old.cs

// Ta có 3 class: vuông, tròn, tam giác, kế thừa class Shape
public class Shape 
{
}
public class Square : Shape
{
  public double Height { get; set; }
}
public class Circle : Shape
{
  public double Radius { get; set; }
}
public class Triangle : Shape
{
  public double FirstSide { get; set; }
  public double SecondSide { get; set; }
  public double ThirdSide { get; set; }
}

// Module in ra diện tích các hình
public class AreaDisplay
{
  public double ShowArea(List<Shape> shapes)
  {
    foreach (var shape in shapes) {
      // Nếu yêu cầu thay đổi, thêm shape khác, ta phải sửa module Area Calculator
      if (shape is Square) {
        Square square = (Square)shape;
        var area += Math.Sqrt(square.Height);
        Console.WriteLine(area);
      }
      if (shape is Triangle) {
        Triangle triangle = (Triangle)shape;
        double TotalHalf = (triangle.FirstSide + triangle.SecondSide + triangle.ThirdSide) / 2;
        var area += Math.Sqrt(TotalHalf * (TotalHalf - triangle.FirstSide) * 
        (TotalHalf - triangle.SecondSide) * (TotalHalf - triangle.ThirdSide));
        Console.WriteLine(area);
      }
      if (shape is Circle) {
        Circle circle = (Circle)shape;
        var area += circle.Radius * circle.Radius * Math.PI;
        Console.WriteLine(area);
      }
    }
  }
}