As Keyword, conversion of derived class to base class type

[C# Programming]

Here is example code I made for this question:
using System;
namespace TestingAs
{
class baseClass
{
public string foo
{
get;
set;
}
}
class derivedClass : baseClass
{
public string bar
{
get;
set;
}
}

class Program
{
static void Main(string[] args)
{
derivedClass derived = new derivedClass();
baseClass b = derived as baseClass;
Console.WriteLine("type is {0}", b.GetType());
Console.ReadLine();
}
}
}

I also tried just casting baseclass b = (baseClass)derived; which gives the same result:
type is TestingAs.derivedClass

Why b is not type of baseClass? How would I "convert" my derived class into base class type?

Thank you in advance!