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• Closure vs Anonymous function (difference?)

  - by Maxim Gershkovich

  Hi, I have been unable to find a definition that clearly explains the differences between a closure and an anonymous function. Most references I have seen clearly specify that they are distinct "things" yet I can't seem to get my head around why. Could someone please simplify it for me? What are the specific differences between these two language features? Which one is more appropriate in what scenarios?

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• When .VHD get updated?

  - by Recce2jp

  Hello, I have a VM with a .vhd file however, when I connet to the VM (by RDP) and make some changes (create a directory and add a small file with notepad) the .VHD file stays unchanged. The 'Date modified' of the .VHD stays at the starttime of the VM. 1/How can we force an update of het VM while the VM is running? 2/Where aree the temporary changes kept? Hypervisor: Microsoft Hyper-V server 2008R2 (free download version) regards, Maxim

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• Des fonctions d'Ubuntu 13.04 seront développées à huis clos, vers une fermeture de l'OS ?

  Des fonctions d'Ubuntu 13.04 développées à huis clos Canonical adopte une nouvelle méthode de développement pour Raring Ringtail, vers une fermeture de l'OS ? À peine Canonical a publié Ubuntu 12.10, que la firme livre déjà quelques informations sur son successeur. Comme il est de coutume, l'OS est d'abord passé par la traditionnelle étape de définition de son nom de code. Ubuntu 13.04 sera baptisé « Raring Ringtail », d'après un billet de blog de Mark Shuttleworth, fondateur de Canonical. Cette mouture sera essentiellement centrée sur la mobilité. Le coeur du système d'exploitation sera nettoyé au maxim...

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• 7zip: how to extract to std output?

  - by Jason S

  I have 7z 4.65 and am trying to extract a single file to standard output. The 7z command-line help says -so is the command-line parameter to extract to standard output, but when I try this: >>> 7z e -so dist\dlogpkg.jar META-INF/MANIFEST.MF 7-Zip 4.65 Copyright (c) 1999-2009 Igor Pavlov 2009-02-03 Error: I won't write data and program's messages to same terminal how can I fix this? There doesn't seem to be a command line param to suppress the normal 7z stdout messages. (edit: the equivalent operation in "unzip" would be unzip -p dist\dlogpkg.jar META-INF/MANIFEST.MF which works fine. But I'd like to use 7z for various reasons.)

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• Iva&rsquo;s internship story

  - by anca.rosu

  Hello, my name is Iva and I am a member of the Internship program at Oracle Czech. When I joined Oracle, I initially worked as an Alliances and Channel Marketing Assistant at Oracle Czech Republic, but most recently, I have been working in the Demand Generation Team. I am a student of the Economics and Management Faculty at Czech University of Life Sciences in Prague, specializing in Marketing, Business and Administration. I have recently passed my Bachelor exams. I received the information about Oracle’s Internship opportunity from a friend. I joined Oracle in September 2008 and worked as an Alliances and Channel Marketing Assistant until May 2009. Here I was responsible for the Open Market Model (OMM) and at the same time I was covering communication with Partners, Oracle Events and Team Buildings as well as creating Partner Databases and Reports. At the moment, I support our Demand Generation Team to execute Direct Marketing campaigns in Czech Republic, Slovak Republic and Hungary. In addition to this, I help with Reporting and Contact Data Management for the whole of the European Enlargement (EE) and Commonwealth of Independent States (CIS) regions. I enjoy my job and I appreciate the experience. Every day is interesting, because every day I learn something new. I am very happy that I was presented with an opportunity to work at Oracle and cooperate with friendly people in a multicultural environment. Oracle gives me the chance to develop my skills and start building my career. I am able to attend interesting training classes, improve my language skills and enjoy sporting activities, such as squash, swimming and aerobics, at the same time. If you dream of working in an international company and you would like to join a very dynamic industry, I really can recommend Oracle without a doubt, even if you have no IT background! If you have any questions related to this article feel free to contact  [email protected].  You can find our job opportunities via http://campus.oracle.com   Technorati Tags: Internship program,Oracle Czech,Economics,Management Faculty,Czech University of Life Sciences in Prague,Demand Generation Team

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• Compressing and copying large files on Windows Server?

