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  • not able to log on to balcberry gmail even after change of psswd and Captcha unlock

    - by user34895
    I have custom domain with gmail, and successfully configured gmail account to receive mail in blackberry. However, after deleting the account, I am not able to configure again, and it keep on repeating same problem: invalid email account. I have changed the password, and also did unlocking of captsha many times, but it just dont connect with blackberry account. Can I get some advise?

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  • Manually closing a port from commandline

    - by codingfreak
    I want to close an open port which is in listening mode between my client and server application. Is there any manual command line option in Linux to close a port ?? NOTE: I came to know that "only the application which owns the connected socket should close it, which will happen when the application terminates." I dont understand why it is only possible by the application which opens it ... But still eager to know if there is any another way to do it ??

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  • Dialog in linux

    - by user35319
    Hi everyone, I want to show the contents of file on Dialog box for which i have use the "--textbox" dialog and "--tailbox" dialog but it dont show the whole contents of file just show some data not the whole data of file...i searched alot but found nothing so if anyone have any idea plz let me know bcoz i have been trying so much to fix the problem...

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  • How copy with shell commands(Linux) from 1 computer have (permanent url + open port) to 2 computer (secure way)?

    - by BenBen
    How copy with shell commands(Linux) from remote(my office computer) computer (permanent url + open port) to my (home) computer home/remote_computer_user/Desktop/test1.txt to my home computer home/home_computer_user/Downloads/ ? What I am doing: 1. ssh <user>@<computer1address> -p <port> :: success to get remote computer shell () 2. (I think I should use scp , but I dont how exactly in my case) Please if you can, write the exact commands that i should to from the shell Thanks in advance

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  • Svn hook on commit makes lock

    - by dynback.com
    My post-commit hook looks like this: pushd C:\Websites\Project svn update I am updating my server copy of repository. When I commit client stopped on sending content and locked or I dont know. Its is waiting for something. So when I cancel and try to update manually on server, I see: Working copy "." lockedsvn And only after manual cleanup and update again, I get updated revision, that was really commited. What I do wrong?

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  • What is . and .. in a directory?

    Based on the qestion How to make using command prompt less painful what is the . and .. in the most voted answer? I see it when i do a dir command but it isn't visible to the user in the form of a file. In case you dont know what I mean heres the example . .. Su.exe Sup.txt SuperUser.COM

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  • want to restore windows 7 from linux ubuntu

    - by elisi
    Hi, I want to recovery Win7 from Linux and I dont have the Win7 CD or any previous back up files. Please tell me if there is a way to recovery Win7 from Linux because I do not want to boot it from the beginning cause I have important files and they are in one partition.

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  • linux output show only right of ':'

    - by acidzombie24
    I forgot a lot of my command line. I am doing cat file | grep "error" and i would like it to show everything to the right of G:/ including G:/ if possible. I figure its an awk command but i dont know what. I tried awk '{print $8+}' but + does not work like i hoped and guessed.

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  • Lost network on ubuntu server

    - by user1838473
    I have a virtual machine on Vsphere 5.0 running Ubuntu 12.04 when i put dinamic IP (/etc/network/interfaces) iface eth0 inet dhcp Ubuntu have network and i can do ping to google for example (8.8.8.8) but when i put static IP and configure resolv.conf My interfaces file: auto eth0 iface eth0 inet static address 192.168.1.54 gateway 192.168.1.1 netmask 255.255.255.0 it lost the network and i cant do ping to anything...i dont understand where is the problem... Thanks a lot

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  • Should I refactor this code?

