Question

I want to create a generic List<> whose type is declared at runtime.

I can do the following, but since it’s dynamic, I suspect there is a speed penalty. I’m writing a wrapper to an exotic database, so speed is critical.

List<dynamic> gdb = new List<dynamic>()

I read this post in dynamic generic types, but can’t get it to work. Specifically, the object is not appearing as a List and hence has no add method.

    Type ac;

    switch (trail[dataPos].Type)
    {
        case GlobalsSubscriptTypes.Int32:
            ac = typeof(System.Int32);

            break;
        case GlobalsSubscriptTypes.Int64:
            ac = typeof(System.Int64);

            break;

        default:
            ac = typeof(System.String);

            break;
    }

    var genericListType = typeof(List<>);
    var specificListType = genericListType.MakeGenericType(ac);
    var gdb = Activator.CreateInstance(specificListType);

How do I get gdb to appear as one of the following:

List<System.Int32>
List<System.Int64>
List<System.String>

Answer 1

Once you’ve used Activator.CreateInstance, you can cast the result to the appropriate type. You could use IList for example:

var gdb = (IList)Activator.CreateInstance(specificListType);
gdb.Add(1);

Note that the above throws an ArgumentException if the type you’re adding does not match the generic type.