我们在设计用户登录模块时，经常会用到验证码，可以有效地防止黑客软件的恶意破解，现公开我常用的验证码的源代码，生成效果如图： 。

   使用方法：
         1、在Web项目中添加一个类，如“CreateImage.cs”，然后将我公布的源代码Copy进去；
         2、再新建一个Web窗体，如“Image.aspx”，在Page_Load中加入代码 “CreateImage.DrawImage ();”当然别忘了加上对类的引用哦！！
         3、在页面的合适位置上（你想放验证码的位置）上加上如下javascript代码就OK 了，
             <SCRIPT LANGUAGE="JavaScript">
                <!__
                    var numkey = Math.random();
                    numkey = Math.round(numkey*10000);
                    document.write("<img src=\"Image.aspx?k="+ numkey +"\" width=\"52\" height=\"23\" hspace=\"4\"");
                //__>
             </SCRIPT>

源代码如下：

/// <summary>
/// 验证码模块
/// </summary>
public class CreateImage
{
public static void DrawImage()
{
CreateImage img=new CreateImage();
HttpContext.Current.Session["CheckCode"]=img.RndNum(4);
img.CreateImages(HttpContext.Current.Session["CheckCode"].ToString());
}

/// <summary>
/// 生成验证图片
/// </summary>
/// <param name="checkCode">验证字符</param>
private void CreateImages(string checkCode)
{
int iwidth = (int)(checkCode.Length * 13);
System.Drawing.Bitmap image = new System.Drawing.Bitmap(iwidth, 23);
Graphics g = Graphics.FromImage(image);
g.Clear(Color.White);
//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"};
Random rand = new Random();
//随机输出噪点
for(int i=0;i<50;i++)
{
int x = rand.Next(image.Width);
int y = rand.Next(image.Height);
g.DrawRectangle(new Pen(Color.LightGray, 0),x,y,1,1);
}

//输出不同字体和颜色的验证码字符
for(int i=0;i<checkCode.Length;i++)
{
int cindex = rand.Next(7);
int findex = rand.Next(5);

Font f = new System.Drawing.Font(font[findex], 10, System.Drawing.FontStyle.Bold);
Brush b = new System.Drawing.SolidBrush(c[cindex]);
int ii=4;
if((i+1)%2==0)
{
ii=2;
}
g.DrawString(checkCode.Substring(i,1), f, b, 3+(i*12), ii);
}
//画一个边框
g.DrawRectangle(new Pen(Color.Black,0),0,0,image.Width-1,image.Height-1);

//输出到浏览器
System.IO.MemoryStream ms = new System.IO.MemoryStream();
image.Save(ms,System.Drawing.Imaging.ImageFormat.Jpeg);
HttpContext.Current.Response.ClearContent();
//Response.ClearContent();
HttpContext.Current.Response.ContentType = "image/Jpeg";
HttpContext.Current.Response.BinaryWrite(ms.ToArray());
g.Dispose();
image.Dispose();
}

/// <summary>
/// 生成随机的字母
/// </summary>
/// <param name="VcodeNum">生成字母的个数</param>
/// <returns>string</returns>
private string RndNum(int VcodeNum)
{
string Vchar = "0,1,2,3,4,5,6,7,8,9" ;
string[] VcArray = Vchar.Split(',') ;
string VNum = "" ; //由于字符串很短，就不用StringBuilder了
int temp = -1 ; //记录上次随机数值，尽量避免生产几个一样的随机数

//采用一个简单的算法以保证生成随机数的不同
Random rand =new Random();
for ( int i = 1 ; i < VcodeNum+1 ; i++ )
{
if ( temp != -1)
{
rand =new Random(i*temp*unchecked((int)DateTime.Now.Ticks));
}
int t = rand.Next(VcArray.Length ) ;
if (temp != -1 && temp == t)
{
return RndNum( VcodeNum );
}
temp = t ;
VNum += VcArray[t];
}
return VNum ;
}
}