    - by user156814
    The code is for a view debate page. The code is supposed to determine whether or not to show an add reply form to the viewing user. If the user is logged in, and the user is not the creator of the debate, then check if the user already replied to the debate. If the user did not already reply to the debate then show the form... Otherwise, Check If the user wants to edit their already existing reply by looking in the url for the reply id If any of these tests dont pass, Then I save the reason as an int and pass that to a switch statement in the view. The logic seems easy enough, but my code seems a little sloppy. Here's the code.. (using Kohana V2.3.4) public function view($id = 0) { $debate = ORM::factory('debate')->with('user')->with('category')->find($id); if ($debate->loaded == FALSE) { url::redirect(); } // series of tests to show an add reply form if ($this->logged_in) { // is the viewer the creator? if ($this->user->id != $debate->user->id) { // has the user already replied? if (ORM::factory('reply') ->where(array('debate_id' => $id, 'user_id' => $this->user->id)) ->count_all() == 0) { $form = $errors = array ( 'body' => '', 'choice_id' => '', 'add' => '' ); if ($post = $this->input->post()) { $reply = ORM::factory('reply'); // validate and insert the reply if ($reply->add($post, TRUE)) { url::redirect(url::current()); } $form = arr::overwrite($form, $post->as_array()); $errors = arr::overwrite($errors, $post->errors('reply_errors')); } } // editing a reply? else if (($rid = (int) $this->input->get('edit')) AND ($reply = ORM::factory('reply') ->where(array('debate_id' => $id, 'user_id' => $this->user->id)) ->find($rid))) { $form = $errors = array ( 'body' => '', 'choice_id' => '', 'add' => '' ); // autocomplete the form $form = arr::overwrite($form, $reply->as_array()); if ($post = $this->input->post()) { // validate and insert the reply if ($reply->edit($post, TRUE)) { url::redirect(url::current()); } $form = arr::overwrite($form, $post->as_array()); $errors = arr::overwrite($errors, $post->errors('reply_errors')); } } else { // user already replied $reason = 3; } } else { // user started the debate $reason = 2; } } else { // user is not logged in. $reason = 1; } $limits = Kohana::config('app/debate.limits'); $page = (int) $this->input->get('page', 1); $offset = ($page > 0) ? ($page - 1) * $limits['replies'] : 0; $replies = ORM::factory('reply')->with('user')->with('choice')->where('replies.debate_id', $id); $this->template->title = $debate->topic; $this->template->debate = $debate; $this->template->body = View::factory('debate/view') ->set('debate', $debate) ->set('replies', $replies->find_all($limits['replies'], $offset)) ->set('pagination', Pagination::factory(array ( 'style' => 'digg', 'items_per_page' => $limits['replies'], 'query_string' => 'page', 'auto_hide' => TRUE, 'total_items' => $total = $replies->count_last_query() )) ) ->set('total', $total); // are we showing the add reply form? if (isset($form, $errors)) { $this->template->body->add_reply_form = View::factory('reply/add_reply_form') ->set('debate', $debate) ->set('form', $form) ->set('errors', $errors); } else { $this->template->body->reason = $reason; } } Heres the view, theres some logic in here that determines what message to show the user. <!-- Add Reply Form --> <?php if (isset($add_reply_form)): ?> <?php echo $add_reply_form; ?> <?php else: ?> <?php switch ($reason) { case 1 : // not logged in, show a message $message = 'Add your ' . html::anchor('login?url=' . url::current(TRUE), '<b>vote</b>') . ' to this discussion'; break; case 2 : // started the debate. dont show a message for that. $message = NULL; break; case 3: // already replied, show a message $message = 'You have already replied to this debate'; break; default: // unknown reason. dont show a message $message = NULL; break; } ?> <?php echo app::show_message($message, 'h2'); ?> <?php endif; ?> <!-- End Add Reply Form --> Should I refactor the add reply logic into another function or something.... It all works, it just seems real sloppy. Thanks

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  • Is there any better way for creating a dynamic HTML table without using any javascript library like

    - by piemesons
    Dont worry we dont need to find out any bug in this code.. Its working perfectly.:-P My boss came to me and said "Hey just tell me whats the best of way of writing code for a dynamic HTML table (add row, delete row, update row).No need to add any CSS. Just javascript. No Jquery library etc. I was confused that in the middle of the project why he asking for some stupid exercise like this. What ever i wrote the following code and mailed him and after 15 mins i got a mail from him. " I was expecting much better code from a guy like you. Anyways good job monkey.(And with a picture of monkey as attachment.) thats was the mail. Line by line. I want to reply him but before that i want to know about the quality of my code. Is this really shitty...!!! Or he was just making fun of mine. I dont think that code is really shitty. Still correct me if you can.Code is working perfectly fine. Just copy paste it in a HTML file. <html> <head> <title> Exercise CSS </title> <script type="text/javascript"> function add_row() { var table = document.getElementById('table'); var rowCount = table.rows.length; var row = table.insertRow(rowCount); var cell1 = row.insertCell(0); var element1 = document.createElement("input"); element1.type = "text"; cell1.appendChild(element1); var cell2 = row.insertCell(1); var element2 = document.createElement("input"); element2.type = "text"; cell2.appendChild(element2); var cell3 = row.insertCell(2); cell3.innerHTML = ' <span onClick="edit(this)">Edit</span>/<span onClick="delete_row(this)">Delete</span>'; cell3.setAttribute("style", "display:none;"); var cell4 = row.insertCell(3); cell4.innerHTML = '<span onClick="save(this)">Save</span>'; } function save(e) { var elTableCells = e.parentNode.parentNode.getElementsByTagName("td"); elTableCells[0].innerHTML=elTableCells[0].firstChild.value; elTableCells[1].innerHTML=elTableCells[1].firstChild.value; elTableCells[2].setAttribute("style", "display:block;"); elTableCells[3].setAttribute("style", "display:none;"); } function edit(e) { var elTableCells = e.parentNode.parentNode.getElementsByTagName("td"); elTableCells[0].innerHTML='<input type="text" value="'+elTableCells[0].innerHTML+'">'; elTableCells[1].innerHTML='<input type="text" value="'+elTableCells[1].innerHTML+'">'; elTableCells[2].setAttribute("style", "display:none;"); elTableCells[3].setAttribute("style", "display:block;"); } function delete_row(e) { e.parentNode.parentNode.parentNode.removeChild(e.parentNode.parentNode); } </script> </head> <body > <div id="display"> <table id='table'> <tr id='id'> <td> Piemesons </td> <td> 23 </td> <td > <span onClick="edit(this)">Edit</span>/<span onClick="delete_row(this)">Delete</span> </td> <td style="display:none;"> <span onClick="save(this)">Save</span> </td> </tr> </table> <input type="button" value="Add new row" onClick="add_row();" /> </div> </body>

