C# Accessing values of a derived class in an array of the base class -

this not working hope makes clear example:

public abstract class shape {      public int area;      public int perimeter;      public class polygon : shape      {          public int sides;          public polygon(int a, int p, int s){              area = a;              perimeter = p;              sides = s;          }      }      public class circle : shape      {          public int radius;          public circle(int r){               area = 3.14*r*r;               perimeter = 6.28*r;               radius = r;          }      } } 

in main function have this:

shape[] thisarray = new shape[5]; thisarray[0] = new shape.circle(5); thisarray[1] = new shape.polygon(25,20,4); 

my problem when deal thisarray, can't access values other area , perimeter. for example:

if (thisarray[0].area > 10)    //this statement executed  if (thisarray[1].sides == 4)    //this not compile 

how can access sides thisarray[1]? access if did like
shape.polygon randomsquare = new shape.polygon(25,20,4) not if in array of shapes.

if recall correctly accomplished in c++ doing like
polygon->thisarray[1].sides (i forget called) not know how in c#

if can't trying do, how can circumvent problem?

thank reading through intended short, appreciated.

you should use casting:

(thisarray[1] shape.polygon).sides 

note should make sure underlying object instance polygon, otherwise raise exception. can using like:

if(thisarray[1] shape.polygon){     (thisarray[1] shape.polygon).sides }