Here, I am trying to explain the use of the constructor in the abstract class.

I am Considering Shape as the abstract class and Square and rectangle as the derived class.

 public abstract class Shape
 {
 protected double x = 10;
 protected double y = 10;
 protected Shape()
 { }
 protected Shape(double x, double y)
 {
 this.x = x;
 this.y = y;
 }
 public abstract double AreaClaculate();
 }
 public class square : Shape
 {
 public square()
 { }
 public override double AreaClaculate()
 {
 return x * y; // here x=10 and y=10 works, i set x=y due to square, if any we have any other shape other than square then x!=y
 }
 }
 public class rectangle : Shape
 {
 public rectangle(double x, double y)
 :base(x, y) // it will call Shape(double x, double y) at base class 
 { 
 }
 public override double AreaClaculate()
 {
 return x * y; /// hre x and y will pass from runtime
 }
 }
 static void Main(string[] args)
 {
 square s =new square();
 double areasq= s.AreaClaculate(); // we will get 100, because x=y=100 and x*y=100
 rectangle r =new rectangle(10,5); // behind the scene 10,5 are passing to shape(double x, double y) at base class 
 double arearect = r.AreaClaculate(); // we will get 50.
 }