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  • Bandwidth Limit Php Not working

    - by Saxtor
    Hey How are you doing guys, i am trying to limit bandwidth per users not by ipaddress for some reason my code doesnt work i need some help, what i am trying to do is to limit the download of the user that they would only have 10Gb per day to download however it seems to me that my buffer is not working when i use multiple connections it doesnt work, but when i use one connect it works 80% here is my code can you debug the error for me thanks. /** * @author saxtor if you can improve this code email me [email protected] * @copyright 2010 */ /** * CREATE TABLE IF NOT EXISTS `max_traffic` ( `id` int(255) NOT NULL AUTO_INCREMENT, `limit` int(255) NOT NULL, PRIMARY KEY (`id`) ) ENGINE=MyISAM DEFAULT CHARSET=latin1 AUTO_INCREMENT=0 ; */ //SQL Connection [this is hackable for testing] date_default_timezone_set("America/Guyana"); mysql_connect("localhost", "root", "") or die(mysql_error()); mysql_select_db("Quota") or die(mysql_error()); function quota($id) { $result = mysql_query("SELECT `limit` FROM max_traffic WHERE id='$id' ") or die(error_log(mysql_error()));; $row = mysql_fetch_array($result); return $row[0]; } function update_quota($id,$value) { $result = mysql_query("UPDATE `max_traffic` SET `limit`='$value' WHERE id='$id'") or die(mysql_error()); return $value; } if ( quota(1) != 0) $limit = quota(1); else $limit = 0; $multipart = false; //was a part of the file requested? (partial download) $range = $_SERVER["HTTP_RANGE"]; if ($range) { $cookie .= "\r\nRange: $range"; $multipart = true; header("X-UR-RANGE-Range: $range"); } $url = 'http://127.0.0.1/puppy.iso'; $filename = basename($url); //octet-stream + attachment => client always stores file header('Content-type: application/octet-stream'); header('Content-Disposition: attachment; filename="'.$filename.'"'); //always included so clients know this script supports resuming header("Accept-Ranges: bytes"); $user_agent = ini_get("user_agent"); ini_set("user_agent", $user_agent . "\r\nCookie: enc=$cookie"); $httphandle = fopen($url, "r"); $headers = stream_get_meta_data($httphandle); $size = $headers["wrapper_data"][6]; $sizer = explode(' ',$size); $size = $sizer[1]; //let's check the return header of rapidshare for range / length indicators //we'll just pass these to the client foreach ($headers["wrapper_data"] as $header) { $header = trim($header); if (substr(strtolower($header), 0, strlen("content-range")) == "content-range") { // _insert($range); header($header); header("X-RS-RANGE-" . $header); $multipart = true; //content-range indicates partial download } elseif (substr(strtolower($header), 0, strlen("Content-Length")) == "content-length") { // _insert($range); header($header); header("X-RS-CL-" . $header); } } if ($multipart) header('HTTP/1.1 206 Partial Content'); flush(); $speed = 4128; $packet = 1; //this is private dont touch. $bufsize = 128; //this is private dont touch/ $bandwidth = 0; //this is private dont touch. while (!(connection_aborted() || connection_status() == 1) && $size > 0) { while (!feof($httphandle) && $size > 0) { if ($limit <= 0 ) $size = 0; if ( $size < $bufsize && $size != 0 && $limit != 0) { echo fread($httphandle,$size); $bandwidth += $size; } else { if( $limit != 0) echo fread($httphandle,$bufsize); $bandwidth += $bufsize; } $size -= $bufsize; $limit -= $bufsize; flush(); if ($speed > 0 && ($bandwidth > $speed*$packet*103)) { usleep(100000); $packet++; //update_quota(1,$limit); } error_log(update_quota(1,$limit)); $limit = quota(1); //if( $size <= 0 ) // exit; } fclose($httphandle); } exit;

