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  • How do I use Declarations (type, inline, optimize) in Scheme?

    - by kunjaan
    How do I declare the types of the parameters in order to circumvent type checking? How do I optimize the speed to tell the compiler to run the function as fast as possible like (optimize speed (safety 0))? How do I make an inline function in Scheme? How do I use an unboxed representation of a data object? And finally are any of these important or necessary? Can I depend on my compiler to make these optimizations? thanks, kunjaan.

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  • Example of contravariance

    - by Misha
    I am thinking of the following example to illustrate why contravariance is useful. Let's consider a GUI framework with Widgets, Events, and Event Listeners. abstract class Event; class KeyEvent extends Event class MouseEvent extends Event trait EventListener[-Event] { def listen(e:Event) } Let Widgets define the following methods: def addKeyEventListener(listener:EventListener[KeyEvent]) def addMouseEventListener(listener:EventListener[MouseEvent]) These methods accept only "specific" event listeners, which is fine. However I would like to define also "kitchen-sink" listeners, which listen to all events, and pass such listeners to the "add listener" methods above. For instance, I would like to define LogEventListener to log all incoming events class LogEventListener extends EventListener[Event] { def listen(e:Event) { log(event) } } Since the trait EventListener is contravariant in Event we can pass LogEventListener to all those "add listener" methods without losing their type safety. Does it make sense ?

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  • MD5 password twice

    - by NoviceCoding
    I know MD5's safety is under question lately and this is the reason a lot of people are using salt (I dont understand this at all btw) but I was wondering if you wanted to easily implement a safe system in php can you just md5 something twice? like test 098f6bcd4621d373cade4e832627b4f6 fb469d7ef430b0baf0cab6c436e70375 So basically: $val = 'test'; $val = md5($val); $val = md5($val); Would that solve the whole rainbow security stuff? Is there an easy/noob proof way of making secure database passwords in php?

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  • Which are the best techniques to protect a 'homemade' framework from unlogged visitors?

    - by Hermet
    First of all, I would like to say that I have used the search box looking for a similar question unsuccessfully, maybe because of my poor english skills. The way I currently do this is checking in every single page that a session has been opened. If not, the user gets redirected to a 404 page, to seem like the file which has been requested doesn't exist. I really don't know if this is sure or there's a better and more safety way and I'm currently working with kind of confidential data that should never become public. Could you give me some tips? Or leave a link where I could find some? Thank you very much, and again excuse me for kicking the dictionary.

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  • Multithreading with STL container

    - by Steven
    I have an unordered map which stores a pointer of objects. I am not sure whether I am doing the correct thing to maintain the thread safety. typedef std::unordered_map<string, classA*>MAP1; MAP1 map1; pthread_mutex_lock(&mutexA) if(map1.find(id) != map1.end()) { pthread_mutex_unlock(&mutexA); //already exist, not adding items } else { classA* obj1 = new classA; map1[id] = obj1; obj1->obtainMutex(); //Should I create a mutex for each object so that I could obtain mutex when I am going to update fields for obj1? pthread_mutex_unlock(&mutexA); //release mutex for unordered_map so that other threads could access other object obj1->field1 = 1; performOperation(obj1); //takes some time obj1->releaseMutex(); //release mutex after updating obj1 }

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  • Emulating a transaction-safe SEQUENCE in MySQL

    - by Michael Pliskin
    We're using MySQL with InnoDB storage engine and transactions a lot, and we've run into a problem: we need a nice way to emulate Oracle's SEQUENCEs in MySQL. The requirements are: - concurrency support - transaction safety - max performance (meaning minimizing locks and deadlocks) We don't care if some of the values won't be used, i.e. gaps in sequence are ok. There is an easy way to archieve that by creating a separate InnoDB table with a counter, however this means it will take part in transaction and will introduce locks and waiting. I am thinking to try a MyISAM table with manual locks, any other ideas or best practices?

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  • C# type safe and developer friendly list/collection technique

    - by Agile Noob
    I am populating a "Dictionary" with the results of an sp call. The key is the field name and the value is whatever value the sp returns for the field. This is all well and good but I'd like developers to have a predefined list of keys to access this list, for safety and documentation reasons. What I'd like to do is have something like an enum as a key for the dictionary so developers can safely access the list, but still have the ability to access the dictionary with a string key value. I am hoping to have a list of string values that I can access with an enum key AND a string key. Please make sure any suggestions are simple to implement, this is not the kind of thing I'm willing to build a lot of overhead to implement.

