Generating Captcha Code in C#

A guide on how to create a Captcha code using C# to protect web forms and applications from automated access. This tutorial demonstrates how to use the `System.Drawing` library to generate Captcha images.

In this article, you will learn how to create a simple Captcha code using the System.Drawing library in C#. The Captcha code will be generated as a random string and displayed as an image.

C# Code

using System;
using System.Drawing;
using System.Drawing.Imaging;
using System.IO;

class CaptchaGenerator
{
    public static void Main(string[] args)
    {
        string captchaCode = GenerateRandomCode(6);
        Console.WriteLine("Captcha code: " + captchaCode);

        using (Bitmap bitmap = new Bitmap(200, 80))
        using (Graphics g = Graphics.FromImage(bitmap))
        {
            // Set background color
            g.Clear(Color.White);
            Font font = new Font("Arial", 30, FontStyle.Bold, GraphicsUnit.Pixel);
            Brush brush = new SolidBrush(Color.Black);

            // Draw the Captcha string
            g.DrawString(captchaCode, font, brush, new PointF(20, 20));

            // Save Captcha image as a file
            bitmap.Save("captcha.png", ImageFormat.Png);
        }

        Console.WriteLine("Captcha image saved as captcha.png");
    }

    // Generate random Captcha code
    private static string GenerateRandomCode(int length)
    {
        const string chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
        Random random = new Random();
        char[] code = new char[length];

        for (int i = 0; i < length; i++)
        {
            code[i] = chars[random.Next(chars.Length)];
        }

        return new string(code);
    }
}

Detailed explanation:

1. GenerateRandomCode(int length): Generates a random string consisting of alphanumeric characters of a specified length.
2. Main():
   • Generates a random Captcha code using GenerateRandomCode.
   • Creates a Bitmap image with dimensions 200x80.
   • Uses the Graphics class to draw the Captcha code on the image.
   • Saves the image as captcha.png.

System Requirements:

• C# 7.0 or newer
• .NET Framework 4.5 and above
• System.Drawing library

How to install the libraries needed to run the C# code above:

For .NET Core projects, you need to install the System.Drawing.Common package using:

dotnet add package System.Drawing.Common

Tips:

• Add noise and cross lines to the Captcha image to increase security.
• Use different fonts or rotate characters to make the Captcha harder for bots to read.

---