  - by Aaron

  I've been having a hard time copying large database backups from the database server to a test box at another site. I'm open to any ideas that would help me get this database moved without having to resort to a USB hard drive and the mail. The database server is running Windows Server 2003 R2 Enterprise, 16 GB of RAM and two quad-core 3.0 GHz Xeon X5450s. Files are SQL Server 2005 backup files between 100 GB and 250 GB. The pipe is not the fastest and SQL Server backup files typically compress down to 10-40% of the original, so it made sense to me to compress the files first. I've tried a number of methods, including: gzip 1.2.4 (UnxUtils) and 1.3.12 (GnuWin) bzip2 1.0.1 (UnxUtils) and 1.0.5 (Cygwin) WinRAR 3.90 7-Zip 4.65 (7za.exe) I've attempted to use WinRAR and 7-Zip options for splitting into multiple segments. 7za.exe has worked well for me for database backups on another server, which has ~50 GB backups. I've also tried splitting the .BAK file first with various utilities and compressing the resulting segments. No joy with that approach either- no matter the tool I've tried, it ends up butting against the size of the file. Especially frustrating is that I've transferred files of similar size on Unix boxes without problems using rsync+ssh. Installing an SSH server is not an option for the situation I'm in, unfortunately. For example, this is how 7-Zip dies: H:\dbatmp>7za.exe a -t7z -v250m -mx3 h:\dbatmp\zip\db-20100419_1228.7z h:\dbatmp\db-20100419_1228.bak 7-Zip (A) 4.65 Copyright (c) 1999-2009 Igor Pavlov 2009-02-03 Scanning Creating archive h:\dbatmp\zip\db-20100419_1228.7z Compressing db-20100419_1228.bak System error: Unspecified error

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• Compressing and copying large files on Windows Server?

  - by Aaron

  I've been having a hard time copying large database backups from the database server to a test box at another site. I'm open to any ideas that would help me get this database moved without having to resort to a USB hard drive and the mail. The database server is running Windows Server 2003 R2 Enterprise, 16 GB of RAM and two quad-core 3.0 GHz Xeon X5450s. Files are SQL Server 2005 backup files between 100 GB and 250 GB. The pipe is not the fastest and SQL Server backup files typically compress down to 10-40% of the original, so it made sense to me to compress the files first. I've tried a number of methods, including: gzip 1.2.4 (UnxUtils) and 1.3.12 (GnuWin) bzip2 1.0.1 (UnxUtils) and 1.0.5 (Cygwin) WinRAR 3.90 7-Zip 4.65 (7za.exe) I've attempted to use WinRAR and 7-Zip options for splitting into multiple segments. 7za.exe has worked well for me for database backups on another server, which has ~50 GB backups. I've also tried splitting the .BAK file first with various utilities and compressing the resulting segments. No joy with that approach either- no matter the tool I've tried, it ends up butting against the size of the file. Especially frustrating is that I've transferred files of similar size on Unix boxes without problems using rsync+ssh. Installing an SSH server is not an option for the situation I'm in, unfortunately. For example, this is how 7-Zip dies: H:\dbatmp>7za.exe a -t7z -v250m -mx3 h:\dbatmp\zip\db-20100419_1228.7z h:\dbatmp\db-20100419_1228.bak 7-Zip (A) 4.65 Copyright (c) 1999-2009 Igor Pavlov 2009-02-03 Scanning Creating archive h:\dbatmp\zip\db-20100419_1228.7z Compressing db-20100419_1228.bak System error: Unspecified error

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• Can I split one ethernet line coming out of my wall into multiple separate lines?

  - by Burteçin 'Turk' Sapta

  Hi all and thanks in advance. I'll start with some background. I live in an apartment which provides internet service included in the rent. They use company called pavlov for the internet http://pavlovmedia.net/ wireless seems to be working fine but wired connection is at least %30 faster. Ethernet, cat5 outlet is built in the wall, and there is only 1 outlet in each room. I would like to take this 1 outlet coming out the wall and multiply it into 4 wires, for desktop, playstation, tv and laptop without loosing any internet bandwidth. i have absolutely no idea weather this line is coming from a switch or a router but i have been researching Ethernet splitter, routers, switches, hubs and haven't found a solid answer. what is the best solution for me? thank you once again! EDIT: ok this picture cleared few things http://www.home-network-help.com/images/home-network-expanded.jpg so seems that an ethernet switch is to ethernet as a USB hub is to USB. what is really 10/100Mbps Network Switch and what is the cap?