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  • C++ Typing and OOP child classes

    - by Zack
    I'm a bit confused: If I have a base class A, and a class B which extends A, can a variable of the type A hold a value of the type B and vice versa? If yes, why? Aren't they completely different even if B is derived from A? How about type-safety? If this is possible, what things do I have to mind when taking use of this? How would this work out in terms of performance? Note: Sorry if I asked too many questions, just ignore them and just look out for those "marked" with the list decoration dot :) Also, this is not my homework. I'm a hobby programmer and have skills in scripting languages with OOP, yet I'm relatively new to OOP typing in C++.

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  • Writing .NET in dynamic language?

    - by tillda
    I'm confused by the possibility of writing .NET in dynamic languages, such as (Iron)Ruby. Particularly, I've seen code in IronRuby that used generics (...foo[String]), but I'm not aware of this feature in Ruby as it seems nonsense to me in dynamic languages. So, when I write .NET app in IronRuby, how is it with type safety and compilation? I thought that it is just as dynamic as Ruby everywhere else. I thought that if the Ruby syntax is OK all the type checking would be done at the runtime. Also, as far as I know, .NET itself is type-oriented - there are classes that heavily utilize the mentioned generics. How is this handled? And what about delegates? In dynamic languages I can have almost function-spaghetti and sometimes, its just fine (like hacking UI in javascript). Or do I have to care even about generic delegates?

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  • [.NET] What's the point of MarshalByValue Object?

    - by smwikipedia
    Hi awesome! We know that MarshalByRef allow us to create an object in a different AppDomain and use a Proxy object to access it. And the behavior of that object is in a different context of the AppDomain where it actually lives in. Well this sounds faily reseaonable in the regard of isolation and safety. But why is there still MarshalByValue? MarshalByValue just leads to an newly deserialized object which is an exact copy of the object in a different AppDomain. If we need that object, why not just create it in the current AppDomain? Why bother to first create it in a different AppDomain and then get it back by MarshaoByValue? Many thanks.

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  • form Validation white page not showing errors

    - by Jess McKenzie
    In the example below I am wanting to do all of the 'safety' checks on the $_POST variables but it seems when I click submit I get a white page why? I am wanting it to show the errors etc Form Process: /* check if the form is submitted */ if (isset($_POST['submitButton'])) { $fullName = $_POST['fullname']; if($_SERVER['REQUEST_METHOD'] == 'POST' && !empty($fullName)) { if (!ctype_alpha(str_replace(array("'", "-"), "",$fullName))) { $errorfullName .= '<span class="errorfullName">*First name should be alpha characters only.</span>'; } if (strlen($fullName) < 3 OR strlen($fullName) > 40) { $errorfullName .= '<span class="errorfullName">*First name should be within 3-40 characters long.</span>'; } } }

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  • Unsafe, super-fast cross-process memory buffer?

    - by John
    Cross-process memory buffers always have some overhead, and my understanding is this is quite high. But what if you're implementing a cross-process render-buffer, this isn't critically important in the same way as other data so are there techniques we can use to get 'raw' access to a chunk of memory from multiple processes, with no safety nets apart from it not crashing? Or do modern operating systems simply not work with unabstracted memory in a way to make this possible? I'm working in C++ but the question applies to Win XP/Vista/7, MacOSX 10.5+ (& Linux less importantly).

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  • druapal content image not showing

    - by tibin mathew
    hi, I'm doing a drupal site. i want show a facebook image in my page content in home page. i have edited content of that page. i have edited the body of that content through admin and put the text in body like this The BikeGlow™ Safety Light runs on 2-AA batteries. Light tube length is 10'. Complete unit weighs under 2 oz (without batteries). Light tube attaches to frame with included attachment strips. "It's like magic cycling protection from cars!" < img src='bikeglow_com/sites/default/files/5u84f48n.gif' alt="facebook" / but after saving image is not showing there even that alt tag is not coming. can any one help me, what will be reason for his

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  • drupal content image not showing

    - by tibin mathew
    hi, I'm doing a drupal site. i want show a facebook image in my page content in home page. i have edited content of that page. i have edited the body of that content through admin and put the text in body like this The BikeGlow™ Safety Light runs on 2-AA batteries. Light tube length is 10'. Complete unit weighs under 2 oz (without batteries). Light tube attaches to frame with included attachment strips. "It's like magic cycling protection from cars!" < img src='bikeglow_com/sites/default/files/5u84f48n.gif' alt="facebook" / but after saving image is not showing there even that alt tag is not coming. can any one help me, what will be reason for his

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  • How to generate JPA 2.0 metamodel?

