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• Update a PDF to include an encrypted, hidden, unique identifier?

  - by Dave Jarvis

  Background The idea is this: Person provides contact information for online book purchase Book, as a PDF, is marked with a unique hash Person downloads book PDF passwords are annoying and extremely easy to circumvent. The ideal process would be something like: Generate hash based on contact information Store contact information and hash in database Acquire book lock Update an "include" file with hash text Generate book as PDF (using pdflatex) Apply hash to book Release book lock Send email with book download link Technologies The following technologies can be used (other programming languages are possible, but libraries will likely be limited to those supplied by the host): C, Java, PHP LaTeX files PDF files Linux Question What programming techniques (or open source software) should I investigate to: Embed a unique hash (or other mark) to a PDF Create a collusion-attack resistant mark Develop a non-fragile (e.g., PDF -> EPS -> PDF still contains the mark) solution Research I have looked at the following possibilities: Steganography Natural Language Processing (NLP) Convert blank pages in PDF to images; mark those images; reassemble PDF LaTeX watermark package ImageMagick Steganograhy requires keeping a master copy of the images, and I'm not sure if the watermark would survive PDF -> EPS -> PDF, or other types of conversion. LaTeX creates an image cache, so any steganographic process would have to intercept that process somehow. NLP introduces grammatical errors. Inserting blank pages as images is immediately suspect; it is easy to replace suspicious blank pages. The LaTeX watermark package draws visible marks. ImageMagick draws visible marks. What other solutions are possible? Related Links http://www.tcpdf.org/ invisible watermarks in images Thank you!

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• What are the useful UNIX functions that MS doesn't implement? And why? [closed]

  - by prosseek

  When programming with Python, I came across some functions that are not implemented on Windows. os.fork() may be one of them. UNIX came before WinNT, so the WinNT developers (most notably Dave Cutler) must knew about the features and functions of the UNIX. But, to me, it seems that MS didn't like UNIX so much that they mistakenly/intentionally skipped or distorted some of the useful UNIX functions/features; i.e. /abc/def in UNIX, \abc\def in Windows as an easy example. And when I read the Windows System Programming book, I felt uncomfortable as the Windows system functions seem nothing more than a tweak from UNIX. (I might be wrong.) What are those functions/features that MS OSes don't have, but UNIX origninated? Is there any reason for this? Do they just want to differentiate from UNIX world? Or do they think some of the UNIX functions are unnecessary? Is Windows a tweak from UNIX? Or, is there any great OS features that were invented in MS to make Windows better than UNIX?

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• In .NET when Aborting Thread, can this piece of code get corrupted?

  - by bosko

  Little intro: In complex multithreaded aplication (enterprise service bus EBS), I need to use Thread.Abort, because this EBS accepts user written modules which communicates with hardware security modules. So if this module gets deadlocked or hardware stops responding - i need to just unload this module and rest of this server aplication must keep runnnig. So there is abort sync mechanism which ensures that code can be aborted only in user section and this section must be marked as AbortAble. If this happen there is possibility that ThreadAbortException will be thrown in this pieace of code: public void StopAbortSection() { var id = Thread.CurrentThread.ManagedThreadId; lock (threadIdMap[id]) { .... } } If module is on AbortSection and Aplication decides to abort module, but after this decision but before actual Thread.Abort, module enters NonAbortableSection by calling this method, but lock is actualy taken on that locking object. So lock will block until Abort or abort can be executed before reaching this block by this code. But Object with this method is essential and i need to be sure that this pieace of code is safe to abort in any moment. Probably i have to mention that threadIdMap is Dictionary(int,ManualResetEvent), so locking object is instance of ManualResetEvent. I hope you now understad my question. Sorry for its largeness.

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• Why do programmers have to learn for their whole lives and aren't you afraid of that?

  - by serg555

  Programming technologies are evolving so fast that programmers constantly have to learn more and more to catch up whether you want it or not. Often it is not just learning more in the same direction but starting from a scratch. Lets say you were a topnotch programmer in 1999 who quit for 10 years and went to a job interview in 2009 (funny even to imagine) - how much of your knowledge is still needed? And if we take a carpenter, engineer, doctor or even mathematician - they all are still good specialists after 10 years. So why programming is so not stable? Is it because it is just relatively new or because something important is still missing after 50 years and we can't find it to settle in that direction? Do you think after some time situation will change? Learning something new all the time is exciting and all, but it is starting to worry me that as I become older it will be harder and harder. After all "you can't teach an old dog new tricks" and I'm afraid that at the end I just end up behind college students and become one of those "cobol dinosaurs", only it will be probably "java dinosaurs" by that time.