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• First TDD, Simple 2-tier C# Project - what do I unit test?

  - by Joel

  This is probably a stupid question but my googling isn't finding a satisfactory answer. I'm starting a small project in C#, with just a business layer and a data access layer - strangely, the UI will come later, and I have very little (read:no) concept / control over what it will look like. I would like to try TDD for this project. I'm using Visual Studio 2008 (soon to be 2010), I have ReSharper 5, and nUnit. Again, I want to do Test-Driven Development, but not necessarily the entire XP system. My question is - when and where do I write the first unit test? Do I only test logic before I write it, or do I test everything? It seems counter-productive to test things that have no reason to fail (auto-properties, empty constructors)...but it seems like the "No new code without a failing test" maxim requires this. Links or references are fine (but preferably to online resources, not books - I would like to get started ASAP). Thanks in advance for any guidance!

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• Image Drawing on UIView

  - by user1180261

  I'm trying to create an application where I can draw a lot of pictures at a specific point (determined for each image) on one view. I have a coordinates where I need draw a picture, width and height of it For example: I have 2 billion jpeg's images. for each images I have a specific origin point and size. In 1 second I need draw on view 20-50 images in specific point. I have already tryid solve that in the next way: UIGraphicsBeginImageContextWithOptions(self.previewScreen.bounds.size, YES, 0); [self.previewScreen.image drawAtPoint:CGPointMake(0, 0)]; [image drawAtPoint:CGPointMake(nRect.left, nRect.top)]; UIImage *imagew = UIGraphicsGetImageFromCurrentImageContext(); UIGraphicsEndImageContext(); [self.previewScreen setImage:imagew]; but in this solution I have a very big latency with displaying images and big CPU usage WBR Maxim Tartachnik

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• how to make "tweet" button active in Twitter Anywhere TweetBox

  - by user322293

  I added a Twitter Anywhere TweetBox to my blog maxim.tumblr.com like this: var idvar = "tweetbox"; twttr.anywhere(function (T) { T("#tweetbox").tweetBox({ label: "Tweet me:", height: 100, width: 210, defaultContent: "@maxgrinev Hello!", onTweet: function (tweet, htmlTweet) { document.getElementById(idvar).setAttribute("style", "display: none;"); } }); }); Everythink works fine except the tweet button is not active by default. You have to edit the box to make it active (look here: http://maxgrinev.tumblr.com/). How can I make it active by default?

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• Not able to use 7-Zip to compress stdin and output with stdout?

  - by acidzombie24

  I get the error "Not implemented". I want to compress a file using 7-Zip via stdin then take the data via stdout and do more conversions with my application. In the man page it shows this example: % echo foo | 7z a dummy -tgzip -si -so /dev/null I am using Windows and C#. Results: 7-Zip 4.65 Copyright (c) 1999-2009 Igor Pavlov 2009-02-03 Creating archive StdOut System error: Not implemented Code: public static byte[] a7zipBuf(byte[] b) { string line; var p = new Process(); line = string.Format("a dummy -t7z -si -so "); p.StartInfo.Arguments = line; p.StartInfo.FileName = @"C:\Program Files\7-Zip\7z.exe"; p.StartInfo.WindowStyle = ProcessWindowStyle.Hidden; p.StartInfo.CreateNoWindow = true; p.StartInfo.UseShellExecute = false; p.StartInfo.RedirectStandardOutput = true; p.StartInfo.RedirectStandardError = true; p.StartInfo.RedirectStandardInput = true; p.Start(); p.StandardInput.BaseStream.Write(b, 0, b.Length); p.StandardInput.Close(); Console.Write(p.StandardError.ReadToEnd()); //Console.Write(p.StandardOutput.ReadToEnd()); return p.StandardOutput.BaseStream.ReadFully(); } Is there another simple way to read the file into memory? Right now I can 1) write to a temporary file and read (easy and can copy/paste some code) 2) use a file pipe (medium? I have never done it) 3) Something else.

