简单验证码的制作

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。