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• Properties vs. Fields: Need help grasping the uses of Properties over Fields.

  - by pghtech

  First off, I have read through a list of postings on this topic and I don't feel I have grasped properties because of what I had come to understand about encapsulation and field modifiers (private, public..ect). One of the main aspects of C# that I have come to learn is the importance of data protection within your code by the use of encapsulation. I 'thought' I understood that to be because of the ability of the use of the modifiers (private, public, internal, protected). However, after learning about properties I am sort of torn in understanding not only properties uses, but the overall importance/ability of data protection (what I understood as encapsulation) within C#. To be more specific, everything I have read when I got to properties in C# is that you should try to use them in place of fields when you can because of: 1) they allow you to change the data type when you can't when directly accessing the field directly. 2) they add a level of protection to data access However, from what I 'thought' I had come to know about the use of field modifiers did #2, it seemed to me that properties just generated additional code unless you had some reason to change the type (#1) - because you are (more or less) creating hidden methods to access fields as opposed to directly. Then there is the whole modifiers being able to be added to Properties which further complicates my understanding for the need of properties to access data. I have read a number of chapters from different writers on "properties" and none have really explained a good understanding of properties vs. fields vs. encapsulation (and good programming methods). Can someone explain: 1) why I would want to use properties instead of fields (especially when it appears I am just adding additional code 2) any tips on recognizing the use of properties and not seeing them as simply methods (with the exception of the get;set being apparent) when tracing other peoples code? 3) Any general rules of thumb when it comes to good programming methods in relation to when to use what? Thanks and sorry for the long post - I didn't want to just ask a question that has been asked 100x without explaining why I am asking it again.

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• Simplest possible rack app -> permission error

  - by 7stud

  Here's the program(1.rb) blah blah blah blah blah blah: require 'rack' my_rack = lambda { |env| [200, {}, ["Hello. The time is: #{Time.now}"]] } handler = Rack::Handler::WEBrick handler.run(my_rack, :PORT => 12_500) Here's the error (blah blah blah blah blah): ~/ruby_programs$ ruby 1.rb [2012-12-07 21:49:09] INFO WEBrick 1.3.1 [2012-12-07 21:49:09] INFO ruby 1.9.3 (2012-04-20) [x86_64-darwin10.8.0] [2012-12-07 21:49:09] WARN TCPServer Error: Permission denied - bind(2) [2012-12-07 21:49:09] WARN TCPServer Error: Permission denied - bind(2) /Users/7stud/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/webrick/utils.rb:85:in `initialize': Permission denied - bind(2) (Errno::EACCES) from /Users/7stud/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/webrick/utils.rb:85:in `new' from /Users/7stud/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/webrick/utils.rb:85:in `block in create_listeners' from /Users/7stud/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/webrick/utils.rb:82:in `each' from /Users/7stud/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/webrick/utils.rb:82:in `create_listeners' from /Users/7stud/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/webrick/server.rb:82:in `listen' from /Users/7stud/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/webrick/server.rb:70:in `initialize' from /Users/7stud/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/webrick/httpserver.rb:45:in `initialize' from /Users/7stud/.rvm/gems/ruby-1.9.3-p194@programming/gems/rack-1.4.1/lib/rack/handler/webrick.rb:10:in `new' from /Users/7stud/.rvm/gems/ruby-1.9.3-p194@programming/gems/rack-1.4.1/lib/rack/handler/webrick.rb:10:in `run' from 1.rb:5:in `<main>' ~/ruby_programs$ Here's line 85 of ../webrick/utils.rb: sock = TCPServer.new(ai[3], port) If I replace the code in 1.rb with this: require 'socket' server = TCPServer.new 12_000 # Server bind to port 2000 loop do client = server.accept # Wait for a client to connect client.puts "Hello !" client.puts "Time is #{Time.now}" client.close end I don't get any errors, and if I enter the address: http://localhost:12000/ in my browser, I get the expected output: Hello ! Time is 2012-12-07 18:58:15 -0700

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• Listing serial (COM) ports on Windows?

