How does ASP.NET design a simple verification code?

Create a new Image.aspx page without adding any elements to the foreground. The background code is

Static read-only string str = "0123456789 abcdefghijklmnopqrstuvwxyzabc defghijklmnopqrstuvwxyXYZ";

Protected void Page_Load (object sender, EventArgs e)

{

Random rd = new random ();

int a = rd。 Next (100);

int b = rd。 Next (1000);

int m = rd。 Next (10000);

int n = rd。 Next (100000);

String result = STR. substring（a % 62， 1）；

Result+= string. substring（b % 62， 1）；

Result+= string. substring（m % 62， 1）；

Result+= string. substring（n % 62， 1）；

session【“check“】= result；

CreateImage (result);

}

///& lt； Summary & gt

///Create a verification picture of the specified content and output it.

///& lt； /summary & gt；

///& lt； param name =“content“& gt； & lt/param & gt；

Private void CreateImage (string content)

{

//The judgment string is not equal to null and null.

if（content = = null | | content。 trim（）= = String。 Empty)

Return;

//Create a bitmap object

Bitmap image = new bitmap ((int) math. Ceiling ((content. Length * 12.5)), 22);

//Create a graph

Graph g = graph. FromImage;

attempt

{

//Generate a random generator

Random Random = new Random（）；

//Clear the background color of the picture.

G. transparent (color. White);

//Draw the background interference line of the picture.

for(int I = 0； I<2; i++)

{

int x 1 = random。 Next (picture. Width);

Int x2 = random. Next (picture. Width);

int y 1 = random。 Next (picture. Height);

Int y2 = random. Next (picture. Height);

G. new pen (color. Black), x 1, y 1, x2, y2);

}

Font font = new font ("Arial", 12, (FontStyle. Bold);

System. Drawing.drawing2d.lineargradientbrush = new system. Drawing.drawing2d.lineargradientbrush (new rectangle (0,0, image). Width, image. Height), color. Black, color. DarkRed， 1.2f，true）；

G. draw rope (content, font, brush, 2, 2);

//Draw the foreground interference point of the picture

for(int I = 0； I< 100; i++)

{

int x = random。 Next (picture. Width);

int y = random。 Next (picture. Height);

Image. set pixel（x，y，Color。 from argb（random。 next（）））；

}

image = twist image（image，true，4，4）；

//Draw the border line of the picture

G.DrawRectangle (new pen (color. Silver), 0, 0, image. Width-1, image. Height-1);

//output the picture to the page

System. IO. memory flow ms = new system. IO . memory stream（）；

Image. Save (ms, system. drawing . imaging . imageformat . gif）；

Response. clear content（）；

Response. content type =“image/Gif“；

Response. binary write（ms . ToArray（））；

}

finally

{

g.dispose（）；

Image. dispose（）；

}

}

# region produces filter effect.

private const double PI = 3. 14 1592653589793238462643832795；

private const double PI2 = 6.283 185307 17958647692528676559；

///& lt； Summary & gt

///sine wave distortion picture

///& lt； /summary & gt；

///& lt； param name =“srcBmp“& gt； Image path

///& lt； param name =“bXDir“& gt； If it is distorted, please select True.

///& lt； param name =“nMultValue“& gt； The greater the amplitude multiple of the waveform, the higher the distortion, usually 3.

///& lt； param name =“d phase“& gt； Initial phase of waveform, value interval [0-2 * pi]

///& lt； Returns & gt& lt/returns & gt；;

Common bitmap TwistImage (bitmap srcBmp, boolbdxdir, double dMultValue, double dPhase)

{

Bitmap destBmp = new bitmap (srcBmp. Width, srcBmp. Height);

//Fill the bitmap background with white.

System. Drawing. Graphics graphics = graphics. from image（dest BMP）；

Charts. FillRectangle (new SolidBrush (color. White), 0,0, destBmp. Width, destBmp. Height);

Charts. dispose（）；

double dBaseAxisLen = bXDir？ (double) destBmp. Height: (double) destBmp. Width;

for(int I = 0； Me & ltdestBmp. Width; i++)

{

for(int j = 0； j & ltdestBmp。 Height; j++)

{

double dx = 0；

dx = bXDir？ （PI2 *（double）j）/dBaseAxisLen:（PI2 *（double）I）/dBaseAxisLen；

dx+= d phase；

Double dy = math. sin（dx）；

//Get the color of the current point.

int nOldX = 0，nOldY = 0；

nOldX = bXDir？ I+（int）（dy * dMultValue）:I；

nOldY = bXDir？ j:j+（int）（dy * dMultValue）；

System. Drawing. Color color = srcBmp。 get pixel（I，j）；

if（nOldX & gt； = 0 & amp& ampnOldX & ltdestBmp。 width

& amp& ampnOldY & gt= 0 & amp& ampnOldY & ltdestBmp。 Height)

{

destBmp。 set pixel（nOldX，nOldY，color）；

}

}

}

return destBmp

}

# End area

When referenced on another page,

& ltimg id = "img" alt = "verification code" src = "image.aspx"/>.

& ltinput type =“button“onclick =“JavaScript:var img = document . getelementbyid（‘img‘）； img . src = img . src+‘？ ‘。 ; "value=" I can't see clearly, change it to "/>

In your registration button, you can use the session [Check] to determine whether the verification code is correct.