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  • GPL license and eiffel studio

    - by Michael Jenneson
    On https://www2.eiffel.com/download/download_info.aspx?id=eiffelstudio&info=false&mirrors=eiffelstudio you can download the IDE "eiffelstudio". They have GPL as their license but they also specify that "The GPL version is for the purpose of developing open-source software only! If you want to evaluate EiffelStudio for commercial software development, please download our Enterprise Evaluation Edition." However as far as i know this is directly against the gpl. Can they really specify such restrictions while adhering to the GPL?

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  • L'école d'ingénieurs ESME Sudria va ouvrir un nouveau campus à Montparnasse en janvier, avec vue sur la Tour Eiffel

    L'école d'ingénieurs ESME Sudria va inaugurer son nouveau campus à Montparnasse Avec vue sur la Tour Eiffel, en janvier L'ESME Sudria, l'école d'ingénieurs pluridisciplinaire centenaire (membre du groupe IONIS) va inaugurer un nouveau campus Paris-Montparnasse le 16 janvier prochain. Ce campus intègrera un immeuble de 2000 m² au coeur - comme son nom l'indique - du quartier de Montparnasse. Autrement dit, juste à côté des centres de recherche de l'Institut Pasteur. Ce nouveau site, comme ses grands frères de Lille et de Lyon, sera lié au campus de l'ESME Sudria Paris-Ivry, où restera la formation du cycle Master. Situé au 40-42 rue du Docteur Roux, ce nouveau ca...

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  • How are design-by-contract and property-based testing (QuickCheck) related?

    - by Todd Owen
    Is their only similarity the fact that they are not xUnit (or more precisely, not based on enumerating specific test cases), or is it deeper than that? Property-based testing (using QuickCheck, ScalaCheck, etc) seem well-suited to a functional programming style where side-effects are avoided. On the other hand, Design by Contract (as implemented in Eiffel) is more suited to OOP languages: you can express post-conditions about the effects of methods, not just their return values. But both of them involve testing assertions that are true in general (rather than assertions that should be true for a specific test case). And both can be tested using randomly generated inputs (with QuickCheck this is the only way, whereas with Eiffel I believe it is an optional feature of the AutoTest tool). Is there an umbrella term to encompass both approaches? Or am I imagining a relationship that doesn't really exist.

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  • Geocoding non-addresses: Geopy

    - by Phil Donovan
    Using geopy to geocode alcohol outlets in NZ. The problem I have is that some places do not have street addresses but are places in Google Maps. For example, plugging: Furneaux Lodge, Endeavour Inlet, Queen Charlotte Sound, Marlborough 7250 into Google Maps via the browser GUI gives me However, using that in Geopy I get a GQueryError saying this geographic location does not exist. Here is the code for geocoding: def GeoCode(address): g=geocoders.Google(domain="maps.google.co.nz") geoloc = g.geocode(address, exactly_one=False) place, (lat, lng) = geoloc[0] GeoOut = [] GeoOut.extend([place, lat, lng]) return GeoOut GeoCode("Furneaux Lodge, Endeavour Inlet, Queen Charlotte Sound, Marlboroguh 7250") Meanwhile, I notice that "Eiffel Tower" works fine. Is there away to solve this and can someone explain the difference between The Eiffel Tower and Furneaux Lodge within Google 'locations'?

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  • Explanation of the definition of interface inheritance as described in GoF book

    - by Geek
    I am reading the first chapter of the Gof book. Section 1.6 discusses about class vs interface inheritance: Class versus Interface Inheritance It's important to understand the difference between an object's class and its type. An object's class defines how the object is implemented.The class defines the object's internal state and the implementation of its operations.In contrast,an object's type only refers to its interface--the set of requests on which it can respond. An object can have many types, and objects of different classes can have the same type. Of course, there's a close relationship between class and type. Because a class defines the operations an object can perform, it also defines the object's type . When we say that an object is an instance of a class, we imply that the object supports the interface defined by the class. Languages like c++ and Eiffel use classes to specify both an object's type and its implementation. Smalltalk programs do not declare the types of variables; consequently,the compiler does not check that the types of objects assigned to a variable are subtypes of the variable's type. Sending a message requires checking that the class of the receiver implements the message, but it doesn't require checking that the receiver is an instance of a particular class. It's also important to understand the difference between class inheritance and interface inheritance (or subtyping). Class inheritance defines an object's implementation in terms of another object's implementation. In short, it's a mechanism for code and representation sharing. In contrast,interface inheritance(or subtyping) describes when an object can be used in place of another. I am familiar with the Java and JavaScript programming language and not really familiar with either C++ or Smalltalk or Eiffel as mentioned here. So I am trying to map the concepts discussed here to Java's way of doing classes, inheritance and interfaces. This is how I think of of these concepts in Java: In Java a class is always a blueprint for the objects it produces and what interface(as in "set of all possible requests that the object can respond to") an object of that class possess is defined during compilation stage only because the class of the object would have implemented those interfaces. The requests that an object of that class can respond to is the set of all the methods that are in the class(including those implemented for the interfaces that this class implements). My specific questions are: Am I right in saying that Java's way is more similar to C++ as described in the third paragraph. I do not understand what is meant by interface inheritance in the last paragraph. In Java interface inheritance is one interface extending from another interface. But I think the word interface has some other overloaded meaning here. Can some one provide an example in Java of what is meant by interface inheritance here so that I understand it better?