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• Why you shouldn't add methods to interfaces in APIs

  - by Simon Cooper

  It is an oft-repeated maxim that you shouldn't add methods to a publically-released interface in an API. Recently, I was hit hard when this wasn't followed. As part of the work on ApplicationMetrics, I've been implementing auto-reporting of MVC action methods; whenever an action was called on a controller, ApplicationMetrics would automatically report it without the developer needing to add manual ReportEvent calls. Fortunately, MVC provides easy hook when a controller is created, letting me log when it happens - the IControllerFactory interface. Now, the dll we provide to instrument an MVC webapp has to be compiled against .NET 3.5 and MVC 1, as the lowest common denominator. This MVC 1 dll will still work when used in an MVC 2, 3 or 4 webapp because all MVC 2+ webapps have a binding redirect redirecting all references to previous versions of System.Web.Mvc to the correct version, and type forwards taking care of any moved types in the new assemblies. Or at least, it should. IControllerFactory In MVC 1 and 2, IControllerFactory was defined as follows: public interface IControllerFactory { IController CreateController(RequestContext requestContext, string controllerName); void ReleaseController(IController controller); } So, to implement the logging controller factory, we simply wrap the existing controller factory: internal sealed class LoggingControllerFactory : IControllerFactory { private readonly IControllerFactory m_CurrentController; public LoggingControllerFactory(IControllerFactory currentController) { m_CurrentController = currentController; } public IController CreateController( RequestContext requestContext, string controllerName) { // log the controller being used FeatureSessionData.ReportEvent("Controller used:", controllerName); return m_CurrentController.CreateController(requestContext, controllerName); } public void ReleaseController(IController controller) { m_CurrentController.ReleaseController(controller); } } Easy. This works as expected in MVC 1 and 2. However, in MVC 3 this type was throwing a TypeLoadException, saying a method wasn't implemented. It turns out that, in MVC 3, the definition of IControllerFactory was changed to this: public interface IControllerFactory { IController CreateController(RequestContext requestContext, string controllerName); SessionStateBehavior GetControllerSessionBehavior( RequestContext requestContext, string controllerName); void ReleaseController(IController controller); } There's a new method in the interface. So when our MVC 1 dll was redirected to reference System.Web.Mvc v3, LoggingControllerFactory tried to implement version 3 of IControllerFactory, was missing the GetControllerSessionBehaviour method, and so couldn't be loaded by the CLR. Implementing the new method Fortunately, there was a workaround. Because interface methods are normally implemented implicitly in the CLR, if we simply declare a virtual method matching the signature of the new method in MVC 3, then it will be ignored in MVC 1 and 2 and implement the extra method in MVC 3: internal sealed class LoggingControllerFactory : IControllerFactory { ... public virtual SessionStateBehaviour GetControllerSessionBehaviour( RequestContext requestContext, string controllerName) {} ... } However, this also has problems - the SessionStateBehaviour type only exists in .NET 4, and we're limited to .NET 3.5 by support for MVC 1 and 2. This means that the only solutions to support all MVC versions are: Construct the LoggingControllerFactory type at runtime using reflection Produce entirely separate dlls for MVC 1&2 and MVC 3. Ugh. And all because of that blasted extra method! Another solution? Fortunately, in this case, there is a third option - System.Web.Mvc also provides a DefaultControllerFactory type that can provide the implementation of GetControllerSessionBehaviour for us in MVC 3, while still allowing us to override CreateController and ReleaseController. However, this does mean that LoggingControllerFactory won't be able to wrap any calls to GetControllerSessionBehaviour. This is an acceptable bug, given the other options, as very few developers will be overriding GetControllerSessionBehaviour in their own custom controller factory. So, if you're providing an interface as part of an API, then please please please don't add methods to it. Especially if you don't provide a 'default' implementing type. Any code compiled against the previous version that can't be updated will have some very tough decisions to make to support both versions.