    - by yamsha
    In the spirit of type safety associated with the CriteriaQuery JPA 2.0 also has an API to support Metamodel representation of entities. Is anyone aware of a fully functional implementation of this API (to generate the Metamodel... as opposed to creating the metamodel classes manually)? It would be awesome if someone also knows the steps for setting this up in Eclipse (I assume it's as simple as setting up an annotation processor, but you never know). EDIT: Just stumbled across Hibernate JPA 2 Metamodel Generator . But the issue remains since I can't find any download links for the jar

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  • Java: Is it possible to have a generic class that only takes types that can be compared?

    - by mr popo
    I wanted to do something along the lines of: public class MyClass<T implements Comparable> { .... } But I can't, since, apparently, generics only accept restrictions with subclasses, and not interfaces. It's important that I'm able to compare the types inside the class, so how should I go about doing this? Ideally I'd be able to keep the type safety of Generics and not have to convert the T's to Object as well, or just not write a lot of code overall. In other words, the simplest the better.

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  • Do fields need to be explicitly final to have a "proper" immutable object?

    - by Yishai
    You often read about immutable objects requiring final fields to be immutable in Java. Is this in fact the case, or is it simply enough to have no public mutability and not actually mutate the state? For example, if you have an immutable object built by the builder pattern, you could do it by having the builder assign the individual fields as it builds, or having the builder hold the fields itself and ultimately return the immutable object by passing the values to its (private) constructor. Having the fields final has the obvious advantage of preventing implementation errors (such as allowing code to retain a reference to the builder and "building" the object multiple times while in fact mutating an existing object), but having the Builder store its data inside the object as it is built would seem to be DRYer. So the question is: Assuming the Builder does not leak the Object early and stops itself from modifying the object once built (say by setting its reference to the object as null) is there actually anything gained (such as improved thread safety) in the "immutability" of the object if the object's fields were made final instead?

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  • strategy to allocate/free lots of small objects

    - by aaa
    hello I am toying with certain caching algorithm, which is challenging somewhat. Basically, it needs to allocate lots of small objects (double arrays, < 256 elements), with objects accessible through mapped value, map[key] = array. time to initialized array may be quite large, generally more than 10 thousand cpu cycles. By lots I mean around gigabyte in total. objects may need to be popped/pushed as needed, generally in random places, one object at a time. lifetime of an object is generally long, minutes or more, however, object may be subject to allocation/deallocation several times during duration of program. What would be good strategy to avoid memory fragmentation, while still maintaining reasonable allocate deallocate speed? I am using C++, so I can use new and malloc. Thanks. I know there a similar questions on website, http://stackoverflow.com/questions/2156745/efficiently-allocating-many-short-lived-small-objects, are somewhat different, thread safety is not immediate issue for me.

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  • Edit form not being instanciated

    - by 47
    I have two models like this: class OptionsAndFeatures(models.Model): options = models.TextField(blank=True, null=True) entertainment = models.TextField(blank=True, null=True) seats_trim = models.TextField(blank=True, null=True) convenience = models.TextField(blank=True, null=True) body_exterior = models.TextField(blank=True, null=True) lighting = models.TextField(blank=True, null=True) safety = models.TextField(blank=True, null=True) powertrain = models.TextField(blank=True, null=True) suspension_handling = models.TextField(blank=True, null=True) specs_dimensions = models.TextField(blank=True, null=True) offroad_capability = models.TextField(blank=True, null=True) class Vehicle(models.Model): ... options_and_features = models.ForeignKey(OptionsAndFeatures, blank=True, null=True) I have a model form for the OptionsAndFeaturesclass that I'm using in both the add and edit views. In the add view it works just fine. But the edit view renders the OptionsAndFeatures as blank. The code for the edit view is as follows: def edit_vehicle(request, stock_number=None): vehicle = get_object_or_404(Vehicle, stock_number=stock_number) if request.method == 'POST': # save info else: vehicle_form = VehicleForm(instance=vehicle) photos = PhotosFormSet(instance=vehicle) options = OptionsForm(instance=vehicle) #render_to_reponse What could be the problem here?

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  • scala coalesces multiple function call parameters into a Tuple -- can this be disabled?

