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• Generics Java and Shadowing of type parameters

  - by rubixibuc

  This code seems to work fine class Rule<T> { public <T>Rule(T t) { } public <T> void Foo(T t) { } } Does the method type parameter shadow the class type parameter? Also when you create an object does it use the type parameter of the class? example Rule<String> r = new Rule<String>(); Does this normally apply to the type parameter of the class, in the situation where they do not conflict? I mean when only the class has a type parameter, not the constructor, or does this look for a type parameter in the constructor? If they do conflict how does this change? SEE DISCUSSION BELOW if I have a function call x = <Type Parameter>method(); // this is a syntax error even inside the function or class ; I must place a this before it, why is this, and does everything still hold true. Why don't I need to prefix anything for the constructor call. Shouldn't Oracle fix this.

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• self.view.frame.size in viewDidLoad and didRotateFromInterfaceOrientation are different

  - by tokentoken

  I created iphone view-based project, and added UINavigationController by code. - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions { TestViewController *vc = [[TestViewController alloc]init]; navController = [[UINavigationController alloc]initWithRootViewController:vc]; [window addSubview:navController.view]; [window makeKeyAndVisible]; return YES; } After that, I checked self.view.frame.size.height in viewDidLoad and didRotateFromInterfaceOrientation, and they're different. (they're 460 and 416) I suppose the height of NavigationBar is not included in viewDidLoad, but should I add it manually (add 44)?

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• window.focus(), self.focus() not working in firefox

  - by Nisanth

  Hi all i am developing a chat application ... i have multiple chat windows ... i want to know which windw contain new message ... i have the following code .. function getCount() { $.ajax({ type: "POST", url: baseUrl + '/Chat/count', data: "chat_id=" + document.ajax.chat_id.value, success: function(msg){ if(msg == 'new1') { self.focus(); //window.focus(); } } }); } If an operator attending both chat.... for example the url is like http://localhost/nisanth/admin/Chat/index/chatId/15 http://localhost/nisanth/admin/Chat/index/chatId/16 http://localhost/nisanth/user/Chat/index/chatId/15 http://localhost/nisanth/user/Chat/index/chatId/16 if the user 16 enter a message i need focus http://localhost/nisanth/admin/Chat/index/chatId/16 This code is work fine with IE but not in firefox...please give me a solution... the above code is in the same html

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• How to generate a client certificate using a third party CA-NOT Self Signed CA

  - by Bryan

  I am trying to trying to export a client certificate for use with a web browser. The goal is to restrict access using the <Location directive to the admin area. I have seen numerous tutorials on using self signed CAs. How would you do this using a third party? 1) Do I need to include the CA in the client pfx if it is a trusted root CA? I have seen both examples. Without CA: openssl pkcs12 -export -inkey KEYFILENAME -in CERTFILEFILENAME -out XXX.pfx With CA: openssl pkcs12 -export -in my.crt- inkey my.key -certfile my.bundle -out my.pfx 2) Do I need to still include SSLCACertificateFile for trusted CA in the httpd.conf setup? SSLVerifyClient none SSLCACertificateFile conf/ssl.crt/ca.crt <Location /secure/area> SSLVerifyClient require SSLVerifyDepth 1 </Location> http://www.modssl.org/docs/2.8/ssl_howto.html#ToC8

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• Getting hierarchy data from a self-referencing table

  - by Emanuil

  Let's say you have the following table: items(item_id, item_parent) ... and it is a self-referencing table as item_parent refers to item_id. What SQL query would you use to SELECT all items in the table along with their depth where the depth of an item is the sum of all parents and grand parents of that item. If the following is the content of the table: item_id item_parent ----------- ----------- 1 0 2 0 3 2 4 2 5 3 ... the query should retrieve the following set of objects: {"item_id":1,"depth":0} {"item_id":2,"depth":0} {"item_id":3,"depth":1} {"item_id":4,"depth":1} {"item_id":5,"depth":2} P.S. I'm looking for a MySQL supported approach.

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• Drupal - no self-linking

  - by pygorex1

  So, I've added several pages to my Drupal install and linking to the pages using URL aliases via the Path module. So far so good - http://mydrupal/node/1 becomes http://mydrupal/about which is exactly what I'm looking for. Then I'm adding these links to the primary navigation and outputting the primary links via a custom theme (copy and paste the primary link code from the Chameleon theme). Here's the problem: when viewing http://mydrupal/about the page links to itself - that is the "About" link at http://mydrupal/about is still linking to http://mydrupal/about - the behavior I'm looking for is to have the "About" text display without a link. In summary: how do I prevent self-linking of pages when displaying the primary navigation?