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• Ubuntu server or Debian server (to run C++ apps developed on Ubuntu)

  - by skyeagle

  I have written a number of C++ server side daemons for my website, using my Ubuntu 9.10 dev machine. I am now about to venture out to look for a hosting provider etc. This is my problem: I have read on many posts (admittedly old posts) that Debian server is much more robust than Ubuntu server - is this till the case?. In particular, I am constantly "raising elephants" with my Ubuntu 9.10 - this is "ok" for home use, but for a website server, I would not be so forgiving. Also, there seems to be a new "patch" every few weeks - which I would not like on a server (I want to leave the server well alone, and let it get on with its business of serving pages). So in this instance Debian looks a more attractive proposition. I am worried that the C++ apps I have developed on Ubuntu may not be binary compatable with Debian (or I may need to install additional libraries/packages etc to get things to work), and I have zero experience with Debian. Additionally, I dont want to be grappling with the learning curve associated with a new OS, whilst trying to launch a new web site (I am assuming Debian UI is quite different from Ubuntu). In this case, the maxim "the devil you know is better than the one you dont", seems appropriate - and I find Ubuntu a more attractive proposition (atleast I know my apps will run without any probs etc). Can anyone provide some rational advice (based on actual experience), to help me decide which route to take - given the two (conflicting) trains of thought outlined above?

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• C#: System.Collections.Concurrent.ConcurrentQueue vs. Queue

  - by James Michael Hare

  I love new toys, so of course when .NET 4.0 came out I felt like the proverbial kid in the candy store!  Now, some people get all excited about the IDE and it’s new features or about changes to WPF and Silver Light and yes, those are all very fine and grand.  But me, I get all excited about things that tend to affect my life on the backside of development.  That’s why when I heard there were going to be concurrent container implementations in the latest version of .NET I was salivating like Pavlov’s dog at the dinner bell. They seem so simple, really, that one could easily overlook them.  Essentially they are implementations of containers (many that mirror the generic collections, others are new) that have either been optimized with very efficient, limited, or no locking but are still completely thread safe -- and I just had to see what kind of an improvement that would translate into. Since part of my job as a solutions architect here where I work is to help design, develop, and maintain the systems that process tons of requests each second, the thought of extremely efficient thread-safe containers was extremely appealing.  Of course, they also rolled out a whole parallel development framework which I won’t get into in this post but will cover bits and pieces of as time goes by. This time, I was mainly curious as to how well these new concurrent containers would perform compared to areas in our code where we manually synchronize them using lock or some other mechanism.  So I set about to run a processing test with a series of producers and consumers that would be either processing a traditional System.Collections.Generic.Queue or a System.Collection.Concurrent.ConcurrentQueue. Now, I wanted to keep the code as common as possible to make sure that the only variance was the container, so I created a test Producer and a test Consumer.  The test Producer takes an Action<string> delegate which is responsible for taking a string and placing it on whichever queue we’re testing in a thread-safe manner: 1: internal class Producer 2: { 3: public int Iterations { get; set; } 4: public Action<string> ProduceDelegate { get; set; } 5: 6: public void Produce() 7: { 8: for (int i = 0; i < Iterations; i++) 9: { 10: ProduceDelegate(“Hello”); 11: } 12: } 13: } Then likewise, I created a consumer that took a Func<string> that would read from whichever queue we’re testing and return either the string if data exists or null if not.  