asp.net 验证码

Before proceeding with the topic first we must understand "What is a Captcha code?" and "Why do we use them?". Most web sites have a Captcha validation in their sites.

What is the Captcha code?

A Captcha code is simply a combination of some characters and numbers like "Alk13" or "aTu2eP" etc.

Why do we use them?

We use them for validating that the client browser window really has a human typing into it.

Ok, let's get to the topic i.e. creating your own Captcha code in ASP.Net.

Step 1

Go to Visual Studio and create a new project (web site or web application) say "CaptchCode".

Step 2

Now add a new page into your application say "ShowCaptcha.aspx" and add this code into the source for the page:

<div>
<table>
  <tr>
    <td>
      <asp:Image ID="imgCaptcha" runat="server" ImageUrl="~/CreateCaptcha.aspx?New=1"/>
     </td>
   </tr>
   <tr>
    <td>
      <asp:TextBox ID="txtCaptcha" runat="server"></asp:TextBox>
     </td>
    </tr>
    <tr>
     <td>
       <asp:Label ID="lblMessage" runat="server"></asp:Label>
      </td>
     </tr>
     <tr>
      <td>
 <asp:Button ID="btnCaptcha" runat="server" Text="Validate Cpatcha Code"                              onclick="btnCaptcha_Click" />
      </td>
       </tr>
     </table>
    </div>

Note: Here I am given the ImageUrl="~/CreateCaptcha.aspx?New=1" of the image control. In CreateCaptcha.aspx we will write the code for creating the Captcha code.

Step 3

Now add a new page into your application "CreateCaptcha.aspx" and create the following methods in its .cs file:

/// <summary>
 
/// method for create captcha image
 
/// </summary>
 
 private void CreateCaptchaImage()
 
{ 
 
 code = GetRandomText();
 
 Bitmap bitmap = new Bitmap(, , System.Drawing.Imaging.PixelFormat.Format32bppArgb);
 
 Graphics g = Graphics.FromImage(bitmap);
 
 Pen pen = new Pen(Color.Yellow);
 
 Rectangle rect = new Rectangle(, , , );
 
 SolidBrush blue = new SolidBrush(Color.CornflowerBlue);
 
 SolidBrush black = new SolidBrush(Color.Black);
 
 int counter = ;
 
 g.DrawRectangle(pen, rect);
 
 g.FillRectangle(blue, rect);
 
 for (int i = ; i < code.Length; i++)
 
 {
 
    g.DrawString(code[i].ToString(), new Font("Tahoma",  + rand.Next(, ), FontStyle.Italic), black, newPointF( + counter, ));
 
   counter += ;
 
 }
 
 DrawRandomLines(g);          
 
 bitmap.Save(Response.OutputStream, ImageFormat.Gif);
 
 g.Dispose();
 
 bitmap.Dispose();
 
}
 
/// <summary>
 
/// Method for drawing lines
 
/// </summary>
 
/// <param name="g"></param>
 
private void DrawRandomLines(Graphics g)
 
{
 
   SolidBrush yellow = new SolidBrush(Color.Yellow);
 
   for (int i = ; i < ; i++)
 
   {g.DrawLines(new Pen(yellow, ), GetRandomPoints());}
 
}
 
/// <summary>
 
/// method for gettting random point position
 
/// </summary>
 
/// <returns></returns>
 
private Point[] GetRandomPoints()
 
{
 
  Point[] points = { new Point(rand.Next(, ), rand.Next(, )), new    Point(rand.Next(, ), rand.Next(, )) };
 
  return points;
 
}
 
/// <summary>
 
/// Method for generating random text of 5 cahrecters as captcha code
 
/// </summary>
 
/// <returns></returns>
 
private string GetRandomText()
 
{
 
  StringBuilder randomText = new StringBuilder();          
 
  string alphabets = "012345679ACEFGHKLMNPRSWXZabcdefghijkhlmnopqrstuvwxyz";
 
  Random r = new Random();
 
  for (int j = ; j <= ; j++)
 
  {randomText.Append(alphabets[r.Next(alphabets.Length)]);}
 
    Session["CaptchaCode"] = randomText.ToString();
 
     return Session["CaptchaCode"] as String;
 
  }

Step 4 

To validate the Captcha code in the "ShowCaptcha.aspx.cs" page:

protected void btnCaptcha_Click(object sender, EventArgs e)
 
{
 
   //imgCaptcha.ImageUrl = "~/CreateCaptcha.aspx?New=0";
 
  if (Session["CaptchaCode"] != null && txtCaptcha.Text == Session["CaptchaCode"].ToString())
 
  {
 
     lblMessage.ForeColor = Color.Green;
 
     lblMessage.Text = "Captcha code validated successfully!!";
 
  }
 
  else
 
  {
 
   lblMessage.ForeColor = Color.Red;
 
   lblMessage.Text = "Captcha code is wrong!!";
 
  }
 
}

I hope this article will be helpful for you.

Happy Coding!!

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