    - by landon9720
    This is a troublesome violation of type safety in my project, so I'm looking for a way to disable it. It seems that if a function takes an AnyRef (or a java.lang.Object), you can call the function with any combination of parameters, and Scala will coalesce the parameters into a Tuple object and invoke the function. In my case the function isn't expecting a Tuple, and fails at runtime. I would expect this situation to be caught at compile time. object WhyTuple { def main(args: Array[String]): Unit = { fooIt("foo", "bar") } def fooIt(o: AnyRef) { println(o.toString) } } Output: (foo,bar)

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  • "Volkswagen Passat" Navigation

    - by joe_midi
    http://web.vw.com/all-new-passat-experience/ The navigation on this .swf is very impressive, and I was wondering how I would go about replicating it. The way each of the sections Design / Technology / Safety / Performance I want to know how each section can expand and still fit together. I've started with MOUSE_OVER events, but I'm not entirely sure how to order them so each section is the right size for every mouse over Event. This is the code I have for each "Section", I have created a movie clip with a Mask that expands on mouse over and brings the selected clip to the highest depth. this.firstButton01.addEventListener(MouseEvent.MOUSE_OVER, firstButton01OVER) function firstButton01OVER(e:Event):void { this.firstButton01.gotoAndPlay(2); this.setChildIndex(firstButton01,this.numChildren-1); }

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  • Solution to compiler warning for generic varargs

    - by TJR
    A puzzle from this blog. Similar to SO1445233. Given the following source listing, explain why the compiler is producing a warning at invocation to the list method and give a solution for removing the warning without resorting to @SuppressWarnings annotation. public class JavaLanguagePuzzle3 { public static void main(String[] args) { list("1", 2, new BigDecimal("3.5")); } private static <T> List<T> list(T... items) { return Arrays.asList(items); } } Warning: Type safety: A generic array of Object&Serializable&Comparable<?> is created for a varargs parameter

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  • On Writing Blogs

    - by Tony Davis
    Why are so many blogs about IT so difficult to read? Over at SQLServerCentral.com, we do a special subscription-only newsletter called Database Weekly. Every other week, it is my turn to look through all the blogs, news and events that might be of relevance to people working with databases. We provide the title, with the link, and a short abstract of what you can expect to read. It is a popular service with close to a million subscribers. You might think that this is a happy and fascinating task. Sometimes, yes. If a blog comes to the point quickly, and says something both interesting and original, then it has our immediate attention. If it backs up what it says with supporting material, then it is more-or-less home and dry, featured in DBW's list. If it also takes trouble over the formatting and presentation, maybe with an illustration or two and any code well-formatted, then we are agog with joy and it is marked as a must-visit destination in our blog roll. More often, however, a task that should be fun becomes a routine chore, and the effort of trawling so many badly-written blogs is enough to make any conscientious Health & Safety officer whistle through their teeth at the risk to the editor's spiritual and psychological well-being. And yet, frustratingly, most blogs could be improved very easily. There is, I believe, a simple formula for a successful blog. First, choose a single topic that is reasonably fresh and interesting. Second, get to the point quickly; explain in the first paragraph exactly what the blog is about, and then stay on topic. In writing the first paragraph, you must picture yourself as a pilot, hearing the smooth roar of the engines as your plane gracefully takes air. Too often, however, the accompanying sound is that of the engine stuttering before the plane veers off the runway into a field, and a wheel falls off. The author meanders around the topic without getting to the point, and takes frequent off-radar diversions to talk about themselves, or the weather, or which friends have recently tagged them. This might work if you're J.D Salinger, or James Joyce, but it doesn't help a technical blog. Sometimes, the writing is so convoluted that we are entirely defeated in our quest to shoehorn its meaning into a simple summary sentence. Finally, write simply, in plain English, and in a conversational way such that you can read it out loud, and sound natural. That's it! If you could also avoid any references to The Matrix then this is a bonus but is purely personal preference. Cheers, Tony.

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  • Java Spotlight Episode 138: Paul Perrone on Life Saving Embedded Java

