本人初学，弄个程序却运行不了，希望高手看看是什么地方出错了？
using System;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Windows.Forms;
class StarGradienBrush:PrintableForm
{
 public new static void Main()
 {
  Application.Run(new StarGradienBrush());
 }
 public StarGradienBrush()
 {
  Text="Star Gradien Brush";
 }
 protected override void DoPage(Graphics grfx,Color clr,int cx,int cy)
 {
  Point [] apt=new Point[5];
  for(int i=0;i<apt.Length;i++)
  {
   double dAngle=(i*0.8-0.5)*Math.PI;
   apt[i]=new Point(
    (int)(cx*(0.50+0.48*Math.Cos(dAngle))),
    (int)(cy*(0.50+0.48*Math.Sin(dAngle))));
  }
  PathGradientBrush pgbrush=new PathGradientBrush(apt);
  pgbrush.CenterColor=Color.White;
  pgbrush.SurroundColors=new Color[1] {Color.Black};
  grfx.FillRectangle(pgbrush,0,0,cx,cy);
 }
}
运行的结果应该是个五角星形状的多边形．

“ 找不到类型或命名空间名称“PrintableForm”(是否缺少 using 指令或程序集引用？)” 这就是错误！