Then, if the item doesn’t exist, it will do a 10 ms wait before testing again.  Once all the producers are done and join the main thread, a flag will be set in each of the consumers to tell them once the queue is empty they can shut down since no other data is coming: 1: internal class Consumer 2: { 3: public Func<string> ConsumeDelegate { get; set; } 4: public bool HaltWhenEmpty { get; set; } 5: 6: public void Consume() 7: { 8: bool processing = true; 9: 10: while (processing) 11: { 12: string result = ConsumeDelegate(); 13: 14: if(result == null) 15: { 16: if (HaltWhenEmpty) 17: { 18: processing = false; 19: } 20: else 21: { 22: Thread.Sleep(TimeSpan.FromMilliseconds(10)); 23: } 24: } 25: else 26: { 27: DoWork(); // do something non-trivial so consumers lag behind a bit 28: } 29: } 30: } 31: } Okay, now that we’ve done that, we can launch threads of varying numbers using lambdas for each different method of production/consumption.  First let's look at the lambdas for a typical System.Collections.Generics.Queue with locking: 1: // lambda for putting to typical Queue with locking... 2: var productionDelegate = s => 3: { 4: lock (_mutex) 5: { 6: _mutexQueue.Enqueue(s); 7: } 8: }; 9:  10: // and lambda for typical getting from Queue with locking... 11: var consumptionDelegate = () => 12: { 13: lock (_mutex) 14: { 15: if (_mutexQueue.Count > 0) 16: { 17: return _mutexQueue.Dequeue(); 18: } 19: } 20: return null; 21: }; Nothing new or interesting here.  Just typical locks on an internal object instance.  Now let's look at using a ConcurrentQueue from the System.Collections.Concurrent library: 1: // lambda for putting to a ConcurrentQueue, notice it needs no locking! 2: var productionDelegate = s => 3: { 4: _concurrentQueue.Enqueue(s); 5: }; 6:  7: // lambda for getting from a ConcurrentQueue, once again, no locking required. 8: var consumptionDelegate = () => 9: { 10: string s; 11: return _concurrentQueue.TryDequeue(out s) ? s : null; 12: }; So I pass each of these lambdas and the number of producer and consumers threads to launch and take a look at the timing results.  Basically I’m timing from the time all threads start and begin producing/consuming to the time that all threads rejoin.  I won't bore you with the test code, basically it just launches code that creates the producers and consumers and launches them in their own threads, then waits for them all to rejoin.  The following are the timings from the start of all threads to the Join() on all threads completing.  The producers create 10,000,000 items evenly between themselves and then when all producers are done they trigger the consumers to stop once the queue is empty. These are the results in milliseconds from the ordinary Queue with locking: 1: Consumers Producers 1 2 3 Time (ms) 2: ---------- ---------- ------ ------ ------ --------- 3: 1 1 4284 5153 4226 4554.33 4: 10 10 4044 3831 5010 4295.00 5: 100 100 5497 5378 5612 5495.67 6: 1000 1000 24234 25409 27160 25601.00 And the following are the results in milliseconds from the ConcurrentQueue with no locking necessary: 1: Consumers Producers 1 2 3 Time (ms) 2: ---------- ---------- ------ ------ ------ --------- 3: 1 1 3647 3643 3718 3669.33 4: 10 10 2311 2136 2142 2196.33 5: 100 100 2480 2416 2190 2362.00 6: 1000 1000 7289 6897 7061 7082.33 Note that even though obviously 2000 threads is quite extreme, the concurrent queue actually scales really well, whereas the traditional queue with simple locking scales much more poorly. I love the new concurrent collections, they look so much simpler without littering your code with the locking logic, and they perform much better.  All in all, a great new toy to add to your arsenal of multi-threaded processing!