  - by Eli Bendersky

  Hello, I'm looking for a robust way to list the available serial (COM) ports on a Windows machine. There's this post about using WMI, but I would like something less .NET specific - I want to get the list of ports in a Python or a C++ program, without .NET. I currently know of two other approaches: Reading the information in the HARDWARE\\DEVICEMAP\\SERIALCOMM registry key. This looks like a great option, but is it robust? I can't find a guarantee online or in MSDN that this registry cell indeed always holds the full list of available ports. Tryint to call CreateFile on COMN with N a number from 1 to something. This isn't good enough, because some COM ports aren't named COMN. For example, some virtual COM ports created are named CSNA0, CSNB0, and so on, so I wouldn't rely on this method. Any other methods/ideas/experience to share? Edit: by the way, here's a simple Python implementation of reading the port names from registry: import _winreg as winreg import itertools def enumerate_serial_ports(): """ Uses the Win32 registry to return a iterator of serial (COM) ports existing on this computer. """ path = 'HARDWARE\\DEVICEMAP\\SERIALCOMM' try: key = winreg.OpenKey(winreg.HKEY_LOCAL_MACHINE, path) except WindowsError: raise IterationError for i in itertools.count(): try: val = winreg.EnumValue(key, i) yield (str(val[1]), str(val[0])) except EnvironmentError: break

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• How can I add a portrait layout on top of a landscape Camera SurfaceView?

  - by user319919

  I need a Camera SurfaceView for my application. The camera should be set to fixed landscape view which is done by setting android:screenOrientation="landscape" for the activity in the AndroidManifest.xml. After doing some experiments and Google researches trying to use setRotation(int) inside the camera preview implementation, I came to the conclusion, that it is obviously the common practice to get a preview with correct behaviour. Now the camera preview itself looks fine for landscape orientation. But I need to have an overlay that holds a bunch of buttons. Due to usability the user interface should be in portrait view (or even better orientation aware). There seemed no other option to me, but to fix the activity screenOrientation, so that the camera preview looks normal (in portrait mode the whole view is streched and rotated to the left) Is there a workaround to get my buttons back to portrait orientation? Or another overall approach to deal with the camera view? Parameters.setRotation(int) obvisouly didnt work. I am quite new to the Android plattform programming. Of course I dont know much about the programming tricks and workarounds yet. I did a lot of research over the last two weeks, but couldnt find the right solution so far.

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• Java (or possibly other languages) learning path

  - by bgo

  I am familiar (as a self-learner) with C, python and php such that i can solve some problems involving simple steps (for example, i easily do calculations for physics lab reports with python which normally would take 4x-5x times longer with a calculator). The point here is, as doing such things, i learnt the idea / concepts of programming language and problem solving along with oop or fuctional programming etc. Recently i have started Java and, with the familiarity of other languages, i am doing well for starters but i need guidence. -I am thinking of learning syntax from sun java tutorials and then practicing with codingbat.com or similar sites. I need a reference book that i can study deeper aspects of the topics i am learning. What do you suggest about these? -The problem is (and always have been) the lack of practice. I need coding and problem-solving practices sources. I stuck at the point where i can't figure out what to do next. Can you suggest any source (possibly like codingbat)? If i could plan a learning trail, i can progress faster and efficiently. So i need ideas, comments, suggestions. Thanks in advance.

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• Design Pattern for error handling in ASP.NET 3.5 site

  - by Kevin

  I am relatively new to ASP.NET programming, and web programming in general. We have a site we recently ported from .NET 1.1 to 3.5. Currently we have two methods of error handling: either catching the error during data load on a page and displaying the formatted error in a label on the page, or redirecting to a generic error page. Both of these are somewhat annoying, as right now I'm trying to redesign how our errors are displayed. We are soon moving to Master pages, and I'm wondering if there is a way to "build in" an error handling control. What I mean by this is using a ASP.NET user control I've designed that simply gets passed the error string returned from the server. If an error occurs, the page would not display the content, and instead display the error control. This provides us with the ability to retain the current banner/navigation during an error (which we don't get with the generic error page), as well as keeping me from having to add the control to every aspx page we have (which I have to do with using the label-per-page system). Does something like this make sense? Ultimately I just want to have the error control added to a single page, and all other pages have access to it directly. Is this something Master pages help with? Thanks!

