C＃泛型继承和协方差

Question

I'm building a collection library and I'd like to have all the generic collection interfaces require class types and all the collections implementing them to be any type. So on value types the collection would have two methods, one for the value type and one for boxing it. Is this possible?

Like this:

interface ICollection<ItemType> where ItemType : class
{
    void DoSomething(ItemType item);
}

class Collection<ItemType> : ICollection<ItemType>
{
    void DoSomething(Object item);
    void DoSomething(ItemType item);
}

Barring that what would be the best way to get around it? The interfaces being non-generic?

Answer 1

The line:

ICollection<Object> s = new Collection<String>();

(comments) would work with out variance; however, DoSomething(ItemType) would require in variance; so that type can be neither in nor out : variance does not apply here .

The way that is usually handled is by having a generic and non-generic API. People interested in the specific types can use the generic API; people just interested in "an object" can use the non-generic API.

For illustration:

interface ICollection
{
    void DoSomething(object item);
}
interface ICollection<ItemType> : ICollection
{
    void DoSomething(ItemType item);
}

class Collection<ItemType> : ICollection<ItemType>
{
    void ICollection.DoSomething(Object item)
    {
        DoSomething((ItemType)item);
    }
    public void DoSomething(ItemType item)
    {
        //...
    }
}

Then this works:

ICollection s = new Collection<String>();
object o = "abcd";
s.DoSomething(o);

Answer 2

Value types are always boxed as type object . That would force a collection of any boxed value type to be a Collection<object> , which is not really typesafe (noone prevents you from adding eg a string , then). Type safety is meant to be one of the biggest advantages of generic types though. So I would suggest dropping the class constraint, if possible.

BTW: You can still assign these generic interfaces (without class constraint) to their non-generic version:
IList l = new List<int>();