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• Right way to create [self]respawning app in python

  - by grapescan

  I am using jabber bot written in python to log some MUC talks. Sometimes it drops on some network or XMPP problems. In this case I have to start it again by myself. The goal is to make it "self-respawning". I have some variants about how to do it. Bot is one process. Another process monitors its activity and starts it if bot died. Main process spawns bot subprocess and controls it. Also I think daemonizing bot process is useful here. Platform is Linux, as you could guess. What is the right way to solve this problem?

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• telling java to accept self-signed ssl certificate

  - by Nikita Rybak

  It looks like a standard question, but I couldn't find clear directions anywhere. I have java code trying to connect server with probably self-signed (or expired) certificate. It gives something like this [HttpMethodDirector] I/O exception (javax.net.ssl.SSLHandshakeException) caught when processing request: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target As I understand, I have to play around with keytool and tell java that it's ok to allow this connection. But all comments I've found assume I'm fully proficient with keytool, like "generate private key for server and import it into keystore". And I'm not. Is there anybody who could post detailed instructions? I'm running unix, so bash script would be best. Not sure if it's important, but code executed in jboss. Thanks a lot!

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• how could i make working self-hosted openid provider, connected to wordpress

  - by blog.blomqvist.no

  Aim: To have own OpenId provider connected with self-hosted Wordpress blog, so Wordpress admin could use his OpenId to login to Admin Panel, and also to comment other blogs with this OpenId. Blog address: http://blog.blomqvist.no OpenId provider: /id/ OpenId created: same as blog address WP Plugins used: WP-Yadis 1.2.1 XRDS-Simple 1.0 OpenId 3.3.2 I have installed phpMyId as OpenId provider. I have managed to connect created OpenId and management panel login, so Admin could login to management panel with created OpenId. However, i can't use this OpenId anywhere else, not on blogspot, not on other wordpress blogs etc.

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• Self-extracting Delphi program

  - by Steve

  I'm writing an updater program in Delphi7 which will be run once, but needs many files to run. What I'd like the achieve: 1, User runs exe 2, Exe unpacks files, runs updater 3, If updater detects and error, prompts the user to send log in e-mail 4, After the updater is run, temporary files are deleted (some of these files are dlls used by the updater program, so the updater has to be closed before the files can be deleted) Can anyone recommend a good solution? I've thought about using Inno Setup (too complicated for such an easy task) or using a self-extracting zip file (but how to delete the files afterwards)? Thanks!

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• jQuery: Self Executing Plugin?

  - by GnrlBzik

  I am working on expansion of my jQuery plug-in authoring knowledge, just playing around with my own ideas for learning benefits. So i was wondering how you guys tackle the need of plugin executing without any user specific input. So I have a need for plug-in that executes right away after document is ready, without any user's specific input just as long as doc is loaded, so the only way I see how to execute plug-in on it's own is to attach handler to ready listener that executes my function which I extended the jQuery with. And because this needs to be self enclosed, part of the code, I attach handler to event listener within the function. So how else could one tackle this? Any take at this is appreciated. Thank you in advance everyone.

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• Self signed Certificate Based Authentication

  - by Tadi Vijay Kumar

  Hi All.. I have started a project based on Client server communication in java. What i'm doing is client and server have their own self signed certificates. In initial communication both client and server will exchange their certificates. Server has to verify clients certificate and should grant authentication. Where i struck was on what topic server can validate or invalidate a particular client..? I am unable to proceed from here. So please any of you can give brief explanation about it? if possible some Pseudocode in java..?

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• Self hosted WCF console output from service

  - by user989056

  quick one: Is it possible to capture the output stream of a WCF service that is hosted via ServiceHost ( self hosted service) ? I have methods within my WCF service that output useful debugging information, is it possible to send these to it's host's console output? Edit: It appears that I have made an obvious blunder - I was using Debug instead of Console. It is possible to output to the console by using the standard Console output commands in your WCF service class. I have marked the answer that I have found the most useful.