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  • Java : Google introduit la programmation par contrat avec une extension open source qui aiderait aussi à sécuriser les programmes

    Java : Google introduit la programmation par contrat Avec une extension open source qui aiderait aussi à sécuriser les programmes Google vient de passer sous licence open source un projet destiné à introduire la programmation par contrat au langage Java. Ce paradigme de conception logiciel initié pour le langage Eiffel devrait faciliter le développement et aider la détection précoce des imperfections dans les premières phases de créations du code. Ce nouveau projet, appelé Cofoja (contraction de « Contracts For Java ») devrait également permettre de sécuriser les applications contre les exploits de type dépassement de tampon notamment, véritable plaie de sécurité po...

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  • Why does Clojure neglect the uniform access principle?

    - by Alexey
    My background is Ruby, C#, JavaScript and Java. And now I'm learning Clojure. What makes me feel uncomfortable about the later is that idiomatic Clojure seems to neglect the Uniform access principle (wiki, c2) and thus to a certain degree encapsulation as well by suggesting to use maps instead of some sort of "structures" or "classes". It feels like step back. So a couple of questions, if anyone informed: Which other design decisions/concerns it conflicted with and why it was considered less important? Did you have the same concern as well and how it end up when you switched from a language supporting UAP by default (Ruby, Eiffel, Python, C#) to Clojure?

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  • Blue people on flash videos (Youtube, Vimeo, etc...)

    - by Luis Alvarado
    After a recent update I am seeing EVERYONE in any video blue. Only the people are blue. First I was thinking it was an April Fools joke about the song Blue from Eiffel 65 but it has been 4 days already. I tested on another 2 PCs, same problem. It started about a week ago I think. I am using Flash 11.2.202.228 with Firefox 11. The problem is not there if I use Google Chrome. In Chrome the Flash Player is 11.2.31.118

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  • A good free BON diagram generator?

    - by Audel
    Hello I need to create a static BON diagram which represents a set of clusters, classes and their relationships(inheritance and client-supplier). Does anyone know a good diagram generator I can get for free? The BON diagram is similar to the C sharp's class diagram but this one is generated by Eiffel studio =/ any diagram generators recommendations are welcome ;D Thanks in advance .

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  • Debug.Assert replacement for Phone and Store apps

    - by Daniel Moth
    I don’t know about you, but all my code is, and always has been, littered with Debug.Assert statements. I think it all started way back in my (short-lived, but impactful to me) Eiffel days, when I was applying Design by Contract. Anyway, I can’t live without Debug.Assert. Imagine my dismay when I upgraded my Windows Phone 7.x app (Translator By Moth) to Windows Phone 8 and discovered that my Debug.Assert statements would not display anything on the target and would not break in the debugger any longer! Luckily, the solution was simple and in this post I share it with you – feel free to teak it to meet your needs. Steps to use Add a new code file to your project, delete all its contents, and paste in the code from MyDebug.cs Perform a global search in your solution replacing Debug.Assert with MyDebug.Assert Build solution and test Now, I do not know why this functionality was broken, but I do know that it exhibits the same broken characteristics for Windows Store apps. There is a simple workaround there to use Contract.Assert which does display a message and offers an option to break in the debugger (although it doesn’t output the message to the Output window). Because I plan on code sharing between Phone and Windows 8 projects, I prefer to have the conditional compilation centralized, so I added the Contract.Assert workaround directly in MyDebug class, so that you can use this class for both platforms – enjoy and enhance! Comments about this post by Daniel Moth welcome at the original blog.

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  • Is there an alternative to Google Code Search?

    - by blunders
    Per the Official Google Blog: Code Search, which was designed to help people search for open source code all over the web, will be shut down along with the Code Search API on January 15, 2012. Google Code Search is now gone, and since that makes it much harder to understand the features it presented, here's my attempt to render them via information I gathered from a cache of the page for the Search Options: The "In Search Box" just notes the syntax to type the command directly in the main search box instead of using the advance search interface. Package (In Search Box: "package:linux-2.6") Language (In Search Box: "lang:c++") (OPTIONS: any language, actionscript, ada, applescript, asp, assembly, autoconf, automake, awk, basic, bat, c, c#, c++, caja, cobol, coldfusion, configure, css, d, eiffel, erlang, fortran, go, haskell, inform, java, java, javascript, jsp, lex, limbo, lisp, lolcode, lua, m4, makefile, maple, mathematica, matlab, messagecatalog, modula2, modula3, objectivec, ocaml, pascal, perl, php, pod, prolog, proto, python, python, r, rebol, ruby, sas, scheme, scilab, sgml, shell, smalltalk, sml, sql, svg, tcl, tex, texinfo, troff, verilog, vhdl, vim, xslt, xul, yacc) File (In Search Box: "file:^.*.java$") Class (In Search Box: "class:HashMap") Function (In Search Box: "function:toString") License (In Search Box: "license:mozilla") (OPTIONS: null/any-license, aladdin/Aladdin-Public-License, artistic/Artistic-License, apache/Apache-License, apple/Apple-Public-Source-License, bsd/BSD-License, cpl/Common-Public-License, epl/Eclipse-Public-License, agpl/GNU-Affero-General-Public-License, gpl/GNU-General-Public-License, lgpl/GNU-Lesser-General-Public-License, disclaimer/Historical-Permission-Notice-and-Disclaimer, ibm/IBM-Public-License, lucent/Lucent-Public-License, mit/MIT-License, mozilla/Mozilla-Public-License, nasa/NASA-Open-Source-Agreement, python/Python-Software-Foundation-License, qpl/Q-Public-License, sleepycat/Sleepycat-License, zope/Zope-Public-License) Case Sensitive (In Search Box: "case:no") (OPTIONS: yes, no) Also of use in understanding the search tool would be the still live FAQs page for Google Code Search. Is there any code search engine that would fully replace Google Code Search's features?

