using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var listOfThings = AddData();

        for(var i=0; i<listOfThings.Count; i++)

        {

            var o = listOfThings[i];

            if (o is IntId)

            {

                Console.WriteLine("Item {0} is an IntId", i);

            }

            else if (o is StringId)

            {

                Console.WriteLine("Item {0} is a StringId", i);

            }

        }

    }

    private static List<object> AddData()

    {

        var list = new List<object>();

        for(var i=0; i<10; i++)

        {

            if (i%2==0)

            {

                list.Add(new IntId{Id=i});

            }

            else

            {

                list.Add(new StringId{Id=i.ToString()});

            }

        }

        return list;

    }

}

​

public class IntId

{

    public int Id{get;set;}

}

​

public class StringId

{

    public string Id{get;set;}

}