graphics对象释放后仍然可以用，为什么？

Question

                private void Form1_MouseDown(object sender, MouseEventArgs e)
                {
                        Graphics thisMouseG = this.CreateGraphics();
                        Point endPoint = new Point(e.X ,e.Y );
                        thisMouseG.DrawLine(bluePen ,startPoint ,endPoint  );
                        startPoint = endPoint;
                        thisMouseG.Dispose();
                }thisMouseG.Dispose()后，再次点击鼠标，上次画的线还在，按道理线应该已经释放清楚了啊？

Answer 1

          private void Form1_MouseDown(object sender, MouseEventArgs e) 
                { 
                        Graphics thisMouseG = this.CreateGraphics(); 
                        Point endPoint = new Point(e.X ,e.Y ); 
                        thisMouseG.DrawLine(bluePen ,startPoint ,endPoint  ); 
                        startPoint = endPoint; 
                        thisMouseG.Dispose(); 
 this.Refresh();
                }