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  • Command-Query-Separation and multithreading safe interfaces

    - by Tobias Langner
    I like the command query separation pattern (from OOSC / Eiffel - basically you either return a value or you change the state of the class - but not both). This makes reasoning about the class easier and it is easier to write exception safe classes. Now, with multi threading, I run into a major problem: the separation of the query and the command basically invalidates the result from the query as anything can happen between those 2. So my question is: how do you handle command query separation in an multi-threaded environment? Clarification example: A stack with command query separation would have the following methods: push (command) pop (command - but does not return a value) top (query - returns the value) empty (query) The problem here is - I can get empty as status, but then I can not rely on top really retrieving an element since between the call of empty and the call of top, the stack might have been emptied. Same goes for pop & top. If I get an item using top, I can not be sure that the item that I pop is the same. This can be solved using external locks - but that's not exactly what I call threadsafe design.

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  • store everything only once, smarter

    - by hsmit
    In the digital world a lot is stored multiple times. As a thought experiment or creative challenge I want you to think about making this more efficient and maybe reuse more. Think of the following cases: an mp3 track is downloaded multiple times, copied over various devices on website a login form is often rebuild many times, why not reuse more code? words themselves are used many times questions and answers are accidentally saved at many places in parallel images or photos often describe the same data (Eiffel tower, Golden gate, Taj Mahal) etc etc Are you aware of solutions? Or are you thinking about similar topics? Ideas? Blueprints? I'd love to hear from you!

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  • Windows API GUI programming

    - by genesys
    Hi! I'm working on a Project with outdated, very old looking GUI (the used GUI framework is more than 10 years old) Since the used programming language is Eiffel, there are almost no good libraries for GUI development. Although Wrappers for C libraries exist, it's not that easy to wrap something like Qt with them. The current GUI framework uses the Windows API to create windows, widgets and so on. But as stated - it's very old. Now i would like to learn more about how to use the Windows API directly to create state of the art GUI's Can someone recommend any reading material? Thanks!

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  • How to intercept deallocate callbacks of Core Foundation objects in Objective-C.

    - by Matteo
    I'm writing an Eiffel wrapper for AppKit and Foundation and I need to hijack all -dealloc methods. Thanks to the dynamic nature of Objective-C it is pretty easy to do that. But the problem is it only works with some of the Foundation or AppKit objects. There are certain objects (e.g. NSString, NSArray, NSDate, ...) that are actually CF objects so the dealloc method doesn't get called. Instead the deallocate callbacks of the allocator that allocated the CF object is called. Is there a way to intercept that?

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  • Interview question: difference between object and object-oriented languages.

    - by Bar
    My friend was asked the following question: what's the difference between object language and object-oriented language? It's a little unintelligible question. What does term «object language» correspond to? Does that mean «pure» object-oriented language, like the Wikipedia article says: Languages called "pure" OO languages, because everything in them is treated consistently as an object, from primitives such as characters and punctuation, all the way up to whole classes, prototypes, blocks, modules, etc. They were designed specifically to facilitate, even enforce, OO methods. Examples: Smalltalk, Eiffel, Ruby, JADE, VB.NET.

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  • Android: Maps - Best way to provide 'search for location' feature?

    - by r3mo
    Hello all, I've got an android app that uses a map activity and serves up content based on map location. I'm looking for a way to allow the user to search for a location by name (anything from 'New York' to 'Eiffel Tower') - e.g. have a text input field into which they could type 'Rome' - after pressing a button, the user would be brought to the coordinates of Rome on the map. What would be the best way to go about this? I've looked into the google geocoding api (http://code.google.com/apis/maps/documentation/geocoding/), but it has limitations of 2,500 geolocation requests per day - I'm presuming this is per API key? Or is it per user/source IP? 2,500 requests for one android app woudln't last long. Ideally, I would be able to search for English and foreign names of countries. Thanks in advance! r3mo

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  • Better language or checking tool?

