3 types of verification code examples in asp.net (numbers, mixed numbers and letters, Chinese characters)

<table> 
<tr> 
<td class="style1"> 
 (验证码测试)</td> 
<td> 
  <asp:Label 
ID="Label1" runat="server"></asp:Label> 
  
<asp:Image ID="Image1" runat="server" Height="22px" ImageUrl="~/ValidNums.aspx" Width="58px" />  
<asp:Image ID="Image2" runat="server" Height="22px" ImageUrl="~/GetValid.aspx" Width="58px" /></td> 
</tr> <tr> 
<td class="style1"> 
  
</td> 
<td> 
  <asp:Button ID="Button1" runat="server" Text="登录" OnClick="btnOK_Click" /> 
   
<asp:Button ID="Button2" runat="server" Text="取消" /> 
</td> 
</tr> 
</table>

using System; 
using System.Collections.Generic; 
using System.Linq; 
using System.Web; 
using System.Web.UI; 
using System.Web.UI.WebControls; 

public partial class _Default : System.Web.UI.Page 
{ 
protected void Page_Load(object sender, EventArgs e) 
{ 
if (!Page.IsPostBack) 
{ 
string getNums = GetVali(); 
Label1.Text = getNums; 
} 
} 

/// <summary> 
/// 随机生成4位数 
/// </summary> 
/// <returns>返回生成的随机数</returns> 
public string GetVali() 
{ 
string strsvali = "0,1,2,3,4,5,6,7,8,9"; 
string[] ValiArray = strsvali.Split(&#39;,&#39;); 
string ReturnNum = ""; 
int nums = -1; 
Random vrand = new Random(); 
for (int n = 1; n < 5; n++) 
{ 
if (nums != -1) 
{ 
vrand = new Random(n * nums * unchecked((int)DateTime.Now.Ticks)); 
} 
int t = vrand.Next(10); 
nums = t; 
ReturnNum += ValiArray[t]; 
} 
Session["Valid"] = ReturnNum; 
return ReturnNum; 
} 
protected void btnOK_Click(object sender, EventArgs e) 
{ 
if (Session["Valid"].ToString() == TextBox3.Text) 
{ 
ClientScript.RegisterStartupScript(this.GetType(),"ss","<script>alert(&#39;您已经成功通过登录验证！&#39;)</script>"); 
} 
else 
{ 
ClientScript.RegisterStartupScript(this.GetType(), "ss", "<script>alert(&#39;您输入的验证码错误！&#39;)</script>"); 
} 
} 
}

using System; 
using System.Collections; 
using System.Configuration; 
using System.Data; 
using System.Linq; 
using System.Web; 
using System.Web.Security; 
using System.Web.UI; 
using System.Web.UI.HtmlControls; 
using System.Web.UI.WebControls; 
using System.Web.UI.WebControls.WebParts; 
using System.Xml.Linq; 
using System.Text; 
using System.Drawing; 

public partial class GetValid : System.Web.UI.Page 
{ 
protected void Page_Load(object sender, EventArgs e) 
{ 
if (!IsPostBack) 
{ 
string validateNum = GetValids(); //成生4位随机字符串 
CreateImage(validateNum); //将生成的随机字符串绘成图片 
Session["ValidNums"] = validateNum; //保存验证码 
} 
} 
public static string GetValids() 
{ 
//获取GB2312编码页（表） 
Encoding gb = Encoding.GetEncoding("gb2312"); 

//调用函数产生4个随机中文汉字编码 
object[] bytes = CreateRegionCode(4); 

//根据汉字编码的字节数组解码出中文汉字 

string s = String.Empty; 
foreach (object byt in bytes) 
{ 
string str1 = gb.GetString((byte[])Convert.ChangeType(byt, typeof(byte[]))); 
s = s + str1; 
} 

//输出的控制台 
return s; 
} 
public static object[] CreateRegionCode(int strlength) 
{ 
//定义一个字符串数组储存汉字编码的组成元素 
string[] rBase = new String[16] { "0", "1", "2", "3", "4", "5", "6", "7", "8", "9", "a", "b", "c", "d", "e", "f" }; 