    - by Roger Brinkley
    Interview with Paul Perrone, founder and CEO of Perrone Robotics, on using Java Embedded to test autonomous vehicle operations for the Insurance Institute for Highway Safety that will save lives. Right-click or Control-click to download this MP3 file. You can also subscribe to the Java Spotlight Podcast Feed to get the latest podcast automatically. If you use iTunes you can open iTunes and subscribe with this link: Java Spotlight Podcast in iTunes. Show Notes News JDK 8 is Feature Complete Java SE 7 Update 25 Released What should the JCP be doing? 2013 Duke's Choice Award Nominations Another Quick update to Code Signing Article on OTN Events June 24, Austin JUG, Austin, TX June 25, Virtual Developer Day - Java, EMEA, 10AM CEST Jul 16-19, Uberconf, Denver, USA Jul 22-24, JavaOne Shanghai, China Jul 29-31, JVM Summit Language, Santa Clara Sep 11-12, JavaZone, Oslo, Norway Sep 19-20, Strange Loop, St. Louis Sep 22-26 JavaOne San Francisco 2013, USA Feature Interview Paul J. Perrone is founder/CEO of Perrone Robotics. Paul architected the Java-based general-purpose robotics and automation software platform known as “MAX”. Paul has overseen MAX’s application to rapidly field self-driving robotic cars, unmanned air vehicles, factory and road-side automation applications, and a wide range of advanced robots and automaton applications. He fielded a self-driving autonomous robotic dune buggy in the historic 2005 Grand Challenge race across the Mojave desert and a self-driving autonomous car in the 2007 Urban Challenge through a city landscape. His work has been featured in numerous televised and print media including the Discovery Channel, a theatrical documentary, scientific journals, trade magazines, and international press. Since 2008, Paul has also been working as the chief software engineer, CTO, and roboticist automating rock star Neil Young’s LincVolt, a 1959 Lincoln Continental retro-fitted as a fully autonomous extended range electric vehicle. Paul has been an engineer, author of books and articles on Java, frequent speaker on Java, and entrepreneur in the robotics and software space for over 20 years. He is a member of the Java Champions program, recipient of three Duke Awards including a Gold Duke and Lifetime Achievement Award, has showcased Java-based robots at five JavaOne keynotes, and is a frequent JavaOne speaker and show floor participant. He holds a B.S.E.E. from Rutgers University and an M.S.E.E. from the University of Virginia. What’s Cool Shenandoah: A pauseless GC for OpenJDK

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  • C#/.NET Little Wonders: The ConcurrentDictionary