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• Making self-logging modules with Log::Log4perl

  - by Oesor

  Is there a way to use Log::Log4perl to make a smart self-logging module that logs its operations to a file even in the absence of the calling script not initializing Log4perl? As far as I can tell from the documentation, the only way to use Log4perl is to initialize it in the running script from a configuration, then modules implementing Log4perl calls log themselves based on the caller's Log4perl config. Instead, I'd like the modules to provide a default initialization config for Log4perl. This would provide the default file appender for the module's category. Then, I could override this behavior by initing Log4perl in the caller with a different config if needed, and everything would hopefully just work. Is this sort of defensive logging behavior possible or am I going to need to rely on initing Log4perl in every .pl script that calls the module I want logged?

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• Self Executing jQuery Plugin?

  - by GnrlBzik

  I am working on expansion of my jQuery plug-in authoring knowledge, just playing around with my own ideas for learning benefits. So i was wondering how you guys tackle the need of plugin executing without any user specific input. So I have a need for plug-in that executes right away after document is ready, without any user's specific input just as long as doc is loaded, so the only way I see how to execute plug-in on it's own is to attach handler to ready listener that executes my function which I extended the jQuery with. And because this needs to be self enclosed, part of the code, I attach handler to event listener within the function. So how else could one tackle this? Any take at this is appreciated. Thank you in advance everyone.

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• databinding to self without code

  - by jcollum

  I'm building a UserControl in WPF. The control has two properties, Title and Description, that I'd like to bind to two textblocks. Seems real straightforward, and I got it working, but I'm curious about something. To get it to work I had to add this code: void CommandBlock_Loaded(object sender, RoutedEventArgs e) { this.DataContext = this; } My bindings look like this: <TextBlock Text="{Binding Title}" Width="100" ... /> <TextBlock Text="{Binding Description}" Width="100" ... /> What I'm wondering is how come I couldn't get it to work without the this.DataContext = this; and instead use DataContext="{Binding RelativeSource={RelativeSource Self}}" (in the UserControl element of the markup)? I'm sure I'm missing something about DataContexts but don't know what.

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• What's wrong with my self-defined init method?

  - by user313439

  In ClassA: - (ClassA *)initWithID:(NSString *) cID andTitle:(NSString *) cTitle { ClassAID = cID; ClassATitle = cTitle; return self; } In ClassB: - (void)cellDidSelected { ClassA *classAController = [[ClassA alloc] init]; //Program received signal: “EXC_BAD_ACCESS” when executing the following line. classAController = [classAController initWithClassAID:ClassAID andClassATitle:ClassATitle]; NSLog(@"I want to get the value of ID:%@ and Title:%@ here.", [classAController ClassATitle], [classAController ClassAID]) } Could anyone point where is wrong? Thanks a lot.

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• iPad --- managedObject problem when [self.tableViewSection reloadData]

  - by user345777

  Hi , I am creating a news aggregator application that fetches a xml feed every 4 minutes. After the xml is loaded, I clear everything by deleting all the objects in the database, then save the new objects back into the database. I then call [self.tableViewSection reloadData]; to reload the tableView. Thats when I get a problem. As soon as the code gets to a section where i access the new [[managedObject valueForKey:@"event_text"] description] content it fails with a : * Terminating app due to uncaught exception 'NSObjectInaccessibleException', reason: 'CoreData could not fulfill a fault for '0x4b3f6a0 '' Now from the flow I described on top, it seems odd to me that the tableView is populated properly the next time i launch the application, with the "new" data I fetched in the previous session. So its not that data thats corrupt , but seems to me something goes wrong when I refetch the managedObject after saving new data. Also keep in mind thats this app runs fine on the iPhone, its the iPad thats is giving me this problem.

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• How to self-update PHP+MySQL CMS?

  - by SaltLake

  I'm writing a CMS on PHP+MySQL. I want it to be self-updatable (throw one click in admin panel). What are the best practices? How to compare current version of cms and a version of the update (application itself and database). Should it just download zip archive, upzip it and overwrite files? (but what to do with files that are no longer used). How to check if an update is downloaded correctly? Also it supports modules and I want this modules to be downloadable from the admin panel of cms. And how should I update MySQL tables?

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• Declared Properties and assigning values with self

  - by Shaun Budhram

  I understand how declared properties work - I just need a clarification on when Objective C is using the accessor method vs. when it is not. Say I have a property declared using retain: @property (nonatomic, retain) NSDate *date; ... and later... @synthesize date If I say: date = x Is that calling the accessor method? Or is it just setting the variable? self.date = x This seems to call the accessor method (I think but I'm not sure, since it seems like the retain count is increasing). Can anyone clarify this issue? I'm curious because i have some variables that seem to become invalid before I need them (and I have to specifically call retain), and I suspect this is why.

