Simple CAPTCHA, Create your own in C#

Have you ever wondered how to create a Captcha for your web forms, well its really simple and it wont take more than 100+ lines of code.  This sample captcha that I will demostrate will have random Font Type, Font Style, Font Color, Font Rotation, Background Style and Background Colour.   Here are some samples of what this can generate:

Now without too much explanation, here it goes.

Step 1 : In your web project add a Generic Handler thats the file with the.ahsx extension, Ill just use this for this demo for simplicity and also it will compile on demand like a normal aspx file.  Its up to you if you want to create a dll which is better so you can reuse it in any projects you want.  To do that right click on the project on your Solution Explorer.

Then choose your file type, in this sample its a Generic Handler

Step 2 : Once added you can start coding, here are the sample codes which you can freely copy.

<%@ WebHandler Language="C#" %>
using System;
using System.Web;
using System.Drawing;
using System.IO;
using System.Web.SessionState;
using System.Drawing.Imaging;
using System.Drawing.Text;
using System.Drawing.Drawing2D;

public class Captcha : IHttpHandler, IReadOnlySessionState
    public void ProcessRequest(HttpContext context)
        int iHeight = 80;
        int iWidth = 190;
        Random oRandom = new Random();

        int[] aBackgroundNoiseColor = new int[] { 150, 150, 150 };
        int[] aTextColor = new int[] { 0, 0, 0 };
        int[] aFontEmSizes = new int[] { 15, 20, 25, 30, 35 };

        string[] aFontNames = new string[]
         "Comic Sans MS",
         "Times New Roman",

        FontStyle[] aFontStyles = new FontStyle[]

        HatchStyle[] aHatchStyles = new HatchStyle[]
         HatchStyle.BackwardDiagonal, HatchStyle.Cross, HatchStyle.DashedDownwardDiagonal, HatchStyle.DashedHorizontal,
         HatchStyle.DashedUpwardDiagonal, HatchStyle.DashedVertical, HatchStyle.DiagonalBrick, HatchStyle.DiagonalCross,
         HatchStyle.Divot, HatchStyle.DottedDiamond, HatchStyle.DottedGrid, HatchStyle.ForwardDiagonal, HatchStyle.Horizontal,
         HatchStyle.HorizontalBrick, HatchStyle.LargeCheckerBoard, HatchStyle.LargeConfetti, HatchStyle.LargeGrid,
         HatchStyle.LightDownwardDiagonal, HatchStyle.LightHorizontal, HatchStyle.LightUpwardDiagonal, HatchStyle.LightVertical,
         HatchStyle.Max, HatchStyle.Min, HatchStyle.NarrowHorizontal, HatchStyle.NarrowVertical, HatchStyle.OutlinedDiamond,
         HatchStyle.Plaid, HatchStyle.Shingle, HatchStyle.SmallCheckerBoard, HatchStyle.SmallConfetti, HatchStyle.SmallGrid,
         HatchStyle.SolidDiamond, HatchStyle.Sphere, HatchStyle.Trellis, HatchStyle.Vertical, HatchStyle.Wave, HatchStyle.Weave,
         HatchStyle.WideDownwardDiagonal, HatchStyle.WideUpwardDiagonal, HatchStyle.ZigZag

        //Get Captcha in Session
        string sCaptchaText = context.Session["Captcha"].ToString();

        //Creates an output Bitmap
        Bitmap oOutputBitmap = new Bitmap(iWidth, iHeight, PixelFormat.Format24bppRgb);
        Graphics oGraphics = Graphics.FromImage(oOutputBitmap);
        oGraphics.TextRenderingHint = TextRenderingHint.AntiAlias;

        //Create a Drawing area
        RectangleF oRectangleF = new RectangleF(0, 0, iWidth, iHeight);
        Brush oBrush = default(Brush);

        //Draw background (Lighter colors RGB 100 to 255)
        oBrush = new HatchBrush(aHatchStyles[oRandom.Next(aHatchStyles.Length - 1)], Color.FromArgb((oRandom.Next(100, 255)), (oRandom.Next(100, 255)), (oRandom.Next(100, 255))), Color.White);
        oGraphics.FillRectangle(oBrush, oRectangleF);

        System.Drawing.Drawing2D.Matrix oMatrix = new System.Drawing.Drawing2D.Matrix();
        int i = 0;
        for (i = 0; i <= sCaptchaText.Length - 1; i++)
            int iChars = sCaptchaText.Length;
            int x = iWidth / (iChars + 1) * i;
            int y = iHeight / 2;

            //Rotate text Random
            oMatrix.RotateAt(oRandom.Next(-40, 40), new PointF(x, y));
            oGraphics.Transform = oMatrix;

            //Draw the letters with Randon Font Type, Size and Color
            sCaptchaText.Substring(i, 1),
                //Random Font Name and Style
            new Font(aFontNames[oRandom.Next(aFontNames.Length - 1)], aFontEmSizes[oRandom.Next(aFontEmSizes.Length - 1)], aFontStyles[oRandom.Next(aFontStyles.Length - 1)]),
                //Random Color (Darker colors RGB 0 to 100)
            new SolidBrush(Color.FromArgb(oRandom.Next(0, 100), oRandom.Next(0, 100), oRandom.Next(0, 100))),
            oRandom.Next(10, 40)

        MemoryStream oMemoryStream = new MemoryStream();
        oOutputBitmap.Save(oMemoryStream, System.Drawing.Imaging.ImageFormat.Png);
        byte[] oBytes = oMemoryStream.GetBuffer();



    public bool IsReusable
            return false;

There are some commented explanation along the way so you wont get lost, but you can still ask me if you really need.

Step 3 : Use that Captcha.  On your aspx add the following as an image like such

<asp:Image ID="imgCaptcha" ImageUrl="Captcha.ashx" runat="server" />

If you noticed its like your using it as an image, this is because its an image rendered by that handler which you can see on the bottom part of the captcha code


Step 4 : Assign a text on that captcha on your code behind

private void SetCaptchaText()
 Random oRandom = new Random();
 int iNumber = oRandom.Next(100000, 999999);
 Session["Captcha"] = iNumber.ToString();

Note : You may notice that I am using only numbers in this regard, this would be easy so users cannot be confused with l and 1, 0 and o, and other similar looking characters.
Note: By setting the session you are also setting the text to render on the captcha and if you look at the captcha codes it is in this section

string sCaptchaText = context.Session["Captcha"].ToString();

Step 5 : Compare that to value to what the users had keyed in to your text box

if (Session["Captcha"].ToString() != txtCaptcha.Text.Trim())

Thats it, Congratulations you have created your own captcha

30 Responses to Simple CAPTCHA, Create your own in C#

  1. codingtoohard says:

    i cant see an image when i run ur codes..

  2. Mily says:

    Hi!! I do this tutorial, but i have a problem, when I execute I cant see the image pliss help me!! thanks!!!

  3. Francesco says:


    How do i make the numbers fit inside the image.? Few numbers are not visible fully since I changed the height to 50 and the numbers are getting cut

  4. bababa says:

    I figured out why the image doesnt display. I added
    context.Response.ContentType = “image/png”;

    above this line:
    MemoryStream oMemoryStream = new MemoryStream();

    Have a good day.

  5. Vincent says:

    Excuse me .. I’m not understand step 3 onward.. where should i put the code (step 3) ?

  6. Wee Kang says:

    Where should I put this?

    private void SetCaptchaText()
    Random oRandom = new Random();
    int iNumber = oRandom.Next(100000, 999999);
    Session[“Captcha”] = iNumber.ToString();

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