using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var list = new List<BaseClass>();

        list.Add(new ChildClass());

        list.Add(new BaseClass());

        var childList = list

            .Where(x => x is ChildClass)

            .Select(x => x as ChildClass)

            .ToList();

        Console.WriteLine((childList.Count()));

        childList = list.OfType<ChildClass>().ToList();

        Console.WriteLine((childList.Count()));

    }

}

​

public class BaseClass

{

}

​

public class ChildClass : BaseClass

{

}