Wildcards in C# generic constraints

Question

I'm aware that C# doesn't have generic wildcards, and that a similar effect can be achieved by generic methods, but I need to use a wildcard in a field and can't work out if there is any way to encode it.

List<State<Thing>> list; void AddToList<T>() where T : Thing { list.Add(new State<T>()); }

Of course, this doesn't work because the object being added isn't of type State<Thing>, it's of type State<T> where T : Thing. Is it possible to adjust the most internal type of the list to be the Java equivalent of ? extends Thing rather than just Thing?

Can you declare your list as List<State<T>> where T : Thing? — abatishchev
– abatishchev, Commented Dec 8, 2011 at 7:17

Marc Gravell · Accepted Answer · 2011-12-08 07:08:30Z

Note that C# 4 does have additional variance support, but it does not apply in the List<T> case, for various reasons (has both "in" and "out" methods, and is a class).

I think, however, the way to address this is with something like:

interface IState { // non-generic object Value { get; } // or whatever `State<Thing>` needs } class State<T> : IState { public T Value { get { ...} } // or whatever object IState.Value { get { return Value; } } }

and

List<IState> list; ...

which will then allow you to add any State<T>. It doesn't really use much of the T, and a cast would be needed to get from the object to T, but .... it will at least work.

I really don't understand why can't I cast as the following: (State<Thing>)new State<T> while T : Thing! But can (Thing)new T()
@abatishchev because a SomeSpecificThing is a Thing; but it is not true that a State<SomeSpecificThing> is a State<Thing>. There is no inheritance etc. If it were a generic interface, if could potentially be IState<out Thing>, in which case the 4.0 variance rules start to apply, and then yes, there is a cast from IState<SomeSpecificThing> to IState<Thing>.

Poca · Accepted Answer · 2011-12-08 08:32:00Z

You could try something like this :

 interface IState<out T> { } class State<T> : IState<T> { } class Thing {} List<IState<Thing>> list; void AddToList<T>() where T : Thing { list.Add(new State<T>()); }

abatishchev · Accepted Answer · 2011-12-08 09:26:43Z

Declare your list as the following:

class State<T> where T : Thing { ... List<State<T>> list = new List<State<T>>(); // or public class StateList<T> : List<State<T>> { void AddToList(T item); } }

then compiler will be able to convert.

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