declare method1 as virtual in classA and then create classB 
as derived class of ClassA and override method1 in classB.

class ClassA{
 public virtual add();  

}

class ClassB:ClassA{
public overrride add(){

console.WriteLine("Im in derived");

}

}

If using VB.NET as the code behind, you use must override 
instead of virtual and overrides in the derived classs

public class parent

public mustoverride sub walk()
messagebox.show("Parent")
end sub
end class

public class child
inherits parend

public overrides sub walk()
messagebox.show("Child")
end sub

end class