You can also add a text verification code to the login control.

Expand the login control provided by Asp.net, and add a text verification function after the password to Prevent Automatic Registration of programs.
1. Add imagevalidate. aspx and add information to the header.
<% Response. Buffer = true; %>
<% Response. expiresabsolute = datetime. Now. addseconds (-1); %>
<% Response. expires = 0; %>
<% Response. cachecontrol = "no-Cache"; %>
The code in imagevalidate. aspx. CS is as follows:
Protected void page_load (Object sender, eventargs E)
{
// Generate a four-digit Verification Code
/* String TMP = rndnum (4 );
Httpcookie cookie = new httpcookie ("imagev", TMP );
// Response. Cookies ["imagev"]. value = TMP;
Response. Cookies. Add (cookie );
This. validatecode (TMP );*/

Createcheckcodeimage (generatecheckcode ());

}

Private string generatecheckcode ()
{
Int number;
Char code;
String checkcode = string. empty;

System. Random random = new random ();

For (INT I = 0; I <5; I ++)
{
Number = random. Next ();

If (Number % 2 = 0)
Code = (char) ('0' + (char) (Number % 10 ));
Else
Code = (char) ('A' + (char) (Number % 26 ));

Checkcode + = code. tostring ();
}
Session ["checkcode"] = checkcode;


// Response. Cookies. Add (New httpcookie ("checkcode", checkcode ));

Return checkcode;
}

Private void createcheckcodeimage (string checkcode)
{
If (checkcode = NULL | checkcode. Trim () = string. Empty)
Return;

System. Drawing. bitmap image = new system. Drawing. Bitmap (INT) math. Ceiling (checkcode. length * 12.5), 22 );
Graphics G = graphics. fromimage (image );

Try
{
// Generate a random Generator
Random random = new random ();

// Clear the background color of the image
G. Clear (color. White );

// Draw the background noise line of the image
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 ("Arial", 12, (system. Drawing. fontstyle. Bold | system. Drawing. fontstyle. italic ));
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, brush, 2, 2 );

// Foreground noise of the image
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 ()));
}

// Draw the border line of the image
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 ();
}
}
This is the page for generating the verification code
2. Add the page to the login control:
Drag a login control to switch to the mode. In the HTML source file, add the following lines to the password row in the table:
<Tr>
<TD style = "width: 151px">
<Asp: Label id = "label1" runat = "server" associatedcontrolid = "password"> Verification Code: </ASP: Label>
</TD>
</Tr>
<Tr>
<TD style = "width: 151px">
<Asp: textbox id = "txtimgvalid" runat = "server" font-size = "0.8em"> </ASP: textbox>
<Asp: imagebutton id = "imgvalid" imageurl = "~ /Public/imagevalidate. aspx "runat =" server "/>
</TD>
</Tr>

Here, imageurl = "~ The imagevalidate. aspx of/public/imagevalidate. aspx indicates the file created in step 1.
3. Add code to the authenticate event of the login Control
Protected void login=authenticate (Object sender, authenticateeventargs E)
{
Textbox txtimage = login1.findcontrol ("txtimgvalid") as textbox;


If (session ["checkcode"] = NULL)
{
Response. Write ("<script language = 'javascript '> alert ('unknown error! ') </SCRIPT> ");
E. Authenticated = false;
Return;
}

If (string. Compare (session ["checkcode"]. tostring (), txtimage. Text, true )! = 0)
{
Response. Write ("<script language = 'javascript '> alert (' does not match! ') </SCRIPT> ");
E. Authenticated = false;
Return;
}

E. Authenticated = true;

}

Click the log on button to clear the session.
Protected void loginbutton_click (Object sender, eventargs E)
{
Session ["checkcode"] = NULL;
}
Running result