accessing ability subsequent category bottom category c#

in c#, best proceed opening ability subsequent category general list contains only bottom class.

public category classa : baseclass
{
   open vigilant propertya { get; set; }
}

public category classb: baseclass
{
    open vigilant propertyb { get; set; }
}

public category baseclass
{
}

public vacant main
{
    list<baseclass> mylist = new list<baseclass>();
    classa = new classa();
    classb b = new classb();

    mylist.add(a);
    mylist.add(b);

    for(int i = 0; i < mylist.count; i++)
    {
        //i opening propertya abd propertyb subsequent classes        
    }
}