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  • ASp.Net Mvc 1.0 Dynamic Images Returned from Controller taking 154 seconds+ to display in IE8, firef

    - by julian guppy
    I have a curious problem with IE, IIS 6.0 dynamic PNG files and I am baffled as to how to fix.. Snippet from Helper (this returns the URL to the view for requesting the images from my Controller. string url = LinkBuilder.BuildUrlFromExpression(helper.ViewContext.RequestContext, helper.RouteCollection, c = c.FixHeight(ir.Filename, ir.AltText, "FFFFFF")); url = url.Replace("&", "&"); sb.Append(string.Format("<removed id=\"TheImage\" src=\"{0}\" alt=\"\" /", url)+Environment.NewLine); This produces a piece of html as follows:- img id="TheImage" src="/ImgText/FixHeight?sFile=Images%2FUser%2FJulianGuppy%2FMediums%2Fconservatory.jpg&backgroundColour=FFFFFF" alt="" / brackets missing because i cant post an image... even though I dont want to post an image I jsut want to post the markup... sigh Snippet from Controller ImgTextController /// <summary> /// This function fixes the height of the image /// </summary> /// <param name="sFile"></param> /// <param name="alternateText"></param> /// <param name="backgroundColour"></param> /// <returns></returns> [AcceptVerbs(HttpVerbs.Get)] public ActionResult FixHeight(string sFile, string alternateText, string backgroundColour) { #region File if (string.IsNullOrEmpty(sFile)) { return new ImgTextResult(); } // MVC specific change to prepend the new directory if (sFile.IndexOf("Content") == -1) { sFile = "~/Content/" + sFile; } // open the file System.Drawing.Image img; try { img = System.Drawing.Image.FromFile(Server.MapPath(sFile)); } catch { img = null; } // did we fail? if (img == null) { return new ImgTextResult(); } #endregion File #region Width // Sort out the width from the image passed to me Int32 nWidth = img.Width; #endregion Width #region Height Int32 nHeight = img.Height; #endregion Height // What is the ideal height given a width of 2100 this should be 1400. var nIdealHeight = (int)(nWidth / 1.40920096852); // So is the actual height of the image already greater than the ideal height? Int32 nSplit; if (nIdealHeight < nHeight) { // Yes, do nothing, well i need to return the iamge... nSplit = 0; } else { // rob wants to not show the white at the top or bottom, so if we were to crop the image how would be do it // 1. Calculate what the width should be If we dont adjust the heigt var newIdealWidth = (int)(nHeight * 1.40920096852); // 2. This newIdealWidth should be smaller than the existing width... so work out the split on that Int32 newSplit = (nWidth - newIdealWidth) / 2; // 3. Now recrop the image using 0-nHeight as the height (i.e. full height) // but crop the sides so that its the correct aspect ration var newRect = new Rectangle(newSplit, 0, newIdealWidth, nHeight); img = CropImage(img, newRect); nHeight = img.Height; nWidth = img.Width; nSplit = 0; } // No, so I want to place this image on a larger canvas and we do this by Creating a new image to be the size that we want System.Drawing.Image canvas = new Bitmap(nWidth, nIdealHeight, PixelFormat.Format24bppRgb); Graphics g = Graphics.FromImage(canvas); #region Color // Whilst we can set the background colour we shall default to white if (string.IsNullOrEmpty(backgroundColour)) { backgroundColour = "FFFFFF"; } Color bc = ColorTranslator.FromHtml("#" + backgroundColour); #endregion Color // Filling the background (which gives us our broder) Brush backgroundBrush = new SolidBrush(bc); g.FillRectangle(backgroundBrush, -1, -1, nWidth + 1, nIdealHeight + 1); // draw the image at the position var rect = new Rectangle(0, nSplit, nWidth, nHeight); g.DrawImage(img, rect); return new ImgTextResult { Image = canvas, ImageFormat = ImageFormat.Png }; } My ImgTextResult is a class that returns an Action result for me but embedding the image from a memory stream into the response.outputstream. snippet from my ImageResults /// <summary> /// Execute the result /// </summary> /// <param name="context"></param> public override void ExecuteResult(ControllerContext context) { // output context.HttpContext.Response.Clear(); context.HttpContext.Response.ContentType = "image/png"; try { var memStream = new MemoryStream(); Image.Save(memStream, ImageFormat.Png); context.HttpContext.Response.BinaryWrite(memStream.ToArray()); context.HttpContext.Response.Flush(); context.HttpContext.Response.Close(); memStream.Dispose(); Image.Dispose(); } catch (Exception ex) { string a = ex.Message; } } Now all of this works locally and lovely, and indeed all of this works on my production server BUT Only for Firefox, Safari, Chrome (and other browsers) IE has a fit and decides that it either wont display the image or it does display the image after approx 154seconds of waiting..... I have made sure my HTML is XHTML compliant, I have made sure I am getting no Routing errors or crashes in my event log on the server.... Now obviously I have been a muppet and have done something wrong... but what I cant fathom is why in development all works fine, and in production all non IE browsers also work fine, but IE 8 using IIS 6.0 production server is having some kind of problem in returning this PNG and I dont have an error to trace... so what I am looking for is guidance as to how I can debug this problem.

