网站上验证码效果一般制作方法是：
1）使用HttpHandler（一般处理程序）绘制随机验证码的图，以及产生随机码，并输出到页面的OutputStream中。
2）页面中使用异步方式（js等）进行刷新当前页面的验证码。
【示例】
1）创建一个“一般应用处理程序ashx”，代码如下：
[C#]
public class ValidationCode : IHttpHandler
{
//随机发生器
static Random r = new Random(Guid.NewGuid().GetHashCode());
//排除黑色、透明色颜色，因为底色黑色
static PropertyInfo[] colors = (typeof(Brushes).GetProperties(System.Reflection.BindingFlags.Public | System.Reflection.BindingFlags.GetProperty | System.Reflection.BindingFlags.Static)).Where(p => p.Name != "Black" && p.Name != "Transparent").Select(p => p).ToArray();
//排除黑色颜色，因为底色黑色
static PropertyInfo[] linecolors = (typeof(Pens).GetProperties(System.Reflection.BindingFlags.Public | System.Reflection.BindingFlags.GetProperty | System.Reflection.BindingFlags.Static)).Where(p => p.Name != "Black").Select(p => p).ToArray();
//获取静态类Brushes实例对象
static object colorobj = typeof(Brushes).GetConstructor(BindingFlags.NonPublic, null, Type.EmptyTypes, null);
//获取静态类Pens实例对象
static object penobj = typeof(Pens).GetConstructor(BindingFlags.NonPublic, null, Type.EmptyTypes, null);
//每个随机字符的宽度
const float PERNUMBERWIDTH = 40.0f;
//每个字符的高度
const float PERNUMBERHEIGHT = 50.0f;
public void ProcessRequest(HttpContext context)
{
//获取要产生多少随机数（默认产生5个）
int reqNum = 5;
if (context.Request.QueryString["reqNum"] != null)
{
int.TryParse(context.Request.QueryString["reqNum"], out reqNum);
}
//产生多少大的背景图
Bitmap bt = new Bitmap((int)(PERNUMBERWIDTH*reqNum), (int)PERNUMBERHEIGHT);
Graphics g = Graphics.FromImage(bt);
//产生4个随机数（number可以被保存到Session中）
string numbers = "";
//绘制数字
for (int i = 1; i <= reqNum; i++)
{
numbers += r.Next(0, 9).ToString();
var color = (PropertyInfo)colors.GetValue(r.Next(0, colors.Length));
context.Response.Write(color.Name + "<br/>");
Brush randomcolor = (Brush)color.GetValue(colorobj, null);
g.DrawString(numbers[i-1].ToString(), new Font("黑体", PERNUMBERWIDTH),randomcolor, new PointF((i-1)*PERNUMBERWIDTH, 0f));
}
//绘制随机线条
int linenum = r.Next(10, 21);
for (int i = 1; i <= linenum; i++)
{
var linecolor = (PropertyInfo)linecolors.GetValue(r.Next(0, colors.Length));
Pen randomcolor = (Pen)linecolor.GetValue(penobj, null);
g.DrawLine(randomcolor, new PointF((float)(r.NextDouble() * PERNUMBERWIDTH * reqNum), (float)(r.NextDouble() * PERNUMBERHEIGHT)), new PointF((float)(r.NextDouble() * PERNUMBERWIDTH * reqNum), (float)(r.NextDouble() * PERNUMBERHEIGHT)));
}
g.Dispose();
context.Response.Clear();
context.Response.ContentType = "image/jpeg";
bt.Save(context.Response.OutputStream, ImageFormat.Jpeg);
bt.Dispose();
context.Response.End();
}
public bool IsReusable
{
get
{
return false;
}
}
}
[VB.NET]
Public Class ValidationCode
Implements IHttpHandler
'随机发生器
Shared r As New Random(Guid.NewGuid().GetHashCode())
'排除黑色、透明色颜色，因为底色黑色
Shared colors As PropertyInfo() = (GetType(Brushes).GetProperties(System.Reflection.BindingFlags.[Public] Or System.Reflection.BindingFlags.GetProperty Or System.Reflection.BindingFlags.[Static])).Where(Function(p) p.Name <> "Black" AndAlso p.Name <> "Transparent").[Select](Function(p) p).ToArray()
