using System;
using System.Collections;
using System.Configuration;
using System.Data;
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.Drawing;
using System.IO;
namespace jgy_keshi_lyb
{
public partial class checkimage : System.Web.UI.Page
{
Random ran = new Random();
protected void Page_Load(object sender, Eventargs e)
{
string str = getRandomValidate(4);
Session["check"] = str;
//這一步是為了將驗證碼寫入Session，進行驗證，不能缺省，也可一使用cookie
getImageValidate(str);
}
//得到隨機字符串,長度自己定義
private string getRandomValidate(int len)
{
int num;
int tem;
string rtuStr = "";
for (int i = 0; i < len; i++)
{
num = ran.Next();
/*
* 這里可以選擇生成字符和數(shù)字組合的驗證碼
*/
tem = num % 10 + 0;//生成數(shù)字
//tem = num % 26 + a;//生成字符
rtuStr += Convert.ToChar(tem).ToString();
}
return rtuStr;
}
//生成圖像
private void getImageValidate(string strValue)
{
//string str = "OO00"; //前兩個為字母O，后兩個為數(shù)字0
int width = Convert.ToInt32(strValue.Length * 12); //計算圖像寬度
Bitmap img = new Bitmap(width, 23);
Graphics gfc = Graphics.FromImage(img); //產(chǎn)生Graphics對象，進行畫圖
gfc.Clear(Color.White);
drawLine(gfc, img);
//寫驗證碼，需要定義font
font font = new font("arial", 12, fontStyle.Bold);
System.Drawing.Drawing2D.LinearGradientBrush brush =
new System.Drawing.Drawing2D.LinearGradientBrush(new Rectangle(0, 0, img.Width, img.Height), Color.DarkOrchid, Color.Blue, 1.5f, true);
gfc.DrawString(strValue, font, brush, 3, 2);
drawPoint(img);
gfc.DrawRectangle(new Pen(Color.DarkBlue), 0, 0, img.Width - 1, img.Height - 1);
//將圖像添加到頁面
MemoryStream ms = new MemoryStream();
img.Save(ms, System.Drawing.Imaging.ImageFormat.Gif);
//更改Http頭
Response.ClearContent();
Response.ContentType = "image/gif";
Response.BinaryWrite(ms.Toarray());
//Dispose
gfc.Dispose();
img.Dispose();
Response.End();
}
private void drawLine(Graphics gfc, Bitmap img)
{
//選擇畫10條線,也可以增加，也可以不要線，只要隨機雜點即可
for (int i = 0; i < 10; i++)
{
int x1 = ran.Next(img.Width);
int y1 = ran.Next(img.Height);
int x2 = ran.Next(img.Width);
int y2 = ran.Next(img.Height);
gfc.DrawLine(new Pen(Color.Silver), x1, y1, x2, y2); //注意畫筆一定要淺顏色，否則驗證碼看不清楚
}
}
private void drawPoint(Bitmap img)
{
/*
//選擇畫100個點,可以根據(jù)實際情況改變
for (int i = 0; i < 100; i++)
{
int x = ran.Next(img.Width);
int y = ran.Next(img.Height);
img.SetPixel(x,y,Color.Fromargb(ran.Next()));//雜點顏色隨機
}
*/
int col = ran.Next();//在一次的圖片中雜店顏色相同
for (int i = 0; i < 100; i++)
{
int x = ran.Next(img.Width);
int y = ran.Next(img.Height);
img.SetPixel(x, y, Color.Fromargb(col));
}
}

}
}

<img src="checkimage.aspx"
alt=看不清楚，雙擊圖片換一張。
ondblclick="this.src= checkimage.aspxflag= + Math.random() "
border="1" height="24" />