//重写按钮的OnPaint事件
class newbtn : System.Windows.Forms.Button //继承之系统按钮控件
{
      protected override void OnPaint(PaintEventArgs e)
      {
           //重写
           base.OnPaint(e);
           System.Drawing.Pen pen = new Pen(this.BackColor, 3);
           e.Graphics.DrawRectangle(pen, 0, 0, this.Width, this.Height);
           //填充
           pen.Dispose();
      }
}
//然后将designer.cs文件中你要修改的按钮重新继承至newbtn就可。