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• Ant fileset's "dir" missing from absolute path when using a mapper

  - by spaaarky21

  I've been trying to write an Ant task to "compile" Sass scripts in my project using the apply task but I kept getting a "No such file or directory" error. I thought it might have been caused by spaces in the buildpath so I went through the trouble of moving the project only to find that Ant seems to omitting the fileset's root directory when it returns the path. This is what the target looks like: <target name="sass-compile" depends="properties"> <apply executable="sass"> <srcfile /> <targetfile /> <fileset dir="${project.src.dir}" includes="**/*.scss" /> <globmapper from="*.scss" to="*.css" /> </apply> </target> To help troubleshoot, I switched the executable from sass to echo and I noticed that the mapper is transforming paths like this... /Users/me/Documents/Programming/workspace/Project/src/java/com/proj/web/page/template/Template.scss ...into this... /Users/me/Documents/Programming/workspace/Project/java/com/proj/web/page/template/Template.css Notice that the src directory is missing from the target file path. Am I seeing a bug here or is this somehow expected? I would love to know what's going on here. I have also tried using a regexpmapper, and a filtermapper with replacestring. The result is the same. I'm running Ant 1.7.1, which comes bundled with Eclipse Helios, which I'm running on a Mac. I also tried Ant 1.8 on both Mac and Linux. Nothing works. Does anyone have any ideas?

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• background image shows up on right-click show image, but not on webpage

  - by William

  okay, so I'm trying to set up a webpage with a div wrapping two other divs, and the wrapper div has a background, and the other two are transparent. How come this isn't working? here is the CSS: .posttext{ float: left; width: 70%; text-align: left; padding: 5px; background-color: transparent !important; } .postavi{ float: left; width: 100px; height: 100%; text-align: left; background-color: transparent !important; padding: 5px; } .postwrapper{ background-image:url('images/post_bg.png'); background-position:left top; background-repeat:repeat-y; } and here is the HTML: <div class="postwrapper"> <div class="postavi"><img src="http://prime.programming-designs.com/test_forum/images/avatars/hacker.png" alt="hacker"/></div><div class="posttext"><p style="color: #ff0066">You will have bad luck today.</p>lol</div> </div> Edit: at request, here is a link to the site: http://prime.programming-designs.com/test_forum/viewthread.php?thread=33 Edit: edited css to be correct, still suffering same problem

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• Preferred way of application initialization

  - by lisak

  Do you guys have your own little framework for project startups ? I mean, every time one needs to do the same things at the beginning: Context initialization - ideally after arguments are processed. Sometimes without interactive user input, sometimes with input reader. Sometimes we need to load properties, sometimes not. Then we need to get a class out of context and run its method. Programming....programming until writing shell script to place everything on classpath. It's true that it differs according to the actual needs. But it seems to me, that I'm doing always almost the same, again and again from the scratch. Sometimes I realize that I'm postponing my work just because I don't want to do these annoying startups. It would be great if there was some kind of universal Main class doing reflection to specified bean, context initialization, argument parsing, interactive user input reading and have the programmer do the important things...All setup might be done via spring configuration. I think I'll have to do it by myself. I'd appreciate your ideas

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• Do you like Twisted?

  - by Luca

  I use Python Twisted for web development, and I don't like it? I know async programming is a great idea, I know there are may async web servers now, I know it's the only way to solve some problems you'd have with threads but I don't like. The problem is that, you're forced to program in a twisted way. So, the architecture you have in mind, very often have to be modified to fit the way twisted works. The architecture have to follow the technology, I don't think this is good. When we use callback in javascript, we don't have too many difficulties: things are usually simpler, we use a callback in response to an Ajax call. But in a server web app things are, very often, a bit more complex. Writing chain of callbacks don't seem to me a wonderful way of programming. The code is not simple, and so it is difficult to understand and to maintain. Writing twisted code we very often lost the linear intuitive idea of the algorithm we wanted to implement, especially when things grow in complexity. What's your point of view?

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• How can I improve this search usability?

