How to do word image verification while submitting a form

These days word image verification has become very common. Mainly to avoid spam mail activities, automatic generation of accounts etc. Here's a simple method that generates an image using random alphabets.

First method recursively generates random characters, concatenates them into a string and returns it back.
public 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,V,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(36);

if (temp != -1 && temp == t)
{
return CreateRandomCode(codeCount);
}

temp = t;

randomCode += allCharArray[t];
}

return randomCode;
}
 
now the second method generates an image
using System.Drawing;


public void GenerateImage () {
string checkCode = this.CreateRandomCode(6);
Session["WordCheck"] = checkCode;

System.Drawing.Bitmap image = new System.Drawing.Bitmap(Convert.ToInt32(Math.Ceiling((decimal)(checkCode.Length * 14))), 22);

Graphics g = Graphics.FromImage(image);

try
{

Random random = new Random();
g.Clear(Color.AliceBlue);

for (int i = 0; i < 25; i++)
{

int x1 = random.Next(image.Width);
int x2 = random.Next(image.Width);
int y1 = random.Next(image.Height);
int y2 = random.Next(image.Height);

g.DrawLine(new Pen(Color.Silver), x1, y1, x2, y2);
}

Font font = new System.Drawing.Font("Comic Sans MS", 12, System.Drawing.FontStyle.Bold);

System.Drawing.Drawing2D.LinearGradientBrush brush = new System.Drawing.Drawing2D.LinearGradientBrush(new Rectangle(0, 0, image.Width, image.Height), Color.Blue, Color.DarkRed, 1.2f, true);

g.DrawString(checkCode, font, new SolidBrush(Color.Red), 2, 2);

for (int i = 0; i < 100; i++)
{
int x = random.Next(image.Width);
int y = random.Next(image.Height);

image.SetPixel(x, y, Color.FromArgb(random.Next()));
}

g.DrawRectangle(new Pen(Color.Silver), 0, 0, image.Width - 1, image.Height - 1);
System.IO.MemoryStream ms = new System.IO.MemoryStream();
image.Save(ms, System.Drawing.Imaging.ImageFormat.Gif);

Response.ClearContent();
Response.ContentType = "image/Gif";
Response.BinaryWrite(ms.ToArray());

}

finally
{
g.Dispose();
image.Dispose();
}

}