在c#的winform登录窗体中，如何实现窗口验证码

c#的winform可开发qq聊天程序，聊天登录窗体验证码是如何实现的呢？
在winform中，需要增加自己的控件
一个picturebox控件pbverifycode
一个按钮butupdateverifycode
代码如下：
public partial class FrmLogin : Form
{
//随机码的长度
private const int iVerifyCodeLength = 6;
//随机码
private String strVerifyCode = "";
public FrmLogin()
{
InitializeComponent();
UpdateVerifyCode();
}
//更新验证码
private void UpdateVerifyCode()
{
strVerifyCode = CreateRandomCode(iVerifyCodeLength);
CreateImage(strVerifyCode);
}        private string CreateRandomCode(int iLength)
{
int rand;
char code;
string randomCode = String.Empty;
//生成一定长度的验证码
System.Random random = new Random();
for (int i = 0; i < iLength; i++)
{
rand = random.Next();
if (rand % 3 == 0)
{
code = (char)('A' + (char)(rand % 26));
}
else
{
code = (char)('0' + (char)(rand % 10));
}                randomCode += code.ToString();
}
return randomCode;
}
///  创建随机码图片
private void CreateImage(string strVerifyCode)
{
try
{
int iRandAngle = 45;    //随机转动角度
int iMapWidth = (int)(strVerifyCode.Length * 21);
Bitmap map = new Bitmap(iMapWidth, 28);     //创建图片背景

Graphics graph = Graphics.FromImage(map);
graph.Clear(Color.AliceBlue);//清除画面，填充背景
graph.DrawRectangle(new Pen(Color.Black, 0), 0, 0, map.Width - 1, map.Height - 1);//画一个边框
graph.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias;//模式
Random rand = new Random();
//背景噪点生成
Pen blackPen = new Pen(Color.LightGray, 0);
for (int i = 0; i < 50; i++)
{
int x = rand.Next(0, map.Width);
int y = rand.Next(0, map.Height);
graph.DrawRectangle(blackPen, x, y, 1, 1);
}
//验证码旋转，防止机器识别
char[] chars = strVerifyCode.ToCharArray();//拆散字符串成单字符数组
//文字距中
StringFormat format = new StringFormat(StringFormatFlags.NoClip);
format.Alignment = StringAlignment.Center;
format.LineAlignment = StringAlignment.Center;
//定义颜色

Color[] c = { Color.Black, Color.Red, Color.DarkBlue, Color.Green,
Color.Orange, Color.Brown, Color.DarkCyan, Color.Purple };
//定义字体
string[] font = { "Verdana", "Microsoft Sans Serif", "Comic Sans MS", "Arial", "宋体" };
for (int i = 0; i < chars.Length; i++)
{
int cindex = rand.Next(7);
int findex = rand.Next(5);                    Font f = new System.Drawing.Font(font[findex], 13, System.Drawing.FontStyle.Bold);//字体样式(参数2为字体大小)
Brush b = new System.Drawing.SolidBrush(c[cindex]);
Point dot = new Point(16, 16);

float angle = rand.Next(-iRandAngle, iRandAngle);//转动的度数
graph.TranslateTransform(dot.X, dot.Y);//移动光标到指定位置
graph.RotateTransform(angle);
graph.DrawString(chars[i].ToString(), f, b, 1, 1, format);

graph.RotateTransform(-angle);//转回去
graph.TranslateTransform(2, -dot.Y);//移动光标到指定位置
}
pbVerifyCode.Image = map;
}
catch (ArgumentException)
{
MessageBox.Show("创建图片错误。");
}
}

private void butUpdateVerifyCode_Click(object sender, EventArgs e)
{
UpdateVerifyCode();
}
代码全部写完。