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• Are free and open source templates, themes, self content management systems replacing the need of we

  - by metal-gear-solid

  Are free and open source templates, themes, self content management systems replacing the need of web designers? You can find templates online for pretty much anything these days. There are html and css templates, templates for Wordpress, all sorts of different cms templates, e-ecommerce templates, etc. Many of these are pretty cheap and some are even free, so where do web designers fit into this picture? I’ve had many clients and potential customers ask why they should use my services instead of just buying a cheap template. Are web designers being replaced?

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• SQL SERVER – Wait Stats – Wait Types – Wait Queues – Day 0 of 28

  - by pinaldave

  This blog post will have running account of the all the blog post I will be doing in this month related to SQL Server Wait Types and Wait Queues. SQL SERVER – Introduction to Wait Stats and Wait Types – Wait Type – Day 1 of 28 SQL SERVER – Signal Wait Time Introduction with Simple Example – Wait Type – Day 2 of 28 SQL SERVER – DMV – sys.dm_os_wait_stats Explanation – Wait Type – Day 3 of 28 SQL SERVER – DMV – sys.dm_os_waiting_tasks and sys.dm_exec_requests – Wait Type – Day 4 of 28 SQL SERVER – Capturing Wait Types and Wait Stats Information at Interval – Wait Type – Day 5 of 28 SQL SERVER – CXPACKET – Parallelism – Usual Solution – Wait Type – Day 6 of 28 SQL SERVER – CXPACKET – Parallelism – Advanced Solution – Wait Type – Day 7 of 28 SQL SERVER – SOS_SCHEDULER_YIELD – Wait Type – Day 8 of 28 SQL SERVER – PAGEIOLATCH_DT, PAGEIOLATCH_EX, PAGEIOLATCH_KP, PAGEIOLATCH_SH, PAGEIOLATCH_UP – Wait Type – Day 9 of 28 SQL SERVER – IO_COMPLETION – Wait Type – Day 10 of 28 SQL SERVER – ASYNC_IO_COMPLETION – Wait Type – Day 11 of 28 SQL SERVER – PAGELATCH_DT, PAGELATCH_EX, PAGELATCH_KP, PAGELATCH_SH, PAGELATCH_UP – Wait Type – Day 12 of 28 SQL SERVER – FT_IFTS_SCHEDULER_IDLE_WAIT – Full Text – Wait Type – Day 13 of 28 SQL SERVER – BACKUPIO, BACKUPBUFFER – Wait Type – Day 14 of 28 SQL SERVER – LCK_M_XXX – Wait Type – Day 15 of 28 Reference: Pinal Dave (http://blog.SQLAuthority.com) Filed under: Pinal Dave, PostADay, SQL, SQL Authority, SQL Optimization, SQL Performance, SQL Query, SQL Server, SQL Tips and Tricks, SQL Wait Stats, SQL Wait Types, T SQL, Technology

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• Confusion for mime files: magic, magic.mgc, magic.mime

  - by Florence Foo

  I'm using Ubuntu. I'm trying to use ruby gem 'shared-mime-info' for an application I'm writing. I understand that magic.mgc is a compiled version of magic file which has magic number definitions for the different file types. BUT I don't understand why is it /usr/share/mime/magic is in binary format instead of just normal text file with each parameters separated by white space like everywhere else I'm finding on the internet when it's referencing this file? The /usr/share/mime/magic has the word 'MIME-Magic' at the beginning of the file and prioritize the rest of the stuff like. So it doesn't look like magic.mgc at all. [100:application/vnd.scribus] >1=^@^KSCRIBUSUTF8 [90:application/vnd.stardivision.writer] >2089=^@ shared-mime-info seems to want a magic file in the binary non compiled format as above and I wanted to add definition for DOCX but how does one update or generate this file without using a hex editor? There is a reference to the magic file I found at: http://standards.freedesktop.org/shared-mime-info-spec/shared-mime-info-spec-latest.html And it mention this file is updated with update-mime-database but what if I just want to add some new entry to it. hex editor? Anyway I ended up using hexer to make a new magic file in ~/.local/share/mime/ with only the entry I wanted to add and the MIME-Magic header. Seems to work (assuming I will ever deal with docx for now). 00000000: 4d 49 4d 45 2d 4d 61 67 69 63 00 0a 5b 36 30 3a MIME-Magic..[60: 00000010: 61 70 70 6c 69 63 61 74 69 6f 6e 2f 76 6e 64 2e application/vnd. 00000020: 6f 70 65 6e 78 6d 6c 66 6f 72 6d 61 74 73 2d 6f openxmlformats-o 00000030: 66 66 69 63 65 64 6f 63 75 6d 65 6e 74 2e 77 6f fficedocument.wo 00000040: 72 64 70 72 6f 63 65 73 73 69 6e 67 6d 6c 2e 64 rdprocessingml.d 00000050: 6f 63 75 6d 65 6e 74 5d 0a 3e 30 3d 00 08 50 4b ocument].>0=..PK 00000060: 03 04 14 00 06 00 0a -- -- -- -- -- -- -- -- -- .......---------

