Question

I have often wanted to create a list of objects where each object must implement a number of interfaces. For example, I’d like to do something similar to the following:

List<T> where T : IConvertible, IComparable _myList;

Another option I considered was to create a third interface that implements these two so that any object that implements these two interfaces inherently implements mine.

public interface IConvertibleAndComparable
    : IConvertible, IComparable { }

List<IConvertibleAndComparable> _myList;

With this I would be able to add any object that implements both IConvertible and IComparable, including double and int, as well as my own objects. Explicitly implementing IConvertibleAndComparable is not required since it does not add any new functionality beyond the interfaces in inherits.

I understand that the first snippet is illegal and the second, while legal, does not do what I want. Is it possible to achieve what I am trying to do? If not, would either of these be a candidate for a future C# feature?

(Note: This would be legitimate application for empty interfaces.)

Edit

In a more general sense, I’d like to perform one of the following:

private MyGenericClass<T> where T : IA, IB, ... _myClass;

where I can declare all of the restrictions on T that I need, or

public interface IMyCombinedInterface : IA, IB, ... {}
private MyGenericClass<IMyCombinedInterface> _myClass;

where any type that implements IA, IB, and ... inherently (or implicitly) implements IMyCombinedInterface (only when IMyCombinedInterface doesn’t explicitly declare new functionality).

Answer 1

You can, as a workaround, do a kind of superposition wrapper, and store it in the list. Look here for the idea.

To your example you could do:

public class Junction
{
    public IConvertible Convertible { get; private set; }
    public IComparable Comparable { get; private set; }
    private Junction() { }
    public static Junction Create<T>(T value) where T : IConvertible, IComparable
    {
        return new Junction
        {
            Convertible = value,
            Comparable = value
        };
    }
}

And then add Junctions to your list:

var l = new List<Junction>();
l.Add(Junction.Create(1));