验证码函数；

public string yanzhen()
{
Random r = new Random();
string str = null;
for (int i = 0; i < 5; i++)
{
int number = r.Next(0, 9);
str += number;
}
Bitmap bmp = new Bitmap(120, 50);
Graphics g = Graphics.FromImage(bmp);
for (int i = 0; i < 5; i++)
{
Point p = new Point(i * 19, 0);
string[] font = { "微软雅黑", "仿宋", "黑体", "隶书", "楷书" };
Color[] color = { Color.Green, Color.Brown, Color.Red, Color.Pink, Color.RoyalBlue };
g.DrawString(str[i].ToString(), new Font(font[r.Next(0, 5)], 30, FontStyle.Bold), new SolidBrush(color[r.Next(0, 5)]), p);
}
for (int i = 0; i < 20; i++)
{
Point p1 = new Point(r.Next(0, bmp.Width), r.Next(0, bmp.Height));
Point p2 = new Point(r.Next(0, bmp.Width), r.Next(0, bmp.Height));
g.DrawLine(new Pen(Brushes.Green), p1, p2);
}
for (int i = 0; i < 200; i++)
{
Point p = new Point(r.Next(0, bmp.Width), r.Next(0, bmp.Height));
bmp.SetPixel(p.X, p.Y, Color.Black);
}
pictureBox1.Image = bmp;
return str;
}