登陆验证码简单制作

public class ValidateCode : System.Web.UI.Page
    {
        private void Page_Load(object sender, System.EventArgs e)
        {
            //如果要在页面a.aspx生成验证码，则在该页面添加一个图片控件，假设命名为：Image1，然后在page_Load事件中写如下代码：
            //ImageButton1.ImageUrl = "ValidateCode.aspx";
            //这样就可以生成验证码了，ValidateCode.aspx页面可以随便放在哪里，不过要注意Image1.src 要写对，同级可以直接写ValidateCode.aspx，上一级写../ValidateCode.aspx，很方便吧。

            if(!IsPostBack)
            {
                //RndNum是一个自定义函数
                //这里的数字4代表显示的是4位的验证字符串！
                //string VNum=RndNum(4); 
                string VNum=GenerateRandom(4);
                Session["VNum"] = VNum;
                Validate_Code(VNum);
            }
        }

        Web Form Designer generated code

        private void Validate_Code(string VNum) 
        {
            int Gheight=(int)(VNum.Length * 11.5);
            //gheight为图片宽度,根据字符长度自动更改图片宽度
            System.Drawing.Bitmap Img = new System.Drawing.Bitmap(Gheight,20);
            Graphics g = Graphics.FromImage(Img);
            g.DrawString(VNum,new System.Drawing.Font("Arial",10),new System.Drawing.SolidBrush(Color.Red),3,3); 
            //在矩形内绘制字串（字串，字体，画笔颜色，左上x.左上y） 
            System.IO.MemoryStream ms=new System.IO.MemoryStream();
            Img.Save(ms,System.Drawing.Imaging.ImageFormat.Png); 
            Response.ClearContent(); //需要输出图象信息 要修改HTTP头 
            Response.ContentType="image/Png";
            Response.BinaryWrite(ms.ToArray());
            g.Dispose();
            Img.Dispose(); 
            Response.End();
        }
        
        private static char[] constant=
        {
            '0','1','2','3','4','5','6','7','8','9',
            'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z',
            'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'
        };
        public static string GenerateRandom(int Length)
        {   
            System.Text.StringBuilder newRandom = new System.Text.StringBuilder(62);
            Random rd= new Random();
            for(int i=0;i<Length;i++)
            {
                newRandom.Append(constant[rd.Next(62)]);
            }
            return newRandom.ToString();
        }

    }
public cl