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• An Alphabet of Eponymous Aphorisms, Programming Paradigms, Software Sayings, Annoying Alliteration

  - by Brian Schroer

  Malcolm Anderson blogged about “Einstein’s Razor” yesterday, which reminded me of my favorite software development “law”, the name of which I can never remember. It took much Wikipedia-ing to find it (Hofstadter’s Law – see below), but along the way I compiled the following list: Amara’s Law: We tend to overestimate the effect of a technology in the short run and underestimate the effect in the long run. Brook’s Law: Adding manpower to a late software project makes it later. Clarke’s Third Law: Any sufficiently advanced technology is indistinguishable from magic. Law of Demeter: Each unit should only talk to its friends; don't talk to strangers. Einstein’s Razor: “Make things as simple as possible, but not simpler” is the popular paraphrase, but what he actually said was “It can scarcely be denied that the supreme goal of all theory is to make the irreducible basic elements as simple and as few as possible without having to surrender the adequate representation of a single datum of experience”, an overly complicated quote which is an obvious violation of Einstein’s Razor. (You can tell by looking at a picture of Einstein that the dude was hardly an expert on razors or other grooming apparati.) Finagle's Law of Dynamic Negatives: Anything that can go wrong, will—at the worst possible moment. - O'Toole's Corollary: The perversity of the Universe tends towards a maximum. Greenspun's Tenth Rule: Any sufficiently complicated C or Fortran program contains an ad hoc, informally-specified, bug-ridden, slow implementation of half of Common Lisp. (Morris’s Corollary: “…including Common Lisp”) Hofstadter's Law: It always takes longer than you expect, even when you take into account Hofstadter's Law. Issawi’s Omelet Analogy: One cannot make an omelet without breaking eggs - but it is amazing how many eggs one can break without making a decent omelet. Jackson’s Rules of Optimization: Rule 1: Don't do it. Rule 2 (for experts only): Don't do it yet. Kaner’s Caveat: A program which perfectly meets a lousy specification is a lousy program. Liskov Substitution Principle (paraphrased): Functions that use pointers or references to base classes must be able to use objects of derived classes without knowing it Mason’s Maxim: Since human beings themselves are not fully debugged yet, there will be bugs in your code no matter what you do. Nils-Peter Nelson’s Nil I/O Rule: The fastest I/O is no I/O.    Occam's Razor: The simplest explanation is usually the correct one. Parkinson’s Law: Work expands so as to fill the time available for its completion. Quentin Tarantino’s Pie Principle: “…you want to go home have a drink and go and eat pie and talk about it.” (OK, he was talking about movies, not software, but I couldn’t find a “Q” quote about software. And wouldn’t it be cool to write a program so great that the users want to eat pie and talk about it?) Raymond’s Rule: Computer science education cannot make anybody an expert programmer any more than studying brushes and pigment can make somebody an expert painter.  Sowa's Law of Standards: Whenever a major organization develops a new system as an official standard for X, the primary result is the widespread adoption of some simpler system as a de facto standard for X. Turing’s Tenet: We shall do a much better programming job, provided we approach the task with a full appreciation of its tremendous difficulty, provided that we respect the intrinsic limitations of the human mind and approach the task as very humble programmers.  Udi Dahan’s Race Condition Rule: If you think you have a race condition, you don’t understand the domain well enough. These rules didn’t exist in the age of paper, there is no reason for them to exist in the age of computers. When you have race conditions, go back to the business and find out actual rules. Van Vleck’s Kvetching: We know about as much about software quality problems as they knew about the Black Plague in the 1600s. We've seen the victims' agonies and helped burn the corpses. We don't know what causes it; we don't really know if there is only one disease. We just suffer -- and keep pouring our sewage into our water supply. Wheeler’s Law: All problems in computer science can be solved by another level of indirection... Except for the problem of too many layers of indirection. Wheeler also said “Compatibility means deliberately repeating other people's mistakes.”. The Wrong Road Rule of Mr. X (anonymous): No matter how far down the wrong road you've gone, turn back. Yourdon’s Rule of Two Feet: If you think your management doesn't know what it's doing or that your organisation turns out low-quality software crap that embarrasses you, then leave. Zawinski's Law of Software Envelopment: Every program attempts to expand until it can read mail. Zawinski is also responsible for “Some people, when confronted with a problem, think 'I know, I'll use regular expressions.' Now they have two problems.” He once commented about X Windows widget toolkits: “Using these toolkits is like trying to make a bookshelf out of mashed potatoes.”

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• Subterranean IL: Generics and array covariance