    - by rwallace
    This is primarily aimed at programmers who use unmanaged languages like C and C++ in preference to managed languages, forgoing some forms of error checking to obtain benefits like the ability to work in extremely resource constrained systems or the last increment of performance, though I would also be interested in answers from those who use managed languages. Which of the following would be of most value? A language that would optionally compile to CLR byte code or to machine code via C, and would provide things like optional array bounds checking, more support for memory management in environments where you can't use garbage collection, and faster compile times than typical C++ projects. (Think e.g. Ada or Eiffel with Python syntax.) A tool that would take existing C code and perform static analysis to look for things like potential null pointer dereferences and array overflows. (Think e.g. an open source equivalent to Coverity.) Something else I haven't thought of. Or put another way, when you're using C family languages, is the top of your wish list more expressiveness, better error checking or something else? The reason I'm asking is that I have a design and prototype parser for #1, and an outline design for #2, and I'm wondering which would be the better use of resources to work on after my current project is up and running; but I think the answers may be useful for other tools programmers also. (As usual with questions of this nature, if the answer you would give is already there, please upvote it.)

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  • How do I use data from the main window in a sub-window?

    - by eagle
    I've just started working on a photo viewer type desktop AIR app with Flex. From the main window I can launch sub-windows, but in these sub-windows I can't seem to access the data I collected in the main window. How can I access this data? Or, how can I send this data to the sub-window on creation? It doesn't need to be dynamically linked. myMain.mxml <?xml version="1.0" encoding="utf-8"?> <s:WindowedApplication xmlns:fx="http://ns.adobe.com/mxml/2009" xmlns:s="library://ns.adobe.com/flex/spark" xmlns:mx="library://ns.adobe.com/flex/mx" width="260" height="200" title="myMain"> <fx:Declarations> </fx:Declarations> <fx:Script> <![CDATA[ public function openWin():void { new myWindow().open(); } public var myData:Array = new Array('The Eiffel Tower','Paris','John Doe'); ]]> </fx:Script> <s:Button x="10" y="10" width="240" label="open a sub-window" click="openWin();"/> </s:WindowedApplication> myWindow.mxml <?xml version="1.0" encoding="utf-8"?> <mx:Window name="myWindow" title="myWindow" xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute" width="640" height="360"> <mx:Script> <![CDATA[ ]]> </mx:Script> <mx:Label id="comment" x="10" y="10" text=""/> <mx:Label id="location" x="10" y="30" text=""/> <mx:Label id="author" x="10" y="50" text=""/> </mx:Window> I realize this might be a very easy question but I have searched the web, read and watched tutorials on random AIR subjects for a few days and couldn't find it. The risk of looking like a fool is worth it now, I want to get on with my first app!

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  • What are the Open Source alternatives to WPF/XAML?

    - by Evan Plaice
    If we've learned anything from HTML/CSS it's that, declarative languages (like XML) work best to describe User Interfaces because: It's easy to build code preprocessors that can template the code effectively. The code is in a well defined well structured (ideally) format so it's easy to parse. The technology to effectively parse or crawl an XML based source file already exists. The UIs scripted code becomes much simpler and easier to understand. It simple enough that designers are able to design the interface themselves. Programmers suck at creating UIs so it should be made easy enough for designers. I recently took a look at the meat of a WPF application (ie. the XAML) and it looks surprisingly familiar to the declarative language style used in HTML. It's apparent to me that the current state of desktop UI development is largely fractionalized, otherwise there wouldn't be so much duplicated effort in the domain of graphical user interface design (IE. GTK, XUL, Qt, Winforms, WPF, etc). There are 45 GUI platforms for Python alone It's seems reasonable to me that there should be a general purpose, open source, standardized, platform independent, markup language for designing desktop GUIs. Much like what the W3C made HTML/CSS into. WPF, or more specifically XAML seems like a pretty likely step in the right direction. Now that the 'browser wars' are over should we look forward to a future of 'desktop gui wars?' Note: This topic is relatively subjective in the attempt to be 'future-thinking.' I think that desktop GUI development in its current state sucks ((really)hard) and, even though WPF is still in it's infancy, it presents a likely solution to the problem. Update: Thanks a lot for the info, keep it comin'. Here's are the options I've gathered from the comments and answers. GladeXML Editor: Glade Interface Designer OS Platforms: All GUI Platform: GTK+ Languages: C (libglade), C++, C# (Glade#), Python, Ada, Pike, Perl, PHP, Eiffel, Ruby XRC (XML Resource) Editors: wxGlade, XRCed, wxDesigner, DialogBlocks (non-free) OS Platforms: All GUI Platform: wxWidgets Languages: C++, Python (wxPython), Perl (wxPerl), .NET (wx.NET) XML based formats that are either not free, not cross-platform, or language specific XUL Editor: Any basic text editor OS Platforms: Any OS running a browser that supports XUL GUI Platform: Gecko Engine? Languages: C++, Python, Ruby as plugin languages not base languages Note: I'm not sure if XUL deserves mentioning in this list because it's less of a desktop GUI language and more of a make-webapps-run-on-the-desktop language. Plus, it requires a browser to run. IE, it's 'DHTML for the desktop.' CookSwing Editor: Eclipse via WindowBuilder, NetBeans 5.0 (non-free) via Swing GUI Builder aka Matisse OS Platforms: All GUI Platform: Java Languages: Java only XAML (Moonlight) Editor: MonoDevelop OS Platforms: Linux and other Unix/X11 based OSes only GUI Platforms: GTK+ Languages: .NET Note: XAML is not a pure Open Source format because Microsoft controls its terms of use including the right to change the terms at any time. Moonlight can not legally be made to run on Windows or Mac. In addition, the only platform that is exempt from legal action is Novell. See this for a full description of what I mean.