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• Ubuntu Software Center starts, then crashes before fully loaded [closed]

  - by Nathan Weisser

  Possible Duplicate: Software center not opening I am brand new to Linux and Ubuntu, and I couldn't install GIMP without the software center. I looked up earlier how to fix it, and it said to fix my sources list, and I did, but now i get a new error in the terminal. 2012-08-14 15:29:08,941 - softwarecenter.ui.gtk3.app - INFO - setting up proxy 'None' 2012-08-14 15:29:08,954 - softwarecenter.db.database - INFO - open() database: path=None use_axi=True use_agent=True 2012-08-14 15:29:09,407 - softwarecenter.ui.gtk3.app - INFO - building local database 2012-08-14 15:29:09,408 - softwarecenter.db.pkginfo_impl.aptcache - INFO - aptcache.open() 2012-08-14 15:29:17,308 - softwarecenter.db.update - WARNING - Problem creating rebuild path '/var/cache/software-center/xapian_rb'. 2012-08-14 15:29:17,309 - softwarecenter.db.update - WARNING - Please check you have the relevant permissions. 2012-08-14 15:29:17,309 - softwarecenter.db.database - INFO - open() database: path=None use_axi=True use_agent=True 2012-08-14 15:29:18,039 - softwarecenter.backend.reviews - WARNING - Could not get usefulness from server, no username in config file 2012-08-14 15:29:18,431 - softwarecenter.ui.gtk3.app - INFO - show_available_packages: search_text is '', app is None. 2012-08-14 15:29:19,153 - softwarecenter.db.pkginfo_impl.aptcache - INFO - aptcache.open() Traceback (most recent call last): File "/usr/bin/software-center", line 176, in <module> app.run(args) File "/usr/share/software-center/softwarecenter/ui/gtk3/app.py", line 1422, in run self.show_available_packages(args) File "/usr/share/software-center/softwarecenter/ui/gtk3/app.py", line 1352, in show_available_packages self.view_manager.set_active_view(ViewPages.AVAILABLE) File "/usr/share/software-center/softwarecenter/ui/gtk3/session/viewmanager.py", line 154, in set_active_view view_widget.init_view() File "/usr/share/software-center/softwarecenter/ui/gtk3/panes/availablepane.py", line 136, in init_view SoftwarePane.init_view(self) File "/usr/share/software-center/softwarecenter/ui/gtk3/panes/softwarepane.py", line 215, in init_view self.icons, self.show_ratings) File "/usr/share/software-center/softwarecenter/ui/gtk3/views/appview.py", line 69, in __init__ self.helper = AppPropertiesHelper(db, cache, icons) File "/usr/share/software-center/softwarecenter/ui/gtk3/models/appstore2.py", line 109, in __init__ softwarecenter.paths.APP_INSTALL_PATH) File "/usr/share/software-center/softwarecenter/db/categories.py", line 255, in parse_applications_menu category = self._parse_menu_tag(child) File "/usr/share/software-center/softwarecenter/db/categories.py", line 444, in _parse_menu_tag query = self._parse_include_tag(element) File "/usr/share/software-center/softwarecenter/db/categories.py", line 402, in _parse_include_tag xapian.Query.OP_AND) File "/usr/share/software-center/softwarecenter/db/categories.py", line 341, in _parse_and_or_not_tag operator_elem, xapian.Query(), xapian.Query.OP_OR) File "/usr/share/software-center/softwarecenter/db/categories.py", line 385, in _parse_and_or_not_tag q = self.db.xapian_parser.parse_query(s, File "/usr/share/software-center/softwarecenter/db/database.py", line 174, in xapian_parser xapian_parser = self._get_new_xapian_parser() File "/usr/share/software-center/softwarecenter/db/database.py", line 200, in _get_new_xapian_parser xapian_parser.set_database(self.xapiandb) File "/usr/share/software-center/softwarecenter/db/database.py", line 166, in xapiandb self._db_per_thread[thread_name] = self._get_new_xapiandb() File "/usr/share/software-center/softwarecenter/db/database.py", line 179, in _get_new_xapiandb xapiandb = xapian.Database(self._db_pathname) File "/usr/lib/python2.7/dist-packages/xapian/__init__.py", line 3666, in __init__ _xapian.Database_swiginit(self,_xapian.new_Database(*args)) xapian.DatabaseOpeningError: Couldn't detect type of database I'm not sure how to fix the errors, and I couldn't find a topic on them anywhere. Be nice, because I am a two-day old Linux user :/ Tell me if you need my Sources list