    - by James Michael Hare
    Once again we consider some of the lesser known classes and keywords of C#.  In this series of posts, we will discuss how the concurrent collections have been developed to help alleviate these multi-threading concerns.  Last week’s post began with a general introduction and discussed the ConcurrentStack<T> and ConcurrentQueue<T>.  Today's post discusses the ConcurrentDictionary<T> (originally I had intended to discuss ConcurrentBag this week as well, but ConcurrentDictionary had enough information to create a very full post on its own!).  Finally next week, we shall close with a discussion of the ConcurrentBag<T> and BlockingCollection<T>. For more of the "Little Wonders" posts, see the index here. Recap As you'll recall from the previous post, the original collections were object-based containers that accomplished synchronization through a Synchronized member.  While these were convenient because you didn't have to worry about writing your own synchronization logic, they were a bit too finely grained and if you needed to perform multiple operations under one lock, the automatic synchronization didn't buy much. With the advent of .NET 2.0, the original collections were succeeded by the generic collections which are fully type-safe, but eschew automatic synchronization.  This cuts both ways in that you have a lot more control as a developer over when and how fine-grained you want to synchronize, but on the other hand if you just want simple synchronization it creates more work. With .NET 4.0, we get the best of both worlds in generic collections.  A new breed of collections was born called the concurrent collections in the System.Collections.Concurrent namespace.  These amazing collections are fine-tuned to have best overall performance for situations requiring concurrent access.  They are not meant to replace the generic collections, but to simply be an alternative to creating your own locking mechanisms. Among those concurrent collections were the ConcurrentStack<T> and ConcurrentQueue<T> which provide classic LIFO and FIFO collections with a concurrent twist.  As we saw, some of the traditional methods that required calls to be made in a certain order (like checking for not IsEmpty before calling Pop()) were replaced in favor of an umbrella operation that combined both under one lock (like TryPop()). Now, let's take a look at the next in our series of concurrent collections!For some excellent information on the performance of the concurrent collections and how they perform compared to a traditional brute-force locking strategy, see this wonderful whitepaper by the Microsoft Parallel Computing Platform team here. ConcurrentDictionary – the fully thread-safe dictionary The ConcurrentDictionary<TKey,TValue> is the thread-safe counterpart to the generic Dictionary<TKey, TValue> collection.  Obviously, both are designed for quick – O(1) – lookups of data based on a key.  If you think of algorithms where you need lightning fast lookups of data and don’t care whether the data is maintained in any particular ordering or not, the unsorted dictionaries are generally the best way to go. Note: as a side note, there are sorted implementations of IDictionary, namely SortedDictionary and SortedList which are stored as an ordered tree and a ordered list respectively.  While these are not as fast as the non-sorted dictionaries – they are O(log2 n) – they are a great combination of both speed and ordering -- and still greatly outperform a linear search. Now, once again keep in mind that if all you need to do is load a collection once and then allow multi-threaded reading you do not need any locking.  Examples of this tend to be situations where you load a lookup or translation table once at program start, then keep it in memory for read-only reference.  In such cases locking is completely non-productive. However, most of the time when we need a concurrent dictionary we are interleaving both reads and updates.  This is where the ConcurrentDictionary really shines!  It achieves its thread-safety with no common lock to improve efficiency.  It actually uses a series of locks to provide concurrent updates, and has lockless reads!  This means that the ConcurrentDictionary gets even more efficient the higher the ratio of reads-to-writes you have. ConcurrentDictionary and Dictionary differences For the most part, the ConcurrentDictionary<TKey,TValue> behaves like it’s Dictionary<TKey,TValue> counterpart with a few differences.  Some notable examples of which are: Add() does not exist in the concurrent dictionary. This means you must use TryAdd(), AddOrUpdate(), or GetOrAdd().  It also means that you can’t use a collection initializer with the concurrent dictionary. TryAdd() replaced Add() to attempt atomic, safe adds. Because Add() only succeeds if the item doesn’t already exist, we need an atomic operation to check if the item exists, and if not add it while still under an atomic lock. TryUpdate() was added to attempt atomic, safe updates. If we want to update an item, we must make sure it exists first and that the original value is what we expected it to be.  If all these are true, we can update the item under one atomic step. TryRemove() was added to attempt atomic, safe removes. To safely attempt to remove a value we need to see if the key exists first, this checks for existence and removes under an atomic lock. AddOrUpdate() was added to attempt an thread-safe “upsert”. There are many times where you want to insert into a dictionary if the key doesn’t exist, or update the value if it does.  This allows you to make a thread-safe add-or-update. GetOrAdd() was added to attempt an thread-safe query/insert. Sometimes, you want to query for whether an item exists in the cache, and if it doesn’t insert a starting value for it.  This allows you to get the value if it exists and insert if not. Count, Keys, Values properties take a snapshot of the dictionary. Accessing these properties may interfere with add and update performance and should be used with caution. ToArray() returns a static snapshot of the dictionary. That is, the dictionary is locked, and then copied to an array as a O(n) operation.  