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  • How to shoot yourself in the foot (DO NOT Read in the office)

    - by TATWORTH
    Originally posted on: http://geekswithblogs.net/TATWORTH/archive/2013/06/21/how-to-shoot-yourself-in-the-foot-do-not-read.aspxLet me make it absolutely clear - the following is:merely collated by your Geek from http://www.codeproject.com/Lounge.aspx?msg=3917012#xx3917012xxvery, very very funny so you read it in the presence of others at your own riskso here is the list - you have been warned!C You shoot yourself in the foot.   C++ You accidently create a dozen instances of yourself and shoot them all in the foot. Providing emergency medical assistance is impossible since you can't tell which are bitwise copies and which are just pointing at others and saying "That's me, over there."   FORTRAN You shoot yourself in each toe, iteratively, until you run out of toes, then you read in the next foot and repeat. If you run out of bullets, you continue anyway because you have no exception-handling facility.   Modula-2 After realizing that you can't actually accomplish anything in this language, you shoot yourself in the head.   COBOL USEing a COLT 45 HANDGUN, AIM gun at LEG.FOOT, THEN place ARM.HAND.FINGER on HANDGUN.TRIGGER and SQUEEZE. THEN return HANDGUN to HOLSTER. CHECK whether shoelace needs to be retied.   Lisp You shoot yourself in the appendage which holds the gun with which you shoot yourself in the appendage which holds the gun with which you shoot yourself in the appendage which holds...   BASIC Shoot yourself in the foot with a water pistol. On big systems, continue until entire lower body is waterlogged.   Forth Foot yourself in the shoot.   APL You shoot yourself in the foot; then spend all day figuring out how to do it in fewer characters.   Pascal The compiler won't let you shoot yourself in the foot.   Snobol If you succeed, shoot yourself in the left foot. If you fail, shoot yourself in the right foot.   HyperTalk Put the first bullet of the gun into foot left of leg of you. Answer the result.   Prolog You tell your program you want to be shot in the foot. The program figures out how to do it, but the syntax doesn't allow it to explain.   370 JCL You send your foot down to MIS with a 4000-page document explaining how you want it to be shot. Three years later, your foot comes back deep-fried.   FORTRAN-77 You shoot yourself in each toe, iteratively, until you run out of toes, then you read in the next foot and repeat. If you run out of bullets, you continue anyway because you still can't do exception-processing.   Modula-2 (alternative) You perform a shooting on what might be currently a foot with what might be currently a bullet shot by what might currently be a gun.   BASIC (compiled) You shoot yourself in the foot with a BB using a SCUD missile launcher.   Visual Basic You'll really only appear to have shot yourself in the foot, but you'll have so much fun doing it that you won't care.   Forth (alternative) BULLET DUP3 * GUN LOAD FOOT AIM TRIGGER PULL BANG! EMIT DEAD IF DROP ROT THEN (This takes about five bytes of memory, executes in two to ten clock cycles on any processor and can be used to replace any existing function of the language as well as in any future words). (Welcome to bottom up programming - where you, too, can perform compiler pre-processing instead of writing code)   APL (alternative) You hear a gunshot and there's a hole in your foot, but you don't remember enough linear algebra to understand what happened. or @#&^$%&%^ foot   Pascal (alternative) Same as Modula-2 except that the bullet is not the right type for the gun and your hand is blown off.   Snobol (alternative) You grab your foot with your hand, then rewrite your hand to be a bullet. The act of shooting the original foot then changes your hand/bullet into yet another foot (a left foot).   Prolog (alternative) You attempt to shoot yourself in the foot, but the bullet, failing to find its mark, backtracks to the gun, which then explodes in your face.   COMAL You attempt to shoot yourself in the foot with a water pistol, but the bore is clogged, and the pressure build-up blows apart both the pistol and your hand. or draw_pistol aim_at_foot(left) pull_trigger hop(swearing)   Scheme As Lisp, but none of the other appendages are aware of this happening.   Algol You shoot yourself in the foot with a musket. The musket is aesthetically fascinating and the wound baffles the adolescent medic in the emergency room.   Ada If you are dumb enough to actually use this language, the United States Department of Defense will kidnap you, stand you up in front of a firing squad and tell the soldiers, "Shoot at the feet." or The Department of Defense shoots you in the foot after offering you a blindfold and a last cigarette. or After correctly packaging your foot, you attempt to concurrently load the gun, pull the trigger, scream and shoot yourself in the foot. When you try, however, you discover that your foot is of the wrong type. or After correctly packing your foot, you attempt to concurrently load the gun, pull the trigger, scream, and confidently aim at your foot knowing it is safe. However the cordite in the round does an Unchecked Conversion, fires and shoots you in the foot anyway.   Eiffel   You create a GUN object, two FOOT objects and a BULLET object. The GUN passes both the FOOT objects a reference to the BULLET. The FOOT objects increment their hole counts and forget about the BULLET. A little demon then drives a garbage truck over your feet and grabs the bullet (both of it) on the way. Smalltalk You spend so much time playing with the graphics and windowing system that your boss shoots you in the foot, takes away your workstation and makes you develop in COBOL on a character terminal. or You send the message shoot to gun, with selectors bullet and myFoot. A window pops up saying Gunpowder doesNotUnderstand: spark. After several fruitless hours spent browsing the methods for Trigger, FiringPin and IdealGas, you take the easy way out and create ShotFoot, a subclass of Foot with an additional instance variable bulletHole. Object Oriented Pascal You perform a shooting on what might currently be a foot with what might currently be a bullet fired from what might currently be a gun.   