  - by Craig Whitley

  This is the first real programming attempt of mine, and theres some major flaws. It's a learning project, and I'm currently re-writing the entire thing as my php is is really messy. I really want to get an idea on how I can improve the actual usability and accesibility of the site at the same time though - so I know how to implement it correctly. The website is basically a comparison website for gameserver hosting. As I mentioned, its a learning project and I don't actually expect any revenue from it. At the moment theres only test data in it, so in the game input box select either 'Battlefied Bad Company 2' or 'Call of Duty 4: Modern Warfare' and ignore the actual search results. http://www.laglessfrag.com I wasn't really sure how to work the search functionality. Basically when you click a game in the drop down box, it sends an ajax request and finds all the locations available to that specific game in the database. After selecting the country theres another ajax to find all the citys available to the game in that country - which gives me the two unique identifiers I need to create the search results. One major and fundamental flaw is that without javascript enabled, the site ceases to function. I'll overcome that in the next re-write, but without the ajax functionality stopping the user 'going wrong' - how can I implement a search that requires two fields without creating extra steps in new pages after form submissions? I'm also no designer so my whole layout and css is a bit rubbish, but this was mainly a learning project as I'm interested in applications / programming rather than design. It's also slow as its on shared hosting, but if I can get it to work correctly then I'm not impartial to chucking a bit of money at it for faster hosting and maybe a bit of advertising and seeing where it goes (if anywhere!). Any info appreciated.

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• how useful is Turing completeness? are neural nets turing complete?

  - by Albert

  While reading some papers about the Turing completeness of recurrent neural nets (for example: Turing computability with neural nets, Hava T. Siegelmann and Eduardo D. Sontag, 1991), I got the feeling that the proof which was given there was not really that practical. For example the referenced paper needs a neural network which neuron activity must be of infinity exactness (to reliable represent any rational number). Other proofs need a neural network of infinite size. Clearly, that is not really that practical. But I started to wonder now if it does make sense at all to ask for Turing completeness. By the strict definition, no computer system nowadays is Turing complete because none of them will be able to simulate the infinite tape. Interestingly, programming language specification leaves it most often open if they are turing complete or not. It all boils down to the question if they will always be able to allocate more memory and if the function call stack size is infinite. Most specification don't really specify this. Of course all available implementations are limited here, so all practical implementations of programming languages are not Turing complete. So, what you can say is that all computer systems are just equally powerful as finite state machines and not more. And that brings me to the question: How useful is the term Turing complete at all? And back to neural nets: For any practical implementation of a neural net (including our own brain), they will not be able to represent an infinite number of states, i.e. by the strict definition of Turing completeness, they are not Turing complete. So does the question if neural nets are Turing complete make sense at all? The question if they are as powerful as finite state machines was answered already much earlier (1954 by Minsky, the answer of course: yes) and also seems easier to answer. I.e., at least in theory, that was already the proof that they are as powerful as any computer.

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• Reading numpy arrays outside of Python

  - by Abiel

  In a recent question I asked about the fastest way to convert a large numpy array to a delimited string. My reason for asking was because I wanted to take that plain text string and transmit it (over HTTP for instance) to clients written in other programming languages. A delimited string of numbers is obviously something that any client program can work with easily. However, it was suggested that because string conversion is slow, it would be faster on the Python side to do base64 encoding on the array and send it as binary. This is indeed faster. My question now is, (1) how can I make sure my encoded numpy array will travel well to clients on different operating systems and different hardware, and (2) how do I decode the binary data on the client side. For (1), my inclination is to do something like the following import numpy as np import base64 x = np.arange(100, dtype=np.float64) base64.b64encode(x.tostring()) Is there anything else I need to do? For (2), I would be happy to have an example in any programming language, where the goal is to take the numpy array of floats and turn them into a similar native data structure. Assume we have already done base64 decoding and have a byte array, and that we also know the numpy dtype, dimensions, and any other metadata which will be needed. Thanks.

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• How can I provide users with the functionality of the DBUnit DatabaseOperation methods from a web in

  - by reckoner

  I am currently updating a java-based web application which allows database developers to create stored procedure regression test suites for database testing. Currently, for test setup, execution and clean-up stages, the user is provided with text boxes where they are able to enter SQL code which is executed by the isql command. I would like to extend the application to use DB Unit’s DatabaseOperation methods to provide more ways to setup the state of the database than just SQL statements. The main reason for using Db Unit rather than just SQL statements is to be able to create and store xml and xls DataSets on a server where they can be associated with their test cases and used for data setup. My question is: How can I provide users with the functionality of the DBUnit DatabaseOperation methods from a web interface? I have considered: Creating a simple programming language and a parser to read some simple syntax involving the DB Unit method names which accept a parameter being the file location to an xml or xls DataSet. I was thinking of allowing the user to register the files they need with the web app which would catalogue them and provide each file with an identifier which could passed as a parameter to the methods in this simple programming language. Creating an XML DTD which provides the user with the ability to specify operations and parameters. If I went this approach, how can I execute the methods and their parameters that I parse from the XML document? Creating a table in the database which stores the method and a FK relation to a catalogued DataSet file, however I don’t think this would be good solution due to the fact that data entry would be tedious. Thanks for your help.