Random rnd = new Random(); 

//定义一个object数组用来 
object[] bytes = new object[strlength]; 
/*每循环一次产生一个含两个元素的十六进制字节数组，并将其放入bject数组中 
每个汉字有四个区位码组成 
区位码第1位和区位码第2位作为字节数组第一个元素 
区位码第3位和区位码第4位作为字节数组第二个元素 
*/ 
for (int i = 0; i < strlength; i++) 
{ 
//区位码第1位 
int r1 = rnd.Next(11, 14); 
string str_r1 = rBase[r1].Trim(); 

//区位码第2位 
rnd = new Random(r1 * unchecked((int)DateTime.Now.Ticks) + i); 
//更换随机数发生器的种子避免产生重复值 
int r2; 
if (r1 == 13) 
{ 
r2 = rnd.Next(0, 8); 
} 
else 
{ 
r2 = rnd.Next(0, 16); 
} 
string str_r2 = rBase[r2].Trim(); 

//区位码第3位 
rnd = new Random(r2 * unchecked((int)DateTime.Now.Ticks) + i); 
int r3 = rnd.Next(10, 16); 
string str_r3 = rBase[r3].Trim(); 

//区位码第4位 
rnd = new Random(r3 * unchecked((int)DateTime.Now.Ticks) + i); 
int r4; 
if (r3 == 10) 
{ 
r4 = rnd.Next(1, 16); 
} 
else if (r3 == 15) 
{ 
r4 = rnd.Next(0, 15); 
} 
else 
{ 
r4 = rnd.Next(0, 16); 
} 
string str_r4 = rBase[r4].Trim(); 

//定义两个字节变量存储产生的随机汉字区位码 
byte byte1 = Convert.ToByte(str_r1 + str_r2, 16); 
byte byte2 = Convert.ToByte(str_r3 + str_r4, 16); 
//将两个字节变量存储在字节数组中 
byte[] str_r = new byte[] { byte1, byte2 }; 

//将产生的一个汉字的字节数组放入object数组中 
bytes.SetValue(str_r, i); 

} 

return bytes; 

} 
//生成图片 
private void CreateImage(string validateNum) 
{ 
if (validateNum == null || validateNum.Trim() == String.Empty) 
return; 
//生成Bitmap图像 
System.Drawing.Bitmap image = new System.Drawing.Bitmap(validateNum.Length * 12 + 10, 22); 
Graphics g = Graphics.FromImage(image); 

try 
{ 
//生成随机生成器 
Random random = new Random(); 

//清空图片背景色 
g.Clear(Color.White); 

//画图片的背景噪音线 
for (int i = 0; i < 25; i++) 
{ 
int x1 = random.Next(image.Width); 
int x2 = random.Next(image.Width); 
int y1 = random.Next(image.Height); 
int y2 = random.Next(image.Height); 

g.DrawLine(new Pen(Color.Coral), x1, y1, x2, y2); 
} 

Font font = new System.Drawing.Font("Arial", 8); 
System.Drawing.Drawing2D.LinearGradientBrush brush = new System.Drawing.Drawing2D.LinearGradientBrush(new Rectangle(0, 0, image.Width, image.Height), Color.Blue, Color.DarkRed, 1.2f, true); 
g.DrawString(validateNum, font, brush, 2, 2); 

//画图片的前景噪音点 
for (int i = 0; i < 100; i++) 
{ 
int x = random.Next(image.Width); 
int y = random.Next(image.Height); 
image.SetPixel(x, y, Color.FromArgb(random.Next())); 
} 

//画图片的边框线 
g.DrawRectangle(new Pen(Color.Silver), 0, 0, image.Width - 1, image.Height - 1); 

System.IO.MemoryStream ms = new System.IO.MemoryStream(); 
//将图像保存到指定的流 
image.Save(ms, System.Drawing.Imaging.ImageFormat.Gif); 
Response.ClearContent(); 
Response.ContentType = "image/Gif"; 
Response.BinaryWrite(ms.ToArray()); 
} 
finally 
{ 
g.Dispose(); 
image.Dispose(); 
} 
} 
}