GetEnumerator() is thread-safe and efficient, but allows dirty reads. Because reads require no locking, you can safely iterate over the contents of the dictionary.  The only downside is that, depending on timing, you may get dirty reads. Dirty reads during iteration The last point on GetEnumerator() bears some explanation.  Picture a scenario in which you call GetEnumerator() (or iterate using a foreach, etc.) and then, during that iteration the dictionary gets updated.  This may not sound like a big deal, but it can lead to inconsistent results if used incorrectly.  The problem is that items you already iterated over that are updated a split second after don’t show the update, but items that you iterate over that were updated a split second before do show the update.  Thus you may get a combination of items that are “stale” because you iterated before the update, and “fresh” because they were updated after GetEnumerator() but before the iteration reached them. Let’s illustrate with an example, let’s say you load up a concurrent dictionary like this: 1: // load up a dictionary. 2: var dictionary = new ConcurrentDictionary<string, int>(); 3:  4: dictionary["A"] = 1; 5: dictionary["B"] = 2; 6: dictionary["C"] = 3; 7: dictionary["D"] = 4; 8: dictionary["E"] = 5; 9: dictionary["F"] = 6; Then you have one task (using the wonderful TPL!) to iterate using dirty reads: 1: // attempt iteration in a separate thread 2: var iterationTask = new Task(() => 3: { 4: // iterates using a dirty read 5: foreach (var pair in dictionary) 6: { 7: Console.WriteLine(pair.Key + ":" + pair.Value); 8: } 9: }); And one task to attempt updates in a separate thread (probably): 1: // attempt updates in a separate thread 2: var updateTask = new Task(() => 3: { 4: // iterates, and updates the value by one 5: foreach (var pair in dictionary) 6: { 7: dictionary[pair.Key] = pair.Value + 1; 8: } 9: }); Now that we’ve done this, we can fire up both tasks and wait for them to complete: 1: // start both tasks 2: updateTask.Start(); 3: iterationTask.Start(); 4:  5: // wait for both to complete. 6: Task.WaitAll(updateTask, iterationTask); Now, if I you didn’t know about the dirty reads, you may have expected to see the iteration before the updates (such as A:1, B:2, C:3, D:4, E:5, F:6).  However, because the reads are dirty, we will quite possibly get a combination of some updated, some original.  My own run netted this result: 1: F:6 2: E:6 3: D:5 4: C:4 5: B:3 6: A:2 Note that, of course, iteration is not in order because ConcurrentDictionary, like Dictionary, is unordered.  Also note that both E and F show the value 6.  This is because the output task reached F before the update, but the updates for the rest of the items occurred before their output (probably because console output is very slow, comparatively). If we want to always guarantee that we will get a consistent snapshot to iterate over (that is, at the point we ask for it we see precisely what is in the dictionary and no subsequent updates during iteration), we should iterate over a call to ToArray() instead: 1: // attempt iteration in a separate thread 2: var iterationTask = new Task(() => 3: { 4: // iterates using a dirty read 5: foreach (var pair in dictionary.ToArray()) 6: { 7: Console.WriteLine(pair.Key + ":" + pair.Value); 8: } 9: }); The atomic Try…() methods As you can imagine TryAdd() and TryRemove() have few surprises.  Both first check the existence of the item to determine if it can be added or removed based on whether or not the key currently exists in the dictionary: 1: // try add attempts an add and returns false if it already exists 2: if (dictionary.TryAdd("G", 7)) 3: Console.WriteLine("G did not exist, now inserted with 7"); 4: else 5: Console.WriteLine("G already existed, insert failed."); TryRemove() also has the virtue of returning the value portion of the removed entry matching the given key: 1: // attempt to remove the value, if it exists it is removed and the original is returned 2: int removedValue; 3: if (dictionary.TryRemove("C", out removedValue)) 4: Console.WriteLine("Removed C and its value was " + removedValue); 5: else 6: Console.WriteLine("C did not exist, remove failed."); Now TryUpdate() is an interesting creature.  You might think from it’s name that TryUpdate() first checks for an item’s existence, and then updates if the item exists, otherwise it returns false.  Well, note quite... It turns out when you call TryUpdate() on a concurrent dictionary, you pass it not only the new value you want it to have, but also the value you expected it to have before the update.  If the item exists in the dictionary, and it has the value you expected, it will update it to the new value atomically and return true.  If the item is not in the dictionary or does not have the value you expected, it is not modified and false is returned. 1: // attempt to update the value, if it exists and if it has the expected original value 2: if (dictionary.TryUpdate("G", 42, 7)) 3: Console.WriteLine("G existed and was 7, now it's 42."); 4: else 5: Console.WriteLine("G either didn't exist, or wasn't 7."); The composite Add methods The ConcurrentDictionary also has composite add methods that can be used to perform updates and gets, with an add if the item is not existing at the time of the update or get. The first of these, AddOrUpdate(), allows you to add a new item to the dictionary if it doesn’t exist, or update the existing item if it does.  For example, let’s say you are creating a dictionary of counts of stock ticker symbols you’ve subscribed to from a market data feed: 1: public sealed class SubscriptionManager 2: { 3: private readonly ConcurrentDictionary<string, int> _subscriptions = new ConcurrentDictionary<string, int>(); 4:  5: // adds a new subscription, or increments the count of the existing one. 6: public void AddSubscription(string tickerKey) 7: { 8: // add a new subscription with count of 1, or update existing count by 1 if exists 9: var resultCount = _subscriptions.AddOrUpdate(tickerKey, 1, (symbol, count) => count + 1); 10:  11: // now check the result to see if we just incremented the count, or inserted first count 12: if (resultCount == 1) 13: { 14: // subscribe to symbol... 