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• Understanding Symbols In Ruby

  - by Kezzer

  Despite reading this article, I'm still confused as to the representation of the data in memory when it comes to using symbols. If a symbol, two of them contained in different objects, exist in the same memory location, then how is it that they contain different values? I'd have expected the same memory location to contain the same value. As a quote from the link: Unlike strings, symbols of the same name are initialized and exist in memory only once during a session of ruby I just don't understand how it manages to differentiate the values contained in the same memory location. EDIT So let's consider the example: patient1 = { :ruby => "red" } patient2 = { :ruby => "programming" } patient1.each_key {|key| puts key.object_id.to_s} 3918094 patient2.each_key {|key| puts key.object_id.to_s} 3918094 patient1 and patient2 are both hashes, that's fine. :ruby however is a symbol. If we were to output the following: patient1.each_key {|key| puts key.to_s} Then what will be output? "red", or "programming"? FURTHER EDIT I'm still really quite confused. I'm thinking a symbol is a pointer to a value. Let's forget hashes for a second. The questions I have are; can you assign a value to a symbol? Is a symbol just a pointer to a variable with a value in it? If symbols are global, does that mean a symbol always points to one thing?

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• How do I populate a MEF plugin with data that is not hard coded into the assembly?

  - by hjoelr

  I am working on a program that will communicate with various pieces of hardware. Because of the varied nature of the items it communicates and controls, I need to have a different "driver" for each different piece of hardware. This made me think that MEF would be a great way to make those drivers as plugins that can be added even after the product has been released. I've looked at a lot of examples of how to use MEF, but the question that I haven't been able to find an answer to is how to populate a MEF plugin with external data (eg. from a database). All the examples I can find have the "data" hard-coded into the assembly, like the following example: [Export( typeof( IRule ) )] public class RuleInstance : IRule { public void DoIt() {} public string Name { get { return "Rule Instance 3"; } } public string Version { get { return "1.1.0.0"; } } public string Description { get { return "Some Rule Instance"; } } } What if I want Name, Version and Description to come from a database? How would I tell MEF where to get that information? I may be missing something very obvious, but I don't know what it is. Thanks! Joel

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• Microsoft Training in .NET

  - by JD

  Hey guys, A quick question about training. My company are offering to put me through a 5 day course in .NET programming with C#, on the proviso that I work for them for a minimum 12 month period immediately after the training has been completed. IF I don't work for them for 12months I will be expected to pay them back teh cost of the training, minus the amount of months which I have stayed at the company (e.g. stay 6 months and only pay back half). The course will be $3,300 AUD - and will have no qualifications. I have been programming in C# for about 6 months, and in PHP for about 10 years - so I am getting the hang of .NET slowly but surely. Is it worth doing this? Or would I be best advised to learn from reading the MS books from Amazon for $40 and then sitting the 2 exams to get MS certified off my own back? The company are not willing to pay for my certification as well as 'they are not training me to get certified' and hence more employable... Thoughts? How do other companies deal with training? And stop people from taking off after becoming certified?

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• Extension methods for encapsulation and reusability

  - by tzaman

  In C++ programming, it's generally considered good practice to "prefer non-member non-friend functions" instead of instance methods. This has been recommended by Scott Meyers in this classic Dr. Dobbs article, and repeated by Herb Sutter and Andrei Alexandrescu in C++ Coding Standards (item 44); the general argument being that if a function can do its job solely by relying on the public interface exposed by the class, it actually increases encapsulation to have it be external. While this confuses the "packaging" of the class to some extent, the benefits are generally considered worth it. Now, ever since I've started programming in C#, I've had a feeling that here is the ultimate expression of the concept that they're trying to achieve with "non-member, non-friend functions that are part of a class interface". C# adds two crucial components to the mix - the first being interfaces, and the second extension methods: Interfaces allow a class to formally specify their public contract, the methods and properties that they're exposing to the world. Any other class can choose to implement the same interface and fulfill that same contract. Extension methods can be defined on an interface, providing any functionality that can be implemented via the interface to all implementers automatically. And best of all, because of the "instance syntax" sugar and IDE support, they can be called the same way as any other instance method, eliminating the cognitive overhead! So you get the encapsulation benefits of "non-member, non-friend" functions with the convenience of members. Seems like the best of both worlds to me; the .NET library itself providing a shining example in LINQ. However, everywhere I look I see people warning against extension method overuse; even the MSDN page itself states: In general, we recommend that you implement extension methods sparingly and only when you have to. So what's the verdict? Are extension methods the acme of encapsulation and code reuse, or am I just deluding myself?

