A simple and practical verification code

Using system;
Using system. Data;
Using system. configuration;
Using system. collections;
Using system. Web;
Using system. Web. Security;
Using system. Web. UI;
Using system. Web. UI. webcontrols;
Using system. Web. UI. webcontrols. webparts;
Using system. Web. UI. htmlcontrols;
Using system. drawing;
Using system. Drawing. imaging;
Using system. IO;
Public partial class test: system. Web. UI. Page
{
Protected void page_load (Object sender, eventargs E)
{
If (! Ispostback)
{
// 4-digit Verification Code
String str_validatecode = getrandomnumberstring (4 );
// The session used for verification
Session ["validatecode"] = str_validatecode;
Createimage (str_validatecode );
}
}
// Generate a random number string
Public String getrandomnumberstring (INT int_numberlength)
{
String str_number = string. empty;
Random therandomnumber = new random ();

For (INT int_index = 0; int_index <int_numberlength; int_index ++)
Str_number + = therandomnumber. Next (10). tostring ();

Return str_number;
}

Public color getrandomcolor ()
{
Random randomnum_first = new random (INT) datetime. Now. ticks );
// There is nothing to say about the random number of C #.
System. Threading. thread. Sleep (randomnum_first.next (50 ));
Random randomnum_sencond = new random (INT) datetime. Now. ticks );

// Try to generate a dark color for display on a white background
Int int_red = randomnum_first.next (256 );
Int int_green = randomnum_sencond.next (256 );
Int int_blue = (int_red + int_green> 400 )? 0: 400-int_red-int_green;
Int_blue = (int_blue & gt; 255 )? 255: int_blue;

return color. fromargb (int_red, int_green, int_blue);
}

Public void createimage (string str_validatecode)
{< br> int int_imagewidth = str_validatecode.length * 13;
random newrandom = new random ();
// figure height: 20px
bitmap thebitmap = new Bitmap (int_imagewidth, 20);
graphics thegraphics = graphics. fromimage (thebitmap);
// white background
thegraphics. clear (color. white);
// gray border
thegraphics. drawrectangle (new pen (color. lightgray, 1), 0, 0, int_imagewidth-1, 19);

// 10pt font
Font thefont = new font ("Arial", 10 );

For (INT int_index = 0; int_index <str_validatecode.length; int_index ++)
{
String str_char = str_validatecode.substring (int_index, 1 );
Brush newbrush = new solidbrush (getrandomcolor ());
Point thepos = new point (int_index * 13 + 1 + newrandom. Next (3), 1 + newrandom. Next (3 ));
Thegraphics. drawstring (str_char, thefont, newbrush, thepos );
}

// Send the generated image back to the client
Memorystream MS = new memorystream ();
Thebitmap. Save (MS, imageformat. PNG );

Response. clearcontent (); // You Need to output the image information to modify the HTTP header.
Response. contenttype = "image/PNG ";
Response. binarywrite (Ms. toarray ());
Thegraphics. Dispose ();
Thebitmap. Dispose ();
Response. End ();
}
}

webpage call: img src = " *. aspx " />