PL/I You consume all available system resources, including all the offline bullets. The Data Processing & Payroll Department doubles its size, triples its budget, acquires four new mainframes and drops the original one on your foot. Postscript foot bullets 6 locate loadgun aim gun shoot showpage or It takes the bullet ten minutes to travel from the gun to your foot, by which time you're long since gone out to lunch. The text comes out great, though.   PERL You stab yourself in the foot repeatedly with an incredibly large and very heavy Swiss Army knife. or You pick up the gun and begin to load it. The gun and your foot begin to grow to huge proportions and the world around you slows down, until the gun fires. It makes a tiny hole, which you don't feel. Assembly Language You crash the OS and overwrite the root disk. The system administrator arrives and shoots you in the foot. After a moment of contemplation, the administrator shoots himself in the foot and then hops around the room rabidly shooting at everyone in sight. or You try to shoot yourself in the foot only to discover you must first reinvent the gun, the bullet, and your foot.or The bullet travels to your foot instantly, but it took you three weeks to load the round and aim the gun.   BCPL You shoot yourself somewhere in the leg -- you can't get any finer resolution than that. Concurrent Euclid You shoot yourself in somebody else's foot.   Motif You spend days writing a UIL description of your foot, the trajectory, the bullet and the intricate scrollwork on the ivory handles of the gun. When you finally get around to pulling the trigger, the gun jams.   Powerbuilder While attempting to load the gun you discover that the LoadGun system function is buggy; as a work around you tape the bullet to the outside of the gun and unsuccessfully attempt to fire it with a nail. In frustration you club your foot with the butt of the gun and explain to your client that this approximates the functionality of shooting yourself in the foot and that the next version of Powerbuilder will fix it.   Standard ML By the time you get your code to typecheck, you're using a shoot to foot yourself in the gun.   MUMPS You shoot 583149 AK-47 teflon-tipped, hollow-point, armour-piercing bullets into even-numbered toes on odd-numbered feet of everyone in the building -- with one line of code. Three weeks later you shoot yourself in the head rather than try to modify that line.   Java You locate the Gun class, but discover that the Bullet class is abstract, so you extend it and write the missing part of the implementation. Then you implement the ShootAble interface for your foot, and recompile the Foot class. The interface lets the bullet call the doDamage method on the Foot, so the Foot can damage itself in the most effective way. Now you run the program, and call the doShoot method on the instance of the Gun class. First the Gun creates an instance of Bullet, which calls the doFire method on the Gun. The Gun calls the hit(Bullet) method on the Foot, and the instance of Bullet is passed to the Foot. But this causes an IllegalHitByBullet exception to be thrown, and you die.   Unix You shoot yourself in the foot or % ls foot.c foot.h foot.o toe.c toe.o % rm * .o rm: .o: No such file or directory % ls %   370 JCL (alternative) You shoot yourself in the head just thinking about it.   DOS JCL You first find the building you're in in the phone book, then find your office number in the corporate phone book. Then you have to write this down, then describe, in cubits, your exact location, in relation to the door (right hand side thereof). Then you need to write down the location of the gun (loading it is a proprietary utility), then you load it, and the COBOL program, and run them, and, with luck, it may be run tonight.   VMS   $ MOUNT/DENSITY=.45/LABEL=BULLET/MESSAGE="BYE" BULLET::BULLET$GUN SYS$BULLET $ SET GUN/LOAD/SAFETY=OFF/SIGHT=NONE/HAND=LEFT/CHAMBER=1/ACTION=AUTOMATIC/ LOG/ALL/FULL SYS$GUN_3$DUA3:[000000]GUN.GNU $ SHOOT/LOG/AUTO SYS$GUN SYS$SYSTEM:[FOOT]FOOT.FOOT   %DCL-W-ACTIMAGE, error activating image GUN -CLI-E-IMGNAME, image file $3$DUA240:[GUN]GUN.EXE;1 -IMGACT-F-NOTNATIVE, image is not an OpenVMS Alpha AXP image or %SYS-F-FTSHT, foot shot (fifty lines of traceback omitted) sh,csh, etc You can't remember the syntax for anything, so you spend five hours reading manual pages, then your foot falls asleep. You shoot the computer and switch to C.   Apple System 7 Double click the gun icon and a window giving a selection for guns, target areas, plus balloon help with medical remedies, and assorted sound effects. Click "shoot" button and a small bomb appears with note "Error of Type 1 has occurred."   Windows 3.1 Double click the gun icon and wait. Eventually a window opens giving a selection for guns, target areas, plus balloon help with medical remedies, and assorted sound effects. Click "shoot" button and a small box appears with note "Unable to open Shoot.dll, check that path is correct."   Windows 95 Your gun is not compatible with this OS and you must buy an upgrade and install it before you can continue. Then you will be informed that you don't have enough memory.   CP/M I remember when shooting yourself in the foot with a BB gun was a big deal.   DOS You finally found the gun, but can't locate the file with the foot for the life of you.   MSDOS You shoot yourself in the foot, but can unshoot yourself with add-on software.   Access You try to point the gun at your foot, but it shoots holes in all your Borland distribution diskettes instead.   