Create a Captcha Image in C# .NET

Captcha (Completely Automated Public Turing test to tell Computers and Humans Apart.)

The Captcha technology help you to make sure your site is reasonably secure against automated attacks.

Step:1 Write the following code in a class named CaptchaText.cs or you can download it here

public class CaptchaText
{
   public string Text
   {
     get { return this.text; }
   }
   public Bitmap Image
   {
     get { return this.image; }
   }
   public int Width
   {
     get { return this.width; }
   }
   public int Height
   {
     get { return this.height; }
   }

   private string text;
   private int width;
   private int height;
   private string familyName;
   private Bitmap image;
   private Random random = new Random();
   public CaptchaText(string s, int width, int height)
   {
     this.text = s;
     this.SetDimensions(width, height);
     this.GenerateImage();
   }
   public CaptchaText(string s,int width,int height,string familyName)
   {
     this.text = s;
     this.SetDimensions(width, height);
     this.SetFamilyName(familyName);
     this.GenerateImage();
   }
   ~CaptchaText()
   {
     Dispose(false);
   }
   public void Dispose()
   {
     GC.SuppressFinalize(this);
     this.Dispose(true);
   }
   protected virtual void Dispose(bool disposing)
   {
     if (disposing)
     this.image.Dispose();
   }
   private void SetDimensions(int width, int height)
   {
    if (width <= 0)
    throw new ArgumentOutOfRangeException("width",width,"Argument out of range,
                                                                                must be greater than zero.");
    if (height <= 0)
    throw new ArgumentOutOfRangeException("height",height,"Argument out of range,
                                                                                must be greater than zero.");
    this.width = width;
    this.height = height;
   }
   private void SetFamilyName(string familyName)
   {
      try
      {
        Font font = new Font(this.familyName, 12F);
        this.familyName = familyName;
        font.Dispose();
      }
      catch (Exception ex)
      {
        this.familyName = System.Drawing.FontFamily.GenericSerif.Name;
      }
   }
private void GenerateImage()
{
Bitmap bitmap = new Bitmap(this.width,this.height,PixelFormat.Format32bppArgb);
Graphics g = Graphics.FromImage(bitmap);
g.SmoothingMode = SmoothingMode.AntiAlias;
Rectangle rect = new Rectangle(0, 0, this.width, this.height);
HatchBrush hatchBrush=new HatchBrush(HatchStyle.SmallConfetti,Color.LightGray,Color.White);
g.FillRectangle(hatchBrush, rect);
   SizeF size;
   float fontSize = rect.Height + 1;
   Font font;
   do
   {
     fontSize--;
     font = new Font(this.familyName, fontSize, FontStyle.Bold);
     size = g.MeasureString(this.text, font);
   } while (size.Width > rect.Width);

  StringFormat format = new StringFormat();
  format.Alignment = StringAlignment.Center;
  format.LineAlignment = StringAlignment.Center;
 GraphicsPath path = new GraphicsPath();
 path.AddString(this.text,font.FontFamily,(int)font.Style,font.Size,rect,format);
 float v = 4F;
 PointF[] points =
 {
  new PointF(this.random.Next(rect.Width) / v, this.random.Next(rect.Height) / v),
  new PointF(rect.Width - this.random.Next(rect.Width) / v, this.random.Next(rect.Height) / v),
  new PointF(this.random.Next(rect.Width) / v, rect.Height - this.random.Next(rect.Height) / v),
  new PointF(rect.Width - this.random.Next(rect.Width) / v,rect.Height - 
                                                                                      this.random.Next(rect.Height) / v)
  };
    Matrix matrix = new Matrix();
    matrix.Translate(0F, 0F);
    path.Warp(points, rect, matrix, WarpMode.Perspective, 0F);
    hatchBrush = new HatchBrush(HatchStyle.LargeConfetti, Color.LightGray, Color.DarkGray);
    g.FillPath(hatchBrush, path);
    int m = Math.Max(rect.Width, rect.Height);
    for (int i = 0; i < (int) (rect.Width * rect.Height / 30F); i++)
      {
            int x = this.random.Next(rect.Width);
            int y = this.random.Next(rect.Height);
            int w = this.random.Next(m / 50);
            int h = this.random.Next(m / 50);
            g.FillEllipse(hatchBrush, x, y, w, h);
      }
         font.Dispose();
         hatchBrush.Dispose();
         g.Dispose();
         this.image = bitmap;
    }
 } 

Step:2 Create a page named "Captcha.aspx" and add code in Captcha.aspx.cs

protected void Page_Load(object sender, EventArgs e)
{
  if (Session["CaptchaImageText"] != null)
   {
      CaptchaText ci = new CaptchaText(this.Session["CaptchaImageText"].ToString(), 200, 50,
                                "Century Schoolbook");
       this.Response.Clear();
       this.Response.ContentType = "image/jpeg";
       ci.Image.Save(this.Response.OutputStream, ImageFormat.Jpeg);
       ci.Dispose();
   } 
}

Step:3 Now Call Captcha.aspx in that page you want to appear it for ex: "Default.aspx"

Add code in Default.aspx

<table width="100%">
    <tr>
      <td align="left">
        <img id="imgcaptcha" runat="server" src="~/Captcha.aspx" alt="Enter the code shown" />
       </td>
    </tr>
    <tr>
      <td align="left">
          <asp:Label runat="server" ID="lblBox" Text="Enter the code shown"></asp:Label>
           <br />
          <asp:TextBox ID="CodeNumberTextBox" runat="server"></asp:TextBox>
          <asp:Label ID="lblerrCaptcha" runat="server" Visible="false"></asp:Label>
        </td>
     </tr>
    <tr>
     <td align="left">
        <asp:LinkButton ID="lnkGetQuotes" Text="GetQuotes"  runat="server" 
                        OnClick="btnGetQuotes_Click">
     </td>
   </tr>
</table>

Now,Compare that to value to what the users had keyed in to your text box, To do that add code in Default.aspx.cs

private Random random = new Random();
 protected void Page_Load(object sender, EventArgs e)
 {
   if (!IsPostBack)
    {
       Session["CaptchaImageText"] = "";
       Session["CaptchaImageText"] = GenerateRandomCode();
    }
 }
private string GenerateRandomCode()
{
            string s = "";
            for (int i = 0; i < 6; i++)
                s = String.Concat(s, this.random.Next(10).ToString());
            return s;
}
protected void btnGetQuotes_Click(object sender, EventArgs e)
{
   if (Convert.ToString(Session["CaptchaImageText"]) != "" &&
             Convert.ToString(CodeNumberTextBox.Text) != "")
   {
     if (CodeNumberTextBox.Text == Session["CaptchaImageText"].ToString())
     { 
        // add your code for the button
     }
    else
     {
         lblerrCaptcha.Visible = true;
         lblerrCaptcha.Text = "Please enter the correct code";
     }
}
else
   {
          lblerrCaptcha.Visible = true;
          lblerrCaptcha.Text = "Please enter the code"; 
   }
} 

The output is like as:

Thats it, Congratulations you have created your Captcha Image in your website.