'排除黑色颜色，因为底色黑色
Shared linecolors As PropertyInfo() = (GetType(Pens).GetProperties(System.Reflection.BindingFlags.[Public] Or System.Reflection.BindingFlags.GetProperty Or System.Reflection.BindingFlags.[Static])).Where(Function(p) p.Name <> "Black").[Select](Function(p) p).ToArray()
'获取静态类Brushes实例对象
Shared colorobj As Object = GetType(Brushes).GetConstructor(BindingFlags.NonPublic, Nothing, Type.EmptyTypes, Nothing)
'获取静态类Pens实例对象
Shared penobj As Object = GetType(Pens).GetConstructor(BindingFlags.NonPublic, Nothing, Type.EmptyTypes, Nothing)
'每个随机字符的宽度
Const PERNUMBERWIDTH As Single = 40F
'每个字符的高度
Const PERNUMBERHEIGHT As Single = 50F
Public Sub ProcessRequest(context As HttpContext)
'获取要产生多少随机数（默认产生5个）
Dim reqNum As Integer = 5
If context.Request.QueryString("reqNum") IsNot Nothing Then
Integer.TryParse(context.Request.QueryString("reqNum"), reqNum)
End If
'产生多少大的背景图
Dim bt As New Bitmap(CInt(Math.Truncate(PERNUMBERWIDTH * reqNum)), CInt(Math.Truncate(PERNUMBERHEIGHT)))
Dim g As Graphics = Graphics.FromImage(bt)
'产生4个随机数（number可以被保存到Session中）
Dim numbers As String = ""
'绘制数字
For i As Integer = 1 To reqNum
numbers += r.[Next](0, 9).ToString()
Dim color = DirectCast(colors.GetValue(r.[Next](0, colors.Length)), PropertyInfo)
context.Response.Write(Convert.ToString(color.Name) & "<br/>")
Dim randomcolor As Brush = DirectCast(color.GetValue(colorobj, Nothing), Brush)
g.DrawString(numbers(i - 1).ToString(), New Font("黑体", PERNUMBERWIDTH), randomcolor, New PointF((i - 1) * PERNUMBERWIDTH, 0F))
Next
'绘制随机线条
Dim linenum As Integer = r.[Next](10, 21)
For i As Integer = 1 To linenum
Dim linecolor = DirectCast(linecolors.GetValue(r.[Next](0, colors.Length)), PropertyInfo)
Dim randomcolor As Pen = DirectCast(linecolor.GetValue(penobj, Nothing), Pen)
g.DrawLine(randomcolor, New PointF(CSng(r.NextDouble() * PERNUMBERWIDTH * reqNum), CSng(r.NextDouble() * PERNUMBERHEIGHT)), New PointF(CSng(r.NextDouble() * PERNUMBERWIDTH * reqNum), CSng(r.NextDouble() * PERNUMBERHEIGHT)))
Next
g.Dispose()
context.Response.Clear()
context.Response.ContentType = "image/jpeg"
bt.Save(context.Response.OutputStream, ImageFormat.Jpeg)
bt.Dispose()
context.Response.[End]()
End Sub
Public ReadOnly Property IsReusable() As Boolean
Get
Return False
End Get
End Property
End Class
注意：
1）一些诸如Brushes等特定因为是公用的，需要通过反射获取全部的颜色属性列表，因此使用了静态变量，这样不必每次都初始化，节省内存和时间。
2）Brushes避免黑色和透明色（本示例背景色是黑色），Pens只需避免黑色即可。有关Brushes颜色，可以查阅：http://msdn.microsoft.com/zh-cn/library/system.windows.media.brush(v=vs.95).aspx
3）Bitmap类是用于绘制使用的，一般是空白的黑色背景。一般配合Image类+Graphics画布使用，进行绘制。
4）BitMap的Save方法有若干个重载版本，其中之一可以指定输出流以及设置图片格式。本示例就是使用了这个函数。
【应用】
Html中代码（验证码部分，局部）：
<h1>
验证码
</h1>
<script>
function ChangeSD() {
document.getElementById("imgSD").src = "";
document.getElementById("imgSD").src = "/ValidationCode.ashx?reqNum=10";
};
</script>
<img src="/ValidationCode.ashx?reqNum=10" id="imgSD" />
<input type="button" value="Change Validation Key" onclick="ChangeSD()" />
注意，之所以使用js设置img的src两次，是因为重复的路径不会引发请求。
