App_code/CreateImage.cs

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using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Drawing.Imaging;
using System.Drawing;
/// <summary>
/// 验证码模块
/// </summary>
public class CreateImage
{
    public static void DrawImage()
    {
        CreateImage img = new CreateImage();
        HttpContext.Current.Session["CheckCode"] = img.RndNum(4);
        img.CreateImages(HttpContext.Current.Session["CheckCode"].ToString());
    }

    /// <summary>
    /// 生成验证图片
    /// </summary>
    /// <param name="checkCode">验证字符</param>
    private void CreateImages(string checkCode)
    {
        int iwidth = (int)(checkCode.Length * 13);
        System.Drawing.Bitmap image = new System.Drawing.Bitmap(iwidth, 23);
        Graphics g = Graphics.FromImage(image);
        g.Clear(Color.White);
        //定义颜色
        Color[] c = { Color.Black, Color.Red, Color.DarkBlue, Color.Green, Color.Orange, Color.Brown, Color.DarkCyan, Color.Purple };
        //定义字体 
        string[] font = { "Verdana", "Microsoft Sans Serif", "Comic Sans MS", "Arial", "宋体" };
        Random rand = new Random();
        //随机输出噪点
        for (int i = 0; i < 50; i++)
        {
            int x = rand.Next(image.Width);
            int y = rand.Next(image.Height);
            g.DrawRectangle(new Pen(Color.LightGray, 0), x, y, 1, 1);
        }

        //输出不同字体和颜色的验证码字符
        for (int i = 0; i < checkCode.Length; i++)
        {
            int cindex = rand.Next(7);
            int findex = rand.Next(5);

            Font f = new System.Drawing.Font(font[findex], 10, System.Drawing.FontStyle.Bold);
            Brush b = new System.Drawing.SolidBrush(c[cindex]);
            int ii = 4;
            if ((i + 1) % 2 == 0)
            {
                ii = 2;
            }
            g.DrawString(checkCode.Substring(i, 1), f, b, 3 + (i * 12), ii);