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• C# 4.0: Covariance And Contravariance In Generics

  - by Paulo Morgado

  C# 4.0 (and .NET 4.0) introduced covariance and contravariance to generic interfaces and delegates. But what is this variance thing? According to Wikipedia, in multilinear algebra and tensor analysis, covariance and contravariance describe how the quantitative description of certain geometrical or physical entities changes when passing from one coordinate system to another.(*) But what does this have to do with C# or .NET? In type theory, a the type T is greater (>) than type S if S is a subtype (derives from) T, which means that there is a quantitative description for types in a type hierarchy. So, how does covariance and contravariance apply to C# (and .NET) generic types? In C# (and .NET), variance applies to generic type parameters and not to the resulting generic type. A generic type parameter is: covariant if the ordering of the generic types follows the ordering of the generic type parameters: Generic<T> = Generic<S> for T = S. contravariant if the ordering of the generic types is reversed from the ordering of the generic type parameters: Generic<T> = Generic<S> for T = S. invariant if neither of the above apply. If this definition is applied to arrays, we can see that arrays have always been covariant because this is valid code: object[] objectArray = new string[] { "string 1", "string 2" }; objectArray[0] = "string 3"; objectArray[1] = new object(); However, when we try to run this code, the second assignment will throw an ArrayTypeMismatchException. Although the compiler was fooled into thinking this was valid code because an object is being assigned to an element of an array of object, at run time, there is always a type check to guarantee that the runtime type of the definition of the elements of the array is greater or equal to the instance being assigned to the element. In the above example, because the runtime type of the array is array of string, the first assignment of array elements is valid because string = string and the second is invalid because string = object. This leads to the conclusion that, although arrays have always been covariant, they are not safely covariant – code that compiles is not guaranteed to run without errors. In C#, the way to define that a generic type parameter as covariant is using the out generic modifier: public interface IEnumerable<out T> { IEnumerator<T> GetEnumerator(); } public interface IEnumerator<out T> { T Current { get; } bool MoveNext(); } Notice the convenient use the pre-existing out keyword. Besides the benefit of not having to remember a new hypothetic covariant keyword, out is easier to remember because it defines that the generic type parameter can only appear in output positions — read-only properties and method return values. In a similar way, the way to define a type parameter as contravariant is using the in generic modifier: public interface IComparer<in T> { int Compare(T x, T y); } Once again, the use of the pre-existing in keyword makes it easier to remember that the generic type parameter can only be used in input positions — write-only properties and method non ref and non out parameters. Because covariance and contravariance apply only to the generic type parameters, a generic type definition can have both covariant and contravariant generic type parameters in its definition: public delegate TResult Func<in T, out TResult>(T arg); A generic type parameter that is not marked covariant (out) or contravariant (in) is invariant. All the types in the .NET Framework where variance could be applied to its generic type parameters have been modified to take advantage of this new feature. In summary, the rules for variance in C# (and .NET) are: Variance in type parameters are restricted to generic interface and generic delegate types. A generic interface or generic delegate type can have both covariant and contravariant type parameters. Variance applies only to reference types; if you specify a value type for a variant type parameter, that type parameter is invariant for the resulting constructed type. Variance does not apply to delegate combination. That is, given two delegates of types Action<Derived> and Action<Base>, you cannot combine the second delegate with the first although the result would be type safe. Variance allows the second delegate to be assigned to a variable of type Action<Derived>, but delegates can combine only if their types match exactly. If you want to learn more about variance in C# (and .NET), you can always read: Covariance and Contravariance in Generics — MSDN Library Exact rules for variance validity — Eric Lippert Events get a little overhaul in C# 4, Afterward: Effective Events — Chris Burrows Note: Because variance is a feature of .NET 4.0 and not only of C# 4.0, all this also applies to Visual Basic 10.

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