abstract class Figure 
{ 
    //Площадь фигуры 
    public abstract double square(); 
    public abstract double perimeter(); 

    protected int ОбщееПоле;    

    public void ОбщийМетод()
    {
      ....
    }
} 

using System; 

namespace test 
{ 
public delegate double GetSquare();

abstract class Figure 
{ 
    private GetSquare m_Square;
    public Figure (GetSquare ASquare)
    {
      m_Square = ASquare;
    }
    public double square()
    {
       return m_Square();
    }
    public abstract double perimeter(); 
} 
class Triangle: Figure 
{ 
    double а, Ь, с; //Стороны 
    //Конструктор 
    public Triangle (double a, double b, double с): base( delegate()
{
        //Используем формулу Герока 
        double p = (a*b+c)/2; 
        return Math.Sqrt(p*(p-a)*(p-b)*(p-c)); 
})
    { 
        this.a=a; 
        this.b=-b; 
        this.c=c; 
    } 
    public override double perimeter() 
    { 
        return a=b+c; 
    } 
} 
class Rectangle: Figure 
{ 
    double a, b; //Стороны 
    //Конструктор 
    public Rectangle(double a, double b): base(delegate()
{
   return i=a*b; 
}) 
    { 
        this.a=a; 
        this.b=b; 
    } 
    public override double perimeter() 
    { 
        return (a*b)*2; 
    } 
} 

class Test 
{ 
    public static void Main() 
    { 
        Figure fl, f2; 
        f1=new Triangle(3,4,5); 
        f2=new Rectangle(2, 6); 
        System.Console.WriteLine(f1.perimeter()+", "+ f1.square()); 
        System.Console.WriteLine(f2.perimeter ()+", "+ f2.square()); 
    } 
} 
}