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  • OpenGL, draw two polygons in the same time (by mouse clicks)

    - by YoungSalafi
    im trying to draw 2 polygons at the same time depending on user input from the opengl screen... so i made 2 arrays which each one of them will carry the vertices of each polygon ... i think my logic is right but the program still prints only polygon and delete the old polygon if you draw a polygon again . and its acting weird too please check the code yourself here it is : P.S dont mind the delete function right now.. i know it missing something. #include <windows.h> #include <gl/gl.h> #include <gl/glut.h> void Draw(); void Set_Transformations(); void Initialize(int argc, char *argv[]); void OnKeyPress(unsigned char key, int x, int y); void DeleteVer(); void MouseClick(int bin, int state , int x , int y); void GetOGLPos(int x, int y,float* arrY,float* arrX); void DrawPolygon(float* arrX,float* arrY); float xPos[20]; float yPos[20]; float xPos2[20]; float yPos2[20]; float fx = 0,fy = 0; float size = 10; int count = 0; bool done = false; bool flag = true; void Initialize(int argc, char *argv[]) { glutInit(&argc, argv); glutInitDisplayMode(GLUT_RGBA); glutInitWindowPosition(100, 100); glutInitWindowSize(600, 600); glutCreateWindow("OpenGL Lab1"); Set_Transformations(); glutDisplayFunc(Draw); glutMouseFunc(MouseClick); glutKeyboardFunc(OnKeyPress); glutMainLoop(); } void Set_Transformations() { glClearColor(1, 1, 1, 1); glMatrixMode(GL_PROJECTION); glLoadIdentity(); gluOrtho2D(-200, 200, -200, 200); } void OnKeyPress(unsigned char key, int x, int y) { if (key == 27) exit(0); switch(key) { case 13: //enter key it will draw done = true; glutPostRedisplay(); flag=!flag; // this flag to switch to the other array that the vertices will be stored in, in order to draw the second polygon break; } } void MouseClick(int button, int state , int x , int y) { switch (button) { case GLUT_RIGHT_BUTTON: if (state == GLUT_DOWN) { if (count>0) { DeleteVer(); //dont mind this right now } } break; case GLUT_LEFT_BUTTON: if (state == GLUT_DOWN) { if(count<20) { if(flag =true){ // drawing first polygon GetOGLPos(x, y,xPos,yPos);} if (flag=false) //drawing second polygon after Enter is pressed GetOGLPos(x, y,xPos2,yPos2); } } break; } } void GetOGLPos(int x, int y,float* arrY,float* arrX) //getting the vertices from the user { GLint viewport[4]; GLdouble modelview[16]; GLdouble projection[16]; GLfloat winX, winY, winZ; GLdouble posX, posY, posZ; glGetDoublev( GL_MODELVIEW_MATRIX, modelview ); glGetDoublev( GL_PROJECTION_MATRIX, projection ); glGetIntegerv( GL_VIEWPORT, viewport ); winX = (float)x; winY = (float)viewport[3] - (float)y; glReadPixels( x, int(winY), 1, 1, GL_DEPTH_COMPONENT, GL_FLOAT, &winZ ); gluUnProject( winX, winY, winZ, modelview, projection, viewport, &posX, &posY, &posZ); arrX[count] = posX; arrY[count] = posY; count++; glPointSize( 6.0 ); glBegin(GL_POINTS); glVertex2f(posX,posY); glEnd(); glFlush(); } void DeleteVer(){ //dont mind this glColor3f ( 1, 1, 1); glBegin(GL_POINTS); glVertex2f(xPos[count-1],yPos[count-1]); glEnd(); glFlush(); xPos[count] = NULL; yPos[count] = NULL; count--; glColor3f ( 0, 0, 0); } void DrawPolygon(float* arrX,float* arrY) { int n=0; glColor3f ( 0, 0, 0); glBegin(GL_POLYGON); while(n<count) { glVertex2f(arrX[n],arrY[n]); n++; } count=0; glEnd(); glFlush(); } void Draw() //main drawing func { glClear(GL_COLOR_BUFFER_BIT); glColor3f(0, 0, 0); if(done) { DrawPolygon(xPos,yPos); DrawPolygon(xPos2,yPos2); } glFlush(); } int main(int argc, char *argv[]) { Initialize(argc, argv); return 0; }

