How to use Activator to create an instance of a generic Type and casting it back to that type?

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Since the actual type T is available to you only through reflection, you would need to access methods of Store<T> through reflection as well:

Type constructedType = classType.MakeGenericType(typeParams);

object x = Activator.CreateInstance(constructedType, new object[] { someParameter });
var method = constructedType.GetMethod("MyMethodTakingT");
var res = method.Invoke(x, new object[] {someObjectThatImplementsStorable});

EDIT You could also define an additional IStore interface that does not use generics, and uses IStorable instead:

interface IStore {
    int CountItems(IStorable item);
}
class Store<T> : IStore where T : IStorable {
    int CountItems(IStorable item) {
        return count;
    }
}

Your Store<T> would remain generic, but you would get access to its CountItems by casting to IStore:

var x = (IStore)Activator.CreateInstance(constructedType, new object[] { someParameter });
var count = x.CountItems((IStorable)someObjectThatImplementsStorable);

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