Paradox Not only can you shoot yourself in the foot, your users can too.   dBase You squeeze the trigger, but the bullet moves so slowly that by the time your foot feels the pain, you've forgotten why you shot yourself anyway. or You buy a gun. Bullets are only available from another company and are promised to work so you buy them. Then you find out that the next version of the gun is the one scheduled to actually shoot bullets.   DBase IV, V1.0 You pull the trigger, but it turns out that the gun was a poorly designed hand grenade and the whole building blows up.   SQL You cut your foot off, send it out to a service bureau and when it returns, it has a hole in it but will no longer fit the attachment at the end of your leg. or Insert into Foot Select Bullet >From Gun.Hand Where Chamber = 'LOADED' And Trigger = 'PULLED'   Clipper You grab a bullet, get ready to insert it in the gun so that you can shoot yourself in the foot and discover that the gun that the bullets fits has not yet been built, but should be arriving in the mail _REAL_SOON_NOW_. Oracle The menus for coding foot_shooting have not been implemented yet and you can't do foot shooting in SQL.   English You put your foot in your mouth, then bite it off. (For those who don't know, English is a McDonnell Douglas/PICK query language which allegedly requires 110% of system resources to run happily.) Revelation [an implementation of the PICK Operating System] You'll be able to shoot yourself in the foot just as soon as you figure out what all these bullets are for.   FlagShip Starting at the top of your head, you aim the gun at yourself repeatedly until, half an hour later, the gun is finally pointing at your foot and you pull the trigger. A new foot with a hole in it appears but you can't work out how to get rid of the old one and your gun doesn't work anymore.   FidoNet You put your foot in your mouth, then echo it internationally.   PicoSpan [a UNIX-based computer conferencing system] You can't shoot yourself in the foot because you're not a host. or (host variation) Whenever you shoot yourself in the foot, someone opens a topic in policy about it.   Internet You put your foot in your mouth, shoot it, then spam the bullet so that everybody gets shot in the foot.   troff rmtroff -ms -Hdrwp | lpr -Pwp2 & .*place bullet in footer .B .NR FT +3i .in 4 .bu Shoot! .br .sp .in -4 .br .bp NR HD -2i .*   Genetic Algorithms You create 10,000 strings describing the best way to shoot yourself in the foot. By the time the program produces the optimal solution, humans have evolved wings and the problem is moot.   CSP (Communicating Sequential Processes) You only fail to shoot everything that isn't your foot.   MS-SQL Server MS-SQL Server’s gun comes pre-loaded with an unlimited supply of Teflon coated bullets, and it only has two discernible features: the muzzle and the trigger. If that wasn't enough, MS-SQL Server also puts the gun in your hand, applies local anesthetic to the skin of your forefinger and stitches it to the gun's trigger. Meanwhile, another process has set up a spinal block to numb your lower body. It will then proceeded to surgically remove your foot, cryogenically freeze it for preservation, and attach it to the muzzle of the gun so that no matter where you aim, you will shoot your foot. In order to avoid shooting yourself in the foot, you need to unstitch your trigger finger, remove your foot from the muzzle of the gun, and have it surgically reattached. Then you probably want to get some crutches and go out to buy a book on SQL Server Performance Tuning.   Sybase Sybase's gun requires assembly, and you need to go out and purchase your own clip and bullets to load the gun. Assembly is complicated by the fact that Sybase has hidden the gun behind a big stack of reference manuals, but it hasn't told you where that stack is. While you were off finding the gun, assembling it, buying bullets, etc., Sybase was also busy surgically removing your foot and cryogenically freezing it for preservation. Instead of attaching it to the muzzle of the gun, though, it packed your foot on dry ice and sent it UPS-Ground to an unnamed hookah bar somewhere in the middle east. In order to shoot your foot, you must modify your gun with a GPS system for targeting and hire some guy named "Indy" to find the hookah bar and wire the coordinates back to you. By this time, you've probably become so daunted at the tasks stand between you and shooting your foot that you hire a guy who's read all the books on Sybase to help you shoot your foot. If you're lucky, he'll be smart enough both to find your foot and to stop you from shooting it.   Magic software You spend 1 week looking up the correct syntax for GUN. When you find it, you realise that GUN will not let you shoot in your own foot. It will allow you to shoot almost anything but your foot. You then decide to build your own gun. You can't use the standard barrel since this will only allow for standard bullets, which will not fire if the barrel is pointed at your foot. After four weeks, you have created your own custom gun. It blows up in your hand without warning, because you failed to initialise the safety catch and it doesn't know whether the initial state is "0", 0, NULL, "ZERO", 0.0, 0,0, "0.0", or "0,00". You fix the problem with your remaining hand by nesting 12 safety catches, and then decide to build the gun without safety catch. You then shoot the management and retire to a happy life where you code in languages that will allow you to shoot your foot in under 10 days.FirefoxLets you shoot yourself in as many feet as you'd like, while using multiple great addons! IEA moving target in terms of standard ammunition size and doesn't always work properly with non-Microsoft ammunition, so sometimes you shoot something other than your foot. However, it's the corporate world's