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  • Design by Contract with Microsoft .Net Code Contract

    - by Fredrik N
    I have done some talks on different events and summits about Defensive Programming and Design by Contract, last time was at Cornerstone’s Developer Summit 2010. Next time will be at SweNug (Sweden .Net User Group). I decided to write a blog post about of some stuffs I was talking about. Users are a terrible thing! Protect your self from them ”Human users have a gift for doing the worst possible thing at the worst possible time.” – Michael T. Nygard, Release It! The kind of users Michael T. Nygard are talking about is the users of a system. We also have users that uses our code, the users I’m going to focus on is the users of our code. Me and you and another developers. “Any fool can write code that a computer can understand. Good programmers write code that humans can understand.” – Martin Fowler Good programmers also writes code that humans know how to use, good programmers also make sure software behave in a predictable manner despise inputs or user actions. Design by Contract   Design by Contract (DbC) is a way for us to make a contract between us (the code writer) and the users of our code. It’s about “If you give me this, I promise to give you this”. It’s not about business validations, that is something completely different that should be part of the domain model. DbC is to make sure the users of our code uses it in a correct way, and that we can rely on the contract and write code in a way where we know that the users will follow the contract. It will make it much easier for us to write code with a contract specified. Something like the following code is something we may see often: public void DoSomething(Object value) { value.DoIKnowThatICanDoThis(); } .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; } Where “value” can be uses directly or passed to other methods and later be used. What some of us can easily forget here is that the “value” can be “null”. We will probably not passing a null value, but someone else that uses our code maybe will do it. I think most of you (including me) have passed “null” into a method because you don’t know if the argument need to be specified to a valid value etc. I bet most of you also have got the “Null reference exception”. Sometimes this “Null reference exception” can be hard and take time to fix, because we need to search among our code to see where the “null” value was passed in etc. Wouldn’t it be much better if we can as early as possible specify that the value can’t not be null, so the users of our code also know it when the users starts to use our code, and before run time execution of the code? This is where DbC comes into the picture. We can use DbC to specify what we need, and by doing so we can rely on the contract when we write our code. So the code above can actually use the DoIKnowThatICanDoThis() method on the value object without being worried that the “value” can be null. The contract between the users of the code and us writing the code, says that the “value” can’t be null.   Pre- and Postconditions   When working with DbC we are specifying pre- and postconditions.  Precondition is a condition that should be met before a query or command is executed. An example of a precondition is: “The Value argument of the method can’t be null”, and we make sure the “value” isn’t null before the method is called. Postcondition is a condition that should be met when a command or query is completed, a postcondition will make sure the result is correct. An example of a postconditon is “The method will return a list with at least 1 item”. Commands an Quires When using DbC, we need to know what a Command and a Query is, because some principles that can be good to follow are based on commands and queries. A Command is something that will not return anything, like the SQL’s CREATE, UPDATE and DELETE. There are two kinds of Commands when using DbC, the Creation commands (for example a Constructor), and Others. Others can for example be a Command to add a value to a list, remove or update a value etc. //Creation commands public Stack(int size) //Other commands public void Push(object value); public void Remove(); .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; }   A Query, is something that will return something, for example an Attribute, Property or a Function, like the SQL’s SELECT.   There are two kinds of Queries, the Basic Queries  (Quires that aren’t based on another queries), and the Derived Queries, queries that is based on another queries. Here is an example of queries of a Stack: //Basic Queries public int Count; public object this[int index] { get; } //Derived Queries //Is related to Count Query public bool IsEmpty() { return Count == 0; } .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; } To understand about some principles that are good to follow when using DbC, we need to know about the Commands and different Queries. The 6 Principles When working with DbC, it’s advisable to follow some principles to make it easier to define and use contracts. The following DbC principles are: Separate commands and queries. Separate basic queries from derived queries. For each derived query, write a postcondition that specifies what result will be returned, in terms of one or more basic queries. For each command, write a postcondition that specifies the value of every basic query. For every query and command, decide on a suitable precondition. Write invariants to define unchanging properties of objects. Before I will write about each of them I want you to now that I’m going to use .Net 4.0 Code Contract. I will in the rest of the post uses a simple Stack (Yes I know, .Net already have a Stack class) to give you the basic understanding about using DbC. A Stack is a data structure where the first item in, will be the first item out. Here is a basic implementation of a Stack where not contract is specified yet: public class Stack { private object[] _array; //Basic Queries public uint Count; public object this[uint index] { get { return _array[index]; } set { _array[index] = value; } } //Derived Queries //Is related to Count Query public bool IsEmpty() { return Count == 0; } //Is related to Count and this[] Query public object Top() { return this[Count]; } //Creation commands public Stack(uint size) { Count = 0; _array = new object[size]; } //Other commands public void Push(object value) { this[++Count] = value; } public void Remove() { this[Count] = null; Count--; } } .