interface IHexGrid
{
    IEnumerable<Hex> hexs { get; }
}

public class hexGrid : IHexGrid //error CS0738: 'WpfApplication3.hexGrid' does not               
{
    public List<Hex> hexs { get; set; }
}

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        List<HexC> hexList1 = new List<HexC>();
        genmethod(hexList1); //but this compiles fine          
    }

    void genmethod(IEnumerable<Hex> hexList)
    {
        string str1;
        foreach (Hex hex in hexList)
            str1 = hex.terr;            
    }
}
}

The full error message is: 'WpfApplication3.hexGrid' does not implement interface member 'WpfApplication3.IHexGrid.hexs'. 'WpfApplication3.hexGrid.hexs' cannot implement 'WpfApplication3.IHexGrid.hexs' because it does not have the matching return type of 'System.Collections.Generic.IEnumerable'.

Why doesn't List implicitly cast to IEnumerable above? Thanks in advance!

There's no casting involved with property declarations. – BoltClock Sep 21 '11 at 16:33 — BoltClock, Sep 21 '11 at 16:33

score 6 · Answer 1 · edited May 23 '17 at 12:01

6

That kind of casting (covariance?) doesn't apply to class/interface declarations. It can only be done on parameters and return types.

The compiler is looking for the exact signature, isn't finding it, and rightly saying you didn't implement the member. The property/method signature must match exactly.

This answer actually sums it up better than I could explain.

edited May 23 '17 at 12:01

Community

1
1

answered Sep 21 '11 at 16:33

Christopher Currens

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Is there any reason for C# not applying covariance to class/interface declarations? – Rich Oliver Sep 21 '11 at 16:59
Cheers, I think I've sort of got it. but I will have to reflect at leisure to fully comprehend the issue. – Rich Oliver Sep 21 '11 at 18:05

Jordan Parmer · Accepted Answer · 2011-09-21T16:52:54.937

3

Christopher has the right answer, but you could easily work around this by having a private property be the actual list and then the getter on the hexGrid just return the casted IEnumerable interface from the List.

public class hexGrid : IHexGrid               
{
    private List<Hex> _hexs { get; set; }
    public IEnumerable<Hex> hexs
    {
       get { return _hexs as IEnumerable; }
       set { _hexs = new List<Hex>( value ); }
    }
}

edited Sep 21 '11 at 16:52

answered Sep 21 '11 at 16:38

Jordan Parmer

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You should not need the `as` cast. – BoltClock Sep 21 '11 at 16:46
@BoltClock - True, I just added it for clarity. – Jordan Parmer Sep 21 '11 at 16:52
Ah thanks, that did the job. I put the set to: set{} as the whole point of IEnumerable is that the collection is effective read only. I wish I'd discovered IEnumerable earlier, its indispensable when working with List – Rich Oliver Sep 21 '11 at 16:54
@RichOliver - In that case you can probably remove the 'set' property altogether if it isn't needed elsewhere. – Jordan Parmer Sep 21 '11 at 16:57
No I tried that it complains: error CS0535: 'WpfApplication3.hexGrid' does not implement interface member 'WpfApplication3.IHexGrid.hexs.set' – Rich Oliver Sep 21 '11 at 17:08
@RichOliver - Oh, I mean remove the setter from the interface (assuming you have the ability to change that interface and the change doesn't break other code that needs it). – Jordan Parmer Sep 21 '11 at 18:45
Yes my mistake, I realised that just before I read your comment. – Rich Oliver Sep 21 '11 at 19:07

List to IEnumerable problem in interface

2 Answers2