Image Verification Helper Class

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Introduction

This article will help the intermediate ASP.NET developers who may be sometimes in need of implementing a feature like Image Verification when getting inputs from the users. Basically, this is a new technique which will stop the automated programs to process the input forms. But, here I'm not explaining you about the whole technique of Image Verification. This article will explain two things, which we really need :

• How to generate alpha-numeric characters randomly?
• How to make it as an image?

Generating Random Alpha-Numerics

Microsoft .NET framework provides System.Security.Cryptography namespace which contains RandomNumberGenerator to produce random numbers.

The class CImgVerify contains the code to produce the random alphabets/numbers in the function getRandomAlphaNumeric.

RandomNumberGenerator rm; 
rm = RandomNumberGenerator.Create();

The above code will create a RandomNumberGenerator object.

byte[] data = new byte[3]; 
rm.GetNonZeroBytes(data);

The byte type "data" will hold three elements of array to have the randomly generated numbers by GetNonZeroBytes. The GetNonZeroBytes fills an array of bytes with a cryptographically strong random sequence of nonzero values.

The generated random numbers will be stored in a byte variable as an array. Each and every elements will be an integer variable. So, make a proper alphanumeric string. The following code will use a loop to identify whether the randomly generated number falls into alphabets.

for(int nCnt=0;nCnt<=data.Length-1;nCnt++)
{ 
  //First convert it into a integer

  int nVal = Convert.ToInt32(data.GetValue(nCnt)); 
  // Check whether the converted int falls in between alphabets,symbols

  if(nVal > 32 && nVal < 127) 
  {
     sTmp = Convert.ToChar(nVal).ToString(); //Convert to character

  }
  else
  {
     sTmp = nVal.ToString(); //Remain as integer

  }
  sRand += sTmp.ToString(); //Append it to a string

}

Generating Image in the Sky

So, now we got the random alphanumeric to display as a image. The image can be created by the System.Drawing namespace. The following is the code which itself self explanatory.

public Bitmap generateImage(string sTextToImg)
{ // 

  //Here, i haven't used any try..catch 

  
  PixelFormat pxImagePattern = PixelFormat.Format32bppArgb; 
  Bitmap bmpImage = new Bitmap(1,1,pxImagePattern); 
  Font fntImageFont = new Font("Trebuchets",14); 
  Graphics gdImageGrp = Graphics.FromImage(bmpImage); 
  float iWidth = gdImageGrp.MeasureString(sTextToImg,fntImageFont).Width; 
  float iHeight = gdImageGrp.MeasureString(sTextToImg,fntImageFont).Height; 
  bmpImage = new Bitmap((int)iWidth,(int)iHeight,pxImagePattern ); 
  gdImageGrp = Graphics.FromImage(bmpImage); 
  gdImageGrp.Clear(Color.White); 
  gdImageGrp.TextRenderingHint = TextRenderingHint.AntiAlias; 
  gdImageGrp.DrawString(sTextToImg,fntImageFont, new SolidBrush(Color.Red),0,0); 
  gdImageGrp.Flush(); 
  return bmpImage;
}

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