I am getting following error any idea, what could be the possible cause.

class Class1
{
[STAThread]
static void Main(string[] args)
{
Foo b1 = new Bar();
b1.Response();
Console.ReadLine();
}
}
sealed class Foo
{
public virtual void Response()
{
Console.WriteLine("In Foo");
}
}
//Output
//Error: 'TestCmd.Foo.Response()' is a new virtual member in a sealed class 'TestCmd.Foo'