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Navigator: Home - Tutorials - Controls - Verification code to prevent auto signup in ASP.NET(C#)
Verification code to prevent auto signup in ASP.NET(C#)


ASP.NET Controls Tutorial

By entering the verification code shown on the web page when a user sign up, the web site or application can prevent automated registrations. This reduces system loads and ensures better performance and security of web site or application. This tutorial will show you how to create the randomly generated verification code in ASP.NET 2.0 and C#.

In order to create a random verification code, we create a method called CreateRandomCode at first

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;
}

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Then we generate verification code character string as exported image

private void CreateImage(string 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();
}
}

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Transfer function on Page_load

protected void Page_Load(object sender, EventArgs e)
{
string checkCode = this.CreateRandomCode(6);
Session["CheckCode"] = checkCode;
CreateImage(checkCode);
}

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