  - by Simon Cooper

  Arrays in .NET are curious beasts. They are the only built-in collection types in the CLR, and SZ-arrays (single dimension, zero-indexed) have their own commands and IL syntax. One of their stranger properties is they have a kind of built-in covariance long before generic variance was added in .NET 4. However, this causes a subtle but important problem with generics. First of all, we need to briefly recap on array covariance. SZ-array covariance To demonstrate, I'll tweak the classes I introduced in my previous posts: public class IncrementableClass { public int Value; public virtual void Increment(int incrementBy) { Value += incrementBy; } } public class IncrementableClassx2 : IncrementableClass { public override void Increment(int incrementBy) { base.Increment(incrementBy); base.Increment(incrementBy); } } In the CLR, SZ-arrays of reference types are implicitly convertible to arrays of the element's supertypes, all the way up to object (note that this does not apply to value types). That is, an instance of IncrementableClassx2[] can be used wherever a IncrementableClass[] or object[] is required. When an SZ-array could be used in this fashion, a run-time type check is performed when you try to insert an object into the array to make sure you're not trying to insert an instance of IncrementableClass into an IncrementableClassx2[]. This check means that the following code will compile fine but will fail at run-time: IncrementableClass[] array = new IncrementableClassx2[1]; array[0] = new IncrementableClass(); // throws ArrayTypeMismatchException These checks are enforced by the various stelem* and ldelem* il instructions in such a way as to ensure you can't insert a IncrementableClass into a IncrementableClassx2[]. For the rest of this post, however, I'm going to concentrate on the ldelema instruction. ldelema This instruction pops the array index (int32) and array reference (O) off the stack, and pushes a pointer (&) to the corresponding array element. However, unlike the ldelem instruction, the instruction's type argument must match the run-time array type exactly. This is because, once you've got a managed pointer, you can use that pointer to both load and store values in that array element using the ldind* and stind* (load/store indirect) instructions. As the same pointer can be used for both input and output to the array, the type argument to ldelema must be invariant. At the time, this was a perfectly reasonable restriction, and maintained array type-safety within managed code. However, along came generics, and with it the constrained callvirt instruction. So, what happens when we combine array covariance and constrained callvirt? .method public static void CallIncrementArrayValue() { // IncrementableClassx2[] arr = new IncrementableClassx2[1] ldc.i4.1 newarr IncrementableClassx2 // arr[0] = new IncrementableClassx2(); dup newobj instance void IncrementableClassx2::.ctor() ldc.i4.0 stelem.ref // IncrementArrayValue<IncrementableClass>(arr, 0) // here, we're treating an IncrementableClassx2[] as IncrementableClass[] dup ldc.i4.0 call void IncrementArrayValue<class IncrementableClass>(!!0[],int32) // ... ret } .method public static void IncrementArrayValue<(IncrementableClass) T>( !!T[] arr, int32 index) { // arr[index].Increment(1) ldarg.0 ldarg.1 ldelema !!T ldc.i4.1 constrained. !!T callvirt instance void IIncrementable::Increment(int32) ret } And the result: Unhandled Exception: System.ArrayTypeMismatchException: Attempted to access an element as a type incompatible with the array. at IncrementArrayValue[T](T[] arr, Int32 index) at CallIncrementArrayValue() Hmm. We're instantiating the generic method as IncrementArrayValue<IncrementableClass>, but passing in an IncrementableClassx2[], hence the ldelema instruction is failing as it's expecting an IncrementableClass[]. On features and feature conflicts What we've got here is a conflict between existing behaviour (ldelema ensuring type safety on covariant arrays) and new behaviour (managed pointers to object references used for every constrained callvirt on generic type instances). And, although this is an edge case, there is no general workaround. The generic method could be hidden behind several layers of assemblies, wrappers and interfaces that make it a requirement to use array covariance when calling the generic method. Furthermore, this will only fail at runtime, whereas compile-time safety is what generics were designed for! The solution is the readonly. prefix instruction. This modifies the ldelema instruction to ignore the exact type check for arrays of reference types, and so it lets us take the address of array elements using a covariant type to the actual run-time type of the array: .method public static void IncrementArrayValue<(IncrementableClass) T>( !!T[] arr, int32 index) { // arr[index].Increment(1) ldarg.0 ldarg.1 readonly. ldelema !!T ldc.i4.1 constrained. !!T callvirt instance void IIncrementable::Increment(int32) ret } But what about type safety? In return for ignoring the type check, the resulting controlled mutability pointer can only be used in the following situations: As the object parameter to ldfld, ldflda, stfld, call and constrained callvirt instructions As the pointer parameter to ldobj or ldind* As the source parameter to cpobj In other words, the only operations allowed are those that read from the pointer; stind* and similar that alter the pointer itself are banned. This ensures that the array element we're pointing to won't be changed to anything untoward, and so type safety within the array is maintained. This is a typical example of the maxim that whenever you add a feature to a program, you have to consider how that feature interacts with every single one of the existing features. Although an edge case, the readonly. prefix instruction ensures that generics and array covariance work together and that compile-time type safety is maintained. Tune in next time for a look at the .ctor generic type constraint, and what it means.

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