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• Bidirectional/Loopback UDP in .net

  - by Jason Williams

  I've got an app that needs to transmit and receive on the same port. This can happen in two cases: Where the PC is talking to a piece of remote hardware. It "replies to sender", so the datagrams come back in to my PC via the sending port. Where the PC is talking to itself (loopback mode) for testing and demoing (a test app feeds fake data into our main app via UDP). This only seems to fail when trying to achieve loopback. The only way I can get it working is to ensure that the receiver is set up first - something I cannot guarantee. Can anyone help narrow down my search by suggesting a "correct" way to implement the UdpClient(s) to handle the above situations reliably? (The only solution that I've found to work reliably with the remote hardware is to use a single UdpClient in a bidirectional manner, although I'm working with legacy code that may be influencing that finding. I've tried using two UdpClients, but they step on each others toes - In some cases, once one client is started up, the other client cannot connect. With ExclusiveAddressUse/ReuseAddress set up to allow port sharing, I can almost get it to work, apart from the receiver having to start first)

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• general learning methodology

  - by momo

  just wanted to hear on the different general learning paths people embark on when learning a new language/framework. the one i currently use, which is how i learned bash and am currently learning python, is: instant hacking tutorial (very short tutorial introducing the basic syntax, variable declaration, loops, data types, etc. and how they are generally used) in depth tutorial with good programming style and slightly topic-specific (e.g. Mark Pilgrim's Dive into Python), important topics for me personally are regex methods, file IO, and ways the different data types are utilized best (i wrote a very primitive bayesian spam filter using python's dictionaries to keep track of word occurrences) spaced-repition of syntax or short recipes (i use anki, with questions like 'create dictionary with filename and filesize metadata, human-readable' or simpler ones like 'match 0 - 3 occurences of the letter M in a string', or 'return/create an iterator from two sequences') the use of spaced-repitition has been invaluable, and i credit it with the ease that i can recall/create python algorithms. however, i've recently started looking into django, and i've found that spaced-repitition, at least in my case, doesn't work very well for learning a framework, it works best with short code recipes (either that or i should start looking into more basic django framework tutorials). the problem i'm encountering is that since framework programming is not only algorithms, but actually learning the API, which can be quite complex since you have to learn all the methods, modules, the places where they are stored, and the sequence of which things have to be done. for ex. in django to start a project that deals with polls (from the django tutorial), one has to create the project, edit the settings.py file, create the polls app, edit the models.py file (which requires knowing the classes that are present in the module models), edit the urls.py file, etc. i found that my spaced-repition method didn't work very well for this type of learning, so i wanted to ask you guys what method(s) you use for learning the different frameworks/APIs.

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• Passing Derived Class Instances as void* to Generic Callbacks in C++

  - by Matthew Iselin

  This is a bit of an involved problem, so I'll do the best I can to explain what's going on. If I miss something, please tell me so I can clarify. We have a callback system where on one side a module or application provides a "Service" and clients can perform actions with this Service (A very rudimentary IPC, basically). For future reference let's say we have some definitions like so: typedef int (*callback)(void*); // This is NOT in our code, but makes explaining easier. installCallback(string serviceName, callback cb); // Really handled by a proper management system sendMessage(string serviceName, void* arg); // arg = value to pass to callback This works fine for basic types such as structs or builtins. We have an MI structure a bit like this: Device <- Disk <- MyDiskProvider class Disk : public virtual Device class MyDiskProvider : public Disk The provider may be anything from a hardware driver to a bit of glue that handles disk images. The point is that classes inherit Disk. We have a "service" which is to be notified of all new Disks in the system, and this is where things unravel: void diskHandler(void *p) { Disk *pDisk = reinterpret_cast<Disk*>(p); // Uh oh! // Remainder is not important } SomeDiskProvider::initialise() { // Probe hardware, whatever... // Tell the disk system we're here! sendMessage("disk-handler", reinterpret_cast<void*>(this)); // Uh oh! } The problem is, SomeDiskProvider inherits Disk, but the callback handler can't receive that type (as the callback function pointer must be generic). Could RTTI and templates help here? Any suggestions would be greatly appreciated.

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