using System;

public class Program

{

    public static void Main()

    {

        var b = new BClass();

        var a = b as AClass;

        a = (AClass)b;

        Console.WriteLine(a.GetType());

    }

}

​

public class AClass

{

    public string a {get; set;}

}

​

public class BClass : AClass

{

    public string b {get; set;}

}

​