In reading your question you mention that you are trying to select multiple calculators, in your example you are trying to assign your keyboard input to your VolumeSphere Object. Here is a quick and dirty implementation of a menu for your calculator, you can play with it and expand it to meet your needs.
class MainClass
{
static void Main(string[] args)
{
int value;
bool run;
run = true;
while(run)
{
Console.WriteLine("Calculator Menu");
Console.WriteLine("1: Compute Volume of a Sphere");
Console.WriteLine("2: Compute Something else");
Console.WriteLine("etc,etc,etc");
if (int.TryParse(Console.ReadLine(), out value))
{
switch (value)
{
case 1:
VolumeSphere Sphere = new VolumeSphere();
Sphere.Details();
Console.WriteLine(Sphere.GetVolume());
Sphere.Display();
break;
case 2:
break;
case 3:
break;
default:
Console.WriteLine("GoodBye");
run = false;
break;
}
}
}
Console.WriteLine("Press Any Key to Exit");
Console.ReadLine();
}
}