asp.net 验证码实现代码一例

需要新建一个创建验证码图片的页面ValidateCode.aspx，其后台cs文件代码如下：

PageLoad
private void Page_Load(object sender, System.EventArgs e)
{
   string checkCode = CreateRandomCode(4);
   Session["CheckCode"] = checkCode;
   CreateImage(checkCode);
}

//CreateRandomCode 参数代表验证码位数
private string CreateRandomCode(int codeCount)
{
 string allChar = "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,W,X,Y,Z" ;
 string[] allCharArray = allChar.Split(',');
 string randomCode = "";
 int temp = -1;

 Random rand = new Random();
 for(int i = 0; i < codeCount; i++)
 {
     if(temp != -1)
     {
         rand = new Random(i*temp*((int)DateTime.Now.Ticks));
     }
     int t = rand.Next(35);
     if(temp == t)
     {
         return CreateRandomCode(codeCount);
     }
     temp = t;
     randomCode += allCharArray[t];
 }
 return randomCode;
}

//CreateImage 用于生成图
private void CreateImage(string checkCode)
        {
 int iwidth = (int)(checkCode.Length * 11.5);
 System.Drawing.Bitmap image = new System.Drawing.Bitmap(iwidth, 20);
 Graphics g = Graphics.FromImage(image);
 Font f = new System.Drawing.Font("Arial", 10, System.Drawing.FontStyle.Bold);
 Brush b = new System.Drawing.SolidBrush(Color.White);
 //g.FillRectangle(new System.Drawing.SolidBrush(Color.Blue),0,0,image.Width, image.Height);
 g.Clear(Color.Blue);
 g.DrawString(checkCode, f, b, 3, 3);

 Pen blackPen = new Pen(Color.Black, 0);
 Random rand = new Random();
 for (int i=0;i<5;i++)
 {
     int y = rand.Next(image.Height);
     g.DrawLine(blackPen,0,y,image.Width,y);
 }
 
 System.IO.MemoryStream ms = new System.IO.MemoryStream();
 image.Save(ms,System.Drawing.Imaging.ImageFormat.Jpeg);
 Response.ClearContent();
 Response.ContentType = "image/Jpeg";
 Response.BinaryWrite(ms.ToArray());
 g.Dispose();
 image.Dispose();
}
//g.FillRectangle(new System.Drawing.SolidBrush(Color.Blue),0,0,image.Width, image.Height);
g.Clear(Color.Blue);

代码说明：
以上两种方法，均可以改变生成图片的背景颜色。
下面那个for循环用来生成一些随机的水平线。

在使用验证码的页面添加一个<asp:Image>控件即可，不过要把ImageUrl指向生成验证码的页面，如下：

<asp:Image Runat="server" ID="ImageCheck" ImageUrl="ValidateCode.aspx"></asp:Image>