15: } 16: } 17: } Notice the update value factory Func delegate.  If the key does not exist in the dictionary, the add value is used (in this case 1 representing the first subscription for this symbol), but if the key already exists, it passes the key and current value to the update delegate which computes the new value to be stored in the dictionary.  The return result of this operation is the value used (in our case: 1 if added, existing value + 1 if updated). Likewise, the GetOrAdd() allows you to attempt to retrieve a value from the dictionary, and if the value does not currently exist in the dictionary it will insert a value.  This can be handy in cases where perhaps you wish to cache data, and thus you would query the cache to see if the item exists, and if it doesn’t you would put the item into the cache for the first time: 1: public sealed class PriceCache 2: { 3: private readonly ConcurrentDictionary<string, double> _cache = new ConcurrentDictionary<string, double>(); 4:  5: // adds a new subscription, or increments the count of the existing one. 6: public double QueryPrice(string tickerKey) 7: { 8: // check for the price in the cache, if it doesn't exist it will call the delegate to create value. 9: return _cache.GetOrAdd(tickerKey, symbol => GetCurrentPrice(symbol)); 10: } 11:  12: private double GetCurrentPrice(string tickerKey) 13: { 14: // do code to calculate actual true price. 15: } 16: } There are other variations of these two methods which vary whether a value is provided or a factory delegate, but otherwise they work much the same. Oddities with the composite Add methods The AddOrUpdate() and GetOrAdd() methods are totally thread-safe, on this you may rely, but they are not atomic.  It is important to note that the methods that use delegates execute those delegates outside of the lock.  This was done intentionally so that a user delegate (of which the ConcurrentDictionary has no control of course) does not take too long and lock out other threads. This is not necessarily an issue, per se, but it is something you must consider in your design.  The main thing to consider is that your delegate may get called to generate an item, but that item may not be the one returned!  Consider this scenario: A calls GetOrAdd and sees that the key does not currently exist, so it calls the delegate.  Now thread B also calls GetOrAdd and also sees that the key does not currently exist, and for whatever reason in this race condition it’s delegate completes first and it adds its new value to the dictionary.  Now A is done and goes to get the lock, and now sees that the item now exists.  In this case even though it called the delegate to create the item, it will pitch it because an item arrived between the time it attempted to create one and it attempted to add it. Let’s illustrate, assume this totally contrived example program which has a dictionary of char to int.  And in this dictionary we want to store a char and it’s ordinal (that is, A = 1, B = 2, etc).  So for our value generator, we will simply increment the previous value in a thread-safe way (perhaps using Interlocked): 1: public static class Program 2: { 3: private static int _nextNumber = 0; 4:  5: // the holder of the char to ordinal 6: private static ConcurrentDictionary<char, int> _dictionary 7: = new ConcurrentDictionary<char, int>(); 8:  9: // get the next id value 10: public static int NextId 11: { 12: get { return Interlocked.Increment(ref _nextNumber); } 13: } Then, we add a method that will perform our insert: 1: public static void Inserter() 2: { 3: for (int i = 0; i < 26; i++) 4: { 5: _dictionary.GetOrAdd((char)('A' + i), key => NextId); 6: } 7: } Finally, we run our test by starting two tasks to do this work and get the results… 1: public static void Main() 2: { 3: // 3 tasks attempting to get/insert 4: var tasks = new List<Task> 5: { 6: new Task(Inserter), 7: new Task(Inserter) 8: }; 9:  10: tasks.ForEach(t => t.Start()); 11: Task.WaitAll(tasks.ToArray()); 12:  13: foreach (var pair in _dictionary.OrderBy(p => p.Key)) 14: { 15: Console.WriteLine(pair.Key + ":" + pair.Value); 16: } 17: } If you run this with only one task, you get the expected A:1, B:2, ..., Z:26.  But running this in parallel you will get something a bit more complex.  My run netted these results: 1: A:1 2: B:3 3: C:4 4: D:5 5: E:6 6: F:7 7: G:8 8: H:9 9: I:10 10: J:11 11: K:12 12: L:13 13: M:14 14: N:15 15: O:16 16: P:17 17: Q:18 18: R:19 19: S:20 20: T:21 21: U:22 22: V:23 23: W:24 24: X:25 25: Y:26 26: Z:27 Notice that B is 3?  This is most likely because both threads attempted to call GetOrAdd() at roughly the same time and both saw that B did not exist, thus they both called the generator and one thread got back 2 and the other got back 3.  However, only one of those threads can get the lock at a time for the actual insert, and thus the one that generated the 3 won and the 3 was inserted and the 2 got discarded.  This is why on these methods your factory delegates should be careful not to have any logic that would be unsafe if the value they generate will be pitched in favor of another item generated at roughly the same time.  As such, it is probably a good idea to keep those generators as stateless as possible. Summary The ConcurrentDictionary is a very efficient and thread-safe version of the Dictionary generic collection.  It has all the benefits of type-safety that it’s generic collection counterpart does, and in addition is extremely efficient especially when there are more reads than writes concurrently. Tweet Technorati Tags: C#, .NET, Concurrent Collections, Collections, Little Wonders, Black Rabbit Coder,James Michael Hare

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