shell類中有一個虛方法；

而其子類Cylinder、Head及其子類中去實現此虛方法；

這樣，在其它類如Main類中可以通過shell及其子類的例項來呼叫此虛方法；

如：new一個HeadEllipse EH

用方法Draw(EH)來實現；

  public class Main
  {
　　
    public void Draw(Shell shell)
    {
      shell.Draw();
    }
  }

  public class Shell : Part
  {
    public virtual ObjectId Draw()
    {
      
 
return ObjectId.Null;
    }
  }

  public class Cylinder : Shell
  {
    public override ObjectId Draw()
    {
      int identify = this.GetCurrentPartIdentity<Cylinder>() + 1;
      ObjectId id = Draw(identify);
      this.SetCurrentPartIdentity<Cylinder>(identify);
      return id;
    }
  }

  
 
public class Head : Shell
  {

  }

  public class HeadEllipse : Head
  {
    public override ObjectId Draw()
    {
      int identify = this.GetCurrentPartIdentity<HeadEllipse>() + 1;
      ObjectId id = Draw(identify);
      this.SetCurrentPartIdentity<HeadEllipse>(identify);
      return
 
 id;
    }
  }

  public class HeadDished : Head
  {
    public override ObjectId Draw()
    {
      int identify = this.GetCurrentPartIdentity<HeadDished>() + 1;
      ObjectId id = Draw(identify);
      this.SetCurrentPartIdentity<HeadDished>(identify);
      return id;
    }
  }