Question

Sorry if this has been asked before but it’s virtually impossible to google. I think that an int array implements IEnumerable and therefore Thing should be able to implement IThing. How come it doesn’t?

public interface IThing
{
    IEnumerable<int> Collection { get; }
}

public class Thing : IThing
{
    public int[] Collection { get; set; }
}

note that

public class Thing : IThing
{
    public int[] Array { get; set; }
    public IEnumerable<int> Collection
    {
         get
         {
              return this.Array;
         }
    }
}

is fine.

Answer 1

The interface implementation must implement the interface exactly. This prevents you from returning a type that implements that interface as the member.

If you wish to do this, one option is to implement the interface explicitly:

public interface IThing
{
    IEnumerable<int> Collection { get; }
}

public class Thing : IThing
{
    public int[] Collection { get; set; }
    IEnumerable<int> IThing.Collection { get { return this.Collection; } }
}

This allows your public API for the class to use the concrete type, but the interface implementation to be fulfilled correctly.

For example, with the above, you can write:

internal class Test
{
    private static void Main(string[] args)
    {
        IThing thing = new Thing { Collection = new[] { 3, 4, 5 } };

        foreach (var i in thing.Collection)
        {
            Console.WriteLine(i);
        }
        Console.ReadKey();
    }
}