Is it impossible to use Generics dynamically?

Question

I need to create at runtime instances of a class that uses generics, like class<T>, without knowing previously the type T they will have, I would like to do something like that:

public Dictionary<Type, object> GenerateLists(List<Type> types)
{
 Dictionary<Type, object> lists = new Dictionary<Type, object>();

 foreach (Type type in types)
 {
  lists.Add(type, new List<type>()); /* this new List<type>() doesn't work */
 }

    return lists;
}

...but I can't. I think it is not possible to write in C# inside the generic brackets a type variable. Is there another way to do it?

Answer 1

You can't do it like that - the point of generics is mostly compile-time type-safety - but you can do it with reflection:

public Dictionary<Type, object> GenerateLists(List<Type> types)
{
    Dictionary<Type, object> lists = new Dictionary<Type, object>();

    foreach (Type type in types)
    {
        Type genericList = typeof(List<>).MakeGenericType(type);
        lists.Add(type, Activator.CreateInstance(genericList));
    }

    return lists;
}

Answer 2

Depending on how often you're calling this method then using Activator.CreateInstance could be slow. Another option is to do something like this:

private Dictionary> delegates = new Dictionary>();

    public Dictionary<Type, object> GenerateLists(List<Type> types)
    {
        Dictionary<Type, object> lists = new Dictionary<Type, object>();

        foreach (Type type in types)
        {
            if (!delegates.ContainsKey(type))
                delegates.Add(type, CreateListDelegate(type));
            lists.Add(type, delegates[type]());
        }

        return lists;
    }

    private Func<object> CreateListDelegate(Type type)
    {
        MethodInfo createListMethod = GetType().GetMethod("CreateList");
        MethodInfo genericCreateListMethod = createListMethod.MakeGenericMethod(type);
        return Delegate.CreateDelegate(typeof(Func<object>), this, genericCreateListMethod) as Func<object>;
    }

    public object CreateList<T>()
    {
        return new List<T>();
    }

On the first hit it'll create a delegate to the generic method that creates the list and then puts that in a dictionary. On each subsequent hit you'll just call the delegate for that type.

Hope this helps!