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?

Can you give a reason why you want to create a type safe Generic Object at runtime? — Henry B, Oct 24 '08 at 15:27
I've done it several times, particularly when porting Protocol Buffers. — Jon Skeet, Oct 24 '08 at 15:28
I am doing an adapter to a persistence framework here in my job, and the classes needs this information to perform their work. — Victor Rodrigues, Oct 24 '08 at 15:30

score 18 · Accepted Answer · answered Oct 24 '08 at 15:24

18

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;
}

answered Oct 24 '08 at 15:24

Jon Skeet

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thanks again Jon. Does CreateInstance require an empty constructor? I don't have it, actually I need to input a parameter into it. – Victor Rodrigues Oct 24 '08 at 15:35
It does require a constructor. If you don't want to use a constructor you can use FormatterServices.GetUninitializedObject() in the System.Runtime.Serialization assembly. – Jason Jackson Oct 24 '08 at 15:38
1

@Victor: There are overloads of Activator.CreateInstance which take parameters, or you could use Type.GetConstructor and then invoke that. Basically there are plenty of ways of going from a type to an instance of a type - pick one :) – Jon Skeet Oct 24 '08 at 16:10
1

Note that this code isn't creating any instances of type though - just List, which doesn't care about what constructors are available for the type. – Jon Skeet Oct 24 '08 at 16:11

score 4 · Answer 2 · answered Oct 24 '08 at 15:45

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!

Thanks jonni, I call this method just only once, otherwise keeping a delegate would be a good option. — Victor Rodrigues, Oct 24 '08 at 16:01

Is it impossible to use Generics dynamically?

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