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; }   Note: The Stack is implemented in a way to demonstrate the use of Code Contract in a simple way, the implementation may not look like how you would implement it, so don’t think this is the perfect Stack implementation, only used for demonstration.   Before I will go deeper into the principles I will simply mention how we can use the .Net Code Contract. I mention before about pre- and postcondition, is about “Require” something and to “Ensure” something. When using Code Contract, we will use a static class called “Contract” and is located in he “System.Diagnostics.Contracts” namespace. The contract must be specified at the top or our member statement block. To specify a precondition with Code Contract we uses the Contract.Requires method, and to specify a postcondition, we uses the Contract.Ensure method. Here is an example where both a pre- and postcondition are used: public object Top() { Contract.Requires(Count > 0, "Stack is empty"); Contract.Ensures(Contract.Result<object>() == this[Count]); return this[Count]; } .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; }   The contract above requires that the Count is greater than 0, if not we can’t get the item at the Top of a Stack. We also Ensures that the results (By using the Contract.Result method, we can specify a postcondition that will check if the value returned from a method is correct) of the Top query is equal to this[Count].   1. Separate Commands and Queries   When working with DbC, it’s important to separate Command and Quires. A method should either be a command that performs an Action, or returning information to the caller, not both. By asking a question the answer shouldn’t be changed. The following is an example of a Command and a Query of a Stack: public void Push(object value) public object Top() .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; }   The Push is a command and will not return anything, just add a value to the Stack, the Top is a query to get the item at the top of the stack.   2. Separate basic queries from derived queries There are two different kinds of queries,  the basic queries that doesn’t rely on another queries, and derived queries that uses a basic query. The “Separate basic queries from derived queries” principle is about about that derived queries can be specified in terms of basic queries. So this principles is more about recognizing that a query is a derived query or a basic query. It will then make is much easier to follow the other principles. The following code shows a basic query and a derived query: //Basic Queries public uint Count; //Derived Queries //Is related to Count Query public bool IsEmpty() { return Count == 0; } .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; }   We can see that IsEmpty will use the Count query, and that makes the IsEmpty a Derived query.   3. For each derived query, write a postcondition that specifies what result will be returned, in terms of one or more basic queries.   When the derived query is recognize we can follow the 3ed principle. For each derived query, we can create a postcondition that specifies what result our derived query will return in terms of one or more basic queries. Remember that DbC is about contracts between the users of the code and us writing the code. So we can’t use demand that the users will pass in a valid value, we must also ensure that we will give the users what the users wants, when the user is following our contract. The IsEmpty query of the Stack will use a Count query and that will make the IsEmpty a Derived query, so we should now write a postcondition that specified what results will be returned, in terms of using a basic query and in this case the Count query, //Basic Queries public uint Count; //Derived Queries public bool IsEmpty() { Contract.Ensures(Contract.Result<bool>() == (Count == 0)); return Count == 0; } The Contract.Ensures is used to create a postcondition. The above code will make sure that the results of the IsEmpty (by using the Contract.Result to get the result of the IsEmpty method) is correct, that will say that the IsEmpty will be either true or false based on Count is equal to 0 or not. The postcondition are using a basic query, so the IsEmpty is now following the 3ed principle. We also have another Derived Query, the Top query, it will also need a postcondition and it uses all basic queries. The Result of the Top method must be the same value as the this[] query returns. //Basic Queries public uint Count; public object this[uint index] { get { return _array[index]; } set { _array[index] = value; } } //Derived Queries //Is related to Count and this[] Query public object Top() { Contract.Ensures(Contract.Result<object>() == this[Count]); return this[Count]; } .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; }   4. For each command, write a postcondition that specifies the value of every basic query.   For each command we will create a postconditon that specifies the value of basic queries. If we look at the Stack implementation we will have three Commands, one Creation command, the Constructor, and two others commands, Push and Remove. Those commands need a postcondition and they should include basic query to follow the 4th principle. //Creation commands public Stack(uint size) { Contract.Ensures(Count == 0); Count = 0; _array = new object[size]; } //Other commands public void Push(object value) { Contract.Ensures(Count == Contract.OldValue<uint>(Count) + 1); Contract.Ensures(this[Count] == value); this[++Count] = value; } public void Remove() { Contract.Ensures(Count == Contract.OldValue<uint>(Count) - 1); this[Count] = null; Count--; } .