standard foot-shooting apparatus. Hackers seem to enjoy rigging websites up to trigger cascading foot-shooting failures. Windows 98 About the same as Windows 95 in terms of overall bullet capacity and triggering mechanisms. Includes updated DirectShot API. A new version was released later on to support USB guns, Windows 98 SE.WPF:You get your baseball glove and a ball and you head out to your backyard, where you throw balls to your pitchback. Then your unkempt-haired-cargo-shorts-and-sandals-with-white-socks-wearing neighbor uses XAML to sculpt your arm into a gun, the ball into a bullet and the pitchback into your foot. By now, however, only the neighbor can get it to work and he's only around from 6:30 PM - 3:30 AM. LOGO: You very carefully lay out the trajectory of the bullet. Then you start the gun, which fires very slowly. You walk precisely to the point where the bullet will travel and wait, but just before it gets to you, your class time is up and one of the other kids has already used the system to hack into Sony's PS3 network. Flash: Someone has designed a beautiful-looking gun that anyone can shoot their feet with for free. It weighs six hundred pounds. All kinds of people are shooting themselves in the feet, and sending the link to everyone else so that they can too. That is, except for the criminals, who are all stealing iOS devices that the gun won't work with.APL: Its (mostly) all greek to me. Lisp: Place ((gun in ((hand sight (foot then shoot))))) (Lots of Insipid Stupid Parentheses)Apple OS/X and iOS Once a year, Steve Jobs returns from sick leave to tell millions of unwavering fans how they will be able to shoot themselves in the foot differently this year. They retweet and blog about it ad nauseam, and wait in line to be the first to experience "shoot different".Windows ME Usually fails, even at shooting you in the foot. Yo dawg, I heard you like shooting yourself in the foot. So I put a gun in your gun, so you can shoot yourself in the foot while you shoot yourself in the foot. (Okay, I'm not especially proud of this joke.) Windows 2000 Now you really do have to log in, before you are allowed to shoot yourself in the foot.Windows XPYou thought you learned your lesson: Don't use Windows ME. Then, along came this new creature, built on top of Windows NT! So you spend the next couple days installing antivirus software, patches and service packs, just so you can get that driver to install, and then proceed to shoot yourself in the foot. Windows Vista Newer! Glossier! Shootier! Windows 7 The bullets come out a lot smoother. Active Directory Each bullet now has an attached Bullet Identifier, and can be uniquely identified. Policies can be applied to dictate fragmentation, and the gun will occasionally have a confusing delay after the trigger has been pulled. PythonYou try to use import foot; foot.shoot() only to realize that's only available in 3.0, to which you can't yet upgrade from 2.7 because of all those extension libs lacking support. Solaris Shoots best when used on SPARC hardware, but still runs the trigger GUI under Java. After weeks of learning the appropriate STOP command to prevent the trigger from automatically being pressed on boot, you think you've got it under control. Then the one time you ever use dtrace, it hits a bug that fires the gun. MySQL The feature that allows you to shoot yourself in the foot has been in development for about 6 years, and they are adding it into the next version, which is coming out REAL SOON NOW, promise! But you can always check it out of source control and try it yourself (just not in any environment where data integrity is important because it will probably explode.) PostgreSQLAllows you to have a smug look on your face while you shoot yourself in the foot, because those MySQL guys STILL don't have that feature. NoSQL Barrel? Who needs a barrel? Just put the bullet on your foot, and strike it with a hammer. See? It's so much simpler and more efficient that way. You can even strike multiple bullets in one swing if you swing with a good enough arc, because hammers are easy to use. Getting them to synchronize is a little difficult, though.Eclipse There are about a dozen different packages for shooting yourself in the foot, with weird interdependencies on outdated components. Once you finally navigate the morass and get one installed, you then have something to look at while you shoot yourself in the foot with that package: You can watch the screen redraw.Outlook Makes it really easy to let everyone know you shot yourself in the foot!Shooting yourself in the foot using delegates.You really need to shoot yourself in the foot but you hate firearms (you don't want any dependency on the specifics of shooting) so you delegate it to somebody else. You don't care how it is done as long is shooting your foot. You can do it asynchronously in case you know you may faint so you are called back/slapped in the face by your shooter/friend (or background worker) when everything is done.C#You prepare the gun and the bullet, carefully modeling all of the physics of a bullet traveling through a foot. Just before you're about to pull the trigger, you stumble on System.Windows.BodyParts.Foot.ShootAt(System.Windows.Firearms.IGun gun) in the extended framework, realize you just wasted the entire afternoon, and shoot yourself in the head.PHP<?phprequire("foot_safety_check.php");?><!DOCTYPE HTML><html><head> <!--Lower!--><title>Shooting me in the foot</title></head> <body> <!--LOWER!!!--><leg> <!--OK, I made this one up...--><footer><?php echo (dungSift($_SERVER['HTTP_USER_AGENT'], "ie"))?("Your foot is safe, but you might want to wear a hard hat!"):("<div class=\"shot\">BANG!</div>"); ?></footer></leg> </body> </html>

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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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