using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var teste = new Foo { details = new List<Detail>() } ;

        teste.details.Add(new Detail { data = "oi", data2 = "io"} );

        Exec(teste);

    }   

    public class FooBase{

        public string baseValue {get;set;}

    }

    public class Foo : FooBase, IBoo<Detail>{

        public List<Detail> details {get;set;}

    }

    public static void Exec(FooBase obj){

        var a = (dynamic)obj;       

        Console.WriteLine("Count: " + a.details.Count);

        foreach(var item in a.details){

            Console.WriteLine("Data: " + item.data);

            Console.WriteLine("Data: " + item.data2); //como acesso o data 2 ?

        }

    }

    public interface IBoo<T> {

        List<T> details {get;set;}

    }

    public interface IData{

        string data {get;set;}

    }

    public class Detail: IData {

        public string data {get;set;}

        public dynamic data2 {get;set;}

    }

}