public abstract class A
{
  // Class members here.
}

public abstract class A
{
  public abstract void DoWork(int i);
}

// compile with: /target:library
public class D
{
  public virtual void DoWork(int i)
  {
    // Original implementation.
  }
}

public abstract class E : D
{
  public abstract override void DoWork(int i);
}

public class F : E
{
  public override void DoWork(int i)
  {
    // New implementation.
  }
}

csc abstractshape.cs shapes.cs shapetest.cs

// compile with: csc /target:library abstractshape.cs
public abstract class Shape
{
  private string name;

  public Shape(string s)
  {
    // calling the set accessor of the Id property.
    Id = s;
  }

  public string Id
  {
    get
    {
      return name;
    }

    set
    {
      name = value;
    }
  }

  // Area is a read-only property - only a get accessor is needed:
  public abstract double Area
  {
    get;
  }

  public override string ToString()
  {
    return Id + " Area = " + string.Format("{0:F2}", Area);
  }
}

public abstract double Area

// compile with: csc /target:library /reference:abstractshape.dll shapes.cs
public class Square : Shape
{
  private int side;

  public Square(int side, string id)
    : base(id)
  {
    this.side = side;
  }

  public override double Area
  {
    get
    {
      // Given the side, return the area of a square:
      return side * side;
    }
  }
}

public class Circle : Shape
{
  private int radius;

  public Circle(int radius, string id)
    : base(id)
  {
    this.radius = radius;
  }

  public override double Area
  {
    get
    {
      // Given the radius, return the area of a circle:
      return radius * radius * System.Math.PI;
    }
  }
}

public class Rectangle : Shape
{
  private int width;
  private int height;

  public Rectangle(int width, int height, string id)
    : base(id)
  {
    this.width = width;
    this.height = height;
  }

  public override double Area
  {
    get
    {
      // Given the width and height, return the area of a rectangle:
      return width * height;
    }
  }
}

// compile with: csc /reference:abstractshape.dll;shapes.dll shapetest.cs
class TestClass
{
  static void Main()
  {
    Shape[] shapes =
    {
      new Square(5, "Square #1"),
      new Circle(3, "Circle #1"),
      new Rectangle( 4, 5, "Rectangle #1")
    };

    System.Console.WriteLine("Shapes Collection");
    foreach (Shape s in shapes)
    {
      System.Console.WriteLine(s);
    }
  }
}

  Shapes Collection
  Square #1 Area = 25.00
  Circle #1 Area = 28.27
  Rectangle #1 Area = 20.00