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; }   As you can see the Create command will Ensures that Count will be 0 when the Stack is created, when a Stack is created there shouldn’t be any items in the stack. The Push command will take a value and put it into the Stack, when an item is pushed into the Stack, the Count need to be increased to know the number of items added to the Stack, and we must also make sure the item is really added to the Stack. The postconditon of the Push method will make sure the that old value of the Count (by using the Contract.OldValue we can get the value a Query has before the method is called)  plus 1 will be equal to the Count query, this is the way we can ensure that the Push will increase the Count with one. We also make sure the this[] query will now contain the item we pushed into the Stack. The Remove method must make sure the Count is decreased by one when the top item is removed from the Stack. The Commands is now following the 4th principle, where each command now have a postcondition that used the value of basic queries. Note: The principle says every basic Query, the Remove only used one Query the Count, it’s because this command can’t use the this[] query because an item is removed, so the only way to make sure an item is removed is to just use the Count query, so the Remove will still follow the principle.   5. For every query and command, decide on a suitable precondition.   We have now focused only on postcondition, now time for some preconditons. The 5th principle is about deciding a suitable preconditon for every query and command. If we starts to look at one of our basic queries (will not go through all Queries and commands here, just some of them) the this[] query, we can’t pass an index that is lower then 1 (.Net arrays and list are zero based, but not the stack in this blog post ;)) and the index can’t be lesser than the number of items in the stack. So here we will need a preconditon. public object this[uint index] { get { Contract.Requires(index >= 1); Contract.Requires(index <= Count); return _array[index]; } } .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; } Think about the Contract as an documentation about how to use the code in a correct way, so if the contract could be specified elsewhere (not part of the method body), we could simply write “return _array[index]” and there is no need to check if index is greater or lesser than Count, because that is specified in a “contract”. The implementation of Code Contract, requires that the contract is specified in the code. As a developer I would rather have this contract elsewhere (Like Spec#) or implemented in a way Eiffel uses it as part of the language. Now when we have looked at one Query, we can also look at one command, the Remove command (You can see the whole implementation of the Stack at the end of this blog post, where precondition is added to more queries and commands then what I’m going to show in this section). We can only Remove an item if the Count is greater than 0. So we can write a precondition that will require that Count must be greater than 0. public void Remove() { Contract.Requires(Count > 0); Contract.Ensures(Count == Contract.OldValue<uint>(Count) - 1); this[Count] = null; Count--; } .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; }   6. Write invariants to define unchanging properties of objects.   The last principle is about making sure the object are feeling great! This is done by using invariants. When using Code Contract we can specify invariants by adding a method with the attribute ContractInvariantMethod, the method must be private or public and can only contains calls to Contract.Invariant. To make sure the Stack feels great, the Stack must have 0 or more items, the Count can’t never be a negative value to make sure each command and queries can be used of the Stack. Here is our invariant for the Stack object: [ContractInvariantMethod] private void ObjectInvariant() { Contract.Invariant(Count >= 0); } .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; }   Note: The ObjectInvariant method will be called every time after a Query or Commands is called. Here is the full example using Code Contract:   public class Stack { private object[] _array; //Basic Queries public uint Count; public object this[uint index] { get { Contract.Requires(index >= 1); Contract.Requires(index <= Count); return _array[index]; } set { Contract.Requires(index >= 1); Contract.Requires(index <= Count); _array[index] = value; } } //Derived Queries //Is related to Count Query public bool IsEmpty() { Contract.Ensures(Contract.Result<bool>() == (Count == 0)); return Count == 0; } //Is related to Count and this[] Query public object Top() { Contract.Requires(Count > 0, "Stack is empty"); Contract.Ensures(Contract.Result<object>() == this[Count]); return this[Count]; } //Creation commands public Stack(uint size) { Contract.Requires(size > 0); Contract.Ensures(Count == 0); Count = 0; _array = new object[size]; } //Other commands public void Push(object value) { Contract.Requires(value != null); Contract.Ensures(Count == Contract.OldValue<uint>(Count) + 1); Contract.Ensures(this[Count] == value); this[++Count] = value; } public void Remove() { Contract.Requires(Count > 0); Contract.Ensures(Count == Contract.OldValue<uint>(Count) - 1); this[Count] = null; Count--; } [ContractInvariantMethod] private void ObjectInvariant() { Contract.Invariant(Count >= 0); } } .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; } Summary By using Design By Contract we can make sure the users are using our code in a correct way, and we must also make sure the users will get the expected results when they uses our code. This can be done by specifying contracts. To make it easy to use Design By Contract, some principles may be good to follow like the separation of commands an queries. With .Net 4.0 we can use the Code Contract feature to specify contracts.

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