OcpAfter.cs

// Ta có 3 class: vuông, tròn, tam giác, kế thừa class Shape
// Chuyển logic tính diện tích vào mỗi class
public abstract class Shape 
{
  public double Area();
}
public class Square : Shape
{
  public double Height { get; set; }
  public double Area() {
    return Math.Sqrt(this.Height);
  }
}
public class Circle : Shape
{
  public double Radius { get; set; }
  public double Area() {
    return this.Radius * this.Radius * Math.PI;
  }
}
public class Triangle : Shape
{
  public double FirstSide { get; set; }
  public double SecondSide { get; set; }
  public double ThirdSide { get; set; }
  public double Area() {
    double TotalHalf = (this.FirstSide + this.SecondSide + this.ThirdSide) / 2;
    var area += Math.Sqrt(TotalHalf * (TotalHalf - this.FirstSide) * 
        (TotalHalf - this.SecondSide) * (TotalHalf - this.ThirdSide));
    return area;
  }
}

// Module in ra diện tích các hình
public class AreaDisplay
{
  public double ShowArea(List<Shape> shapes)
  {
    foreach (var shape in shapes) {
      Console.WriteLine(shape.Area());
    }
  }
}