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随机生成字母数字验证码，并可以定义验证码的长度，知码网在这有详细的介绍：

usingSystem;usingSystem.Data;usingSystem.Configuration;usingSystem.Collections;usingSystem.Web;usingSystem.Web.Security;usingSystem.Web.UI;usingSystem.Web.UI.WebControls;usingSystem.Web.UI.WebControls.WebParts;usingSystem.Web.UI.HtmlControls;usingSystem.Drawing;usingSystem.Drawing.Imaging;publicpartialclassLogin:System.Web.UI.Page{protectedvoidPage_Load(objectsender,EventArgse){if(!IsPostBack){this.CreateCheckCodeImage(GenerateCheckCode());}}privatestringGenerateCheckCode(){intnumber;charcode;stringcheckCodeString.Empty;System.RandomrandomnewRandom();for(inti0;i15;i++){numberrandom.Next();if(number%20)code(char)(‘0‘+(char)(number%10));elsecode(char)(‘A‘+(char)(number%26));checkCode+code.ToString();}Response.Cookies.Add(newHttpCookie("CheckCode",checkCode));returncheckCode;}privatevoidCreateCheckCodeImage(stringcheckCode){if(checkCodenull||checkCode.Trim()String.Empty)return;System.Drawing.BitmapimagenewSystem.Drawing.Bitmap((int)Math.Ceiling((checkCode.Length*12.5)),22);GraphicsgGraphics.FromImage(image);try{//生成随机生成器RandomrandomnewRandom();//清空图片背景色g.Clear(Color.White);//画图片的背景噪音线for(inti0;i35;i++){intx1random.Next(image.Width);intx2random.Next(image.Width);inty1random.Next(image.Height);inty2random.Next(image.Height);g.DrawLine(newPen(Color.Silver),x1,y1,x2,y2);}FontfontnewSystem.Drawing.Font("Arial",12,(System.Drawing.FontStyle.Bold|System.Drawing.FontStyle.Italic));System.Drawing.Drawing2D.LinearGradientBrushbrushnewSystem.Drawing.Drawing2D.LinearGradientBrush(newRectangle(0,0,image.Width,image.Height),Color.Blue,Color.DarkRed,1.2f,true);g.DrawString(checkCode,font,brush,2,2);//画图片的前景噪音点for(inti0;i180;i++){intxrandom.Next(image.Width);intyrandom.Next(image.Height);image.SetPixel(x,y,Color.FromArgb(random.Next()));}//画图片的边框线g.DrawRectangle(newPen(Color.Silver),0,0,image.Width-1,image.Height-1);image.Save(Server.MapPath("imgs")+"/VaImag.gif",ImageFormat.Gif);}finally{g.Dispose();image.Dispose();}}}本文摘自：www.zhima365.com