Simple production of asp.net verification code (vb.net+C#)

The general method of making verification code effects on the website is:
1) Use HttpHandler (general handler) to draw a random verification code picture, generate a random code, and output it to the OutputStream of the page.
2) Use asynchronous method (js, etc.) in the page to refresh the verification code of the current page.
[Example]
1) Create a "general application handler ashx" with the following code:
[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() 
&#39;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() 
&#39;获取静态类Brushes实例对象 
Shared colorobj As Object = GetType(Brushes).GetConstructor(BindingFlags.NonPublic, Nothing, Type.EmptyTypes, Nothing) 
&#39;获取静态类Pens实例对象 
Shared penobj As Object = GetType(Pens).GetConstructor(BindingFlags.NonPublic, Nothing, Type.EmptyTypes, Nothing) 
&#39;每个随机字符的宽度 
Const PERNUMBERWIDTH As Single = 40F 
&#39;每个字符的高度 
Const PERNUMBERHEIGHT As Single = 50F 
Public Sub ProcessRequest(context As HttpContext) 
&#39;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 
&#39;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) 
&#39;产生4个随机数（number可以被保存到Session中） 
Dim numbers As String = "" 
&#39;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 
&#39;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

Note :
1) Some specific ones such as Brushes are public and need to obtain the entire color attribute list through reflection, so static variables are used so that they do not need to be initialized every time, saving memory and time.
2) Brushes avoid black and transparent colors (the background color in this example is black), and Pens only need to avoid black. For information about Brushes colors, you can check: http://msdn.microsoft.com/zh-cn/library/system.windows.media.brush(v=vs.95).aspx
3) The Bitmap class is used for drawing What is used is usually a blank black background. Generally used with the Image class + Graphics canvas for drawing.
4) BitMap's Save method has several overloaded versions, one of which can specify the output stream and set the image format. This example uses this function.
[Application]

Code in Html (verification code part, partial):

<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()" />

Note that the reason why js is used to set the src two of img times, because duplicate paths do not trigger requests.

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