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  • First Project a big one, How much should we charge?

    - by confuzzled
    Two of my cousins and I started a freelance computer repair/web design business just to make some money on the side during college, and received our first major web design project about three weeks ago. Now we've created websites before, but it was mostly for family businesses and have never really charged money, and most of the websites have been static, and don't really require a CMS. This project, however, was a big one (for us anyways). We created a news site that had several categories, we created the banners, we created a classifieds page (not a web app just something static that they control). Several links, a few graphical assets, CSS drop down menu, RSS feed from a different news site, weather, all the normal stuff you would find on a regular news site. On top of that we put in all the usual Joomla stuff (search, Jcomments, Jslide pictures, JCE, etc.). Then we uploaded the first 10 articles they gave us, and we are going to train them how to use Joomla. Now, at first we decided for 700 dollars. I assumed they just wanted a simple blog like website where they can upload articles. But then we had a meeting, and they asked for a lot more. Note: we did not hard code the template from scratch, but customized the gantry framework to fit their needs. We did code quite a bit however. I estimate that we put in about 50-60 hours in total. I'm wondering if 700 dollars is a bit low, this price is definitely not set in stone. Please keep in mind that this is our first project, and we are newbies, please be kind. Thank You!

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  • What do you wish language designers paid attention to?

    - by Berin Loritsch
    The purpose of this question is not to assemble a laundry list of programming language features that you can't live without, or wish was in your main language of choice. The purpose of this question is to bring to light corners of languge design most language designers might not think about. So, instead of thinking about language feature X, think a little more philisophically. One of my biases, and perhaps it might be controversial, is that the softer side of engineering--the whys and what fors--are many times more important than the more concrete side. For example, Ruby was designed with a stated goal of improving developer happiness. While your opinions may be mixed on whether it delivered or not, the fact that was a goal means that some of the choices in language design were influenced by that philosophy. Please do not post: Syntax flame wars (I could care less whether you use whitespace [Python], keywords [Ruby], or curly braces [Java, C/C++, et. al.] to denote program blocks). That's just an implementation detail. "Any language that doesn't have feature X doesn't deserve to exist" type comments. There is at least one reason for all programming languages to exist--good or bad. Please do post: Philisophical ideas that language designers seem to miss. Technical concepts that seem to be poorly implemented more often than not. Please do provide an example of the pain it causes and if you have any ideas of how you would prefer it to function. Things you wish were in the platform's common library but seldom are. One the same token, things that usually are in a common library that you wish were not. Conceptual features such as built in test/assertion/contract/error handling support that you wish all programming languages would implement properly--and define properly. My hope is that this will be a fun and stimulating topic.

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  • Making money from a custom built interpreter?

    - by annoying_squid
    I have been making considerable progress lately on building an interpreter. I am building it from NASM assembly code (for the core engine) and C (cl.exe the Microsoft compiler for the parser). I really don't have a lot of time but I have a lot of good ideas on how to build this so it appeals to a certain niche market. I'd love to finish this but I need to face reality here ... unless I can make some good monetary return on my investment, there is not a lot of time for me to invest. So I ask the following questions to anyone out there, especially those who have experience in monetizing their programs: 1) How easy is it for a programmer to make good money from one design? (I know this is vague but it will be interesting to hear from those who have experience or know of others' experiences). 2) What are the biggest obstacles to making money from a programming design? 3) For the parser, I am using the Microsoft compiler (no IDE) I got from visual express, so will this be an issue? Will I have to pay royalties or a license fee? 4) As far as I know NASM is a 2-clause BSD licensed application. So this should allow me to use NASM for commericial development unless I am missing something? It's good to know these things before launching into the meat and potatoes of the project.

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  • Testing of visualization projects

    - by paxRoman
    We develop small to large visualization projects for different tasks and industries and sometimes while rewriting them a couple of times in the process we hit walls because we discover that we need to add a lot of code to support new requirements. Now we have established a design process that seems to work well (at least we reduced the development time for each new project quite a bit), but we're still left scratching our heads around this question: what exactly should we test when testing visualizations? If everything that we want to explore is on the screen (bounded visualizations)? If the data is ok - if data is valid (that's one of the nice things about visualizations you can spot errors in your datasets)? Usability? User interaction? Code quality? I can tell you for sure that a simple check of the code quality is certainly not enough! Is there a classic paper / book about how to test visualizations? Also do you happen to know about classic design patterns for visualizations (except the obvious ones like Pub-Sub)?

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  • Is there a canonical source supporting "all-surrogates"?

    - by user61852
    Background The "all-PK-must-be-surrogates" approach is not present in Codd's Relational Model or any SQL Standard (ANSI, ISO or other). Canonical books seems to elude this restrictions too. Oracle's own data dictionary scheme uses natural keys in some tables and surrogate keys in other tables. I mention this because these people must know a thing or two about RDBMS design. PPDM (Professional Petroleum Data Management Association) recommend the same canonical books do: Use surrogate keys as primary keys when: There are no natural or business keys Natural or business keys are bad ( change often ) The value of natural or business key is not known at the time of inserting record Multicolumn natural keys ( usually several FK ) exceed three columns, which makes joins too verbose. Also I have not found canonical source that says natural keys need to be immutable. All I find is that they need to be very estable, i.e need to be changed only in very rare ocassions, if ever. I mention PPDM because these people must know a thing or two about RDBMS design too. The origins of the "all-surrogates" approach seems to come from recommendations from some ORM frameworks. It's true that the approach allows for rapid database modeling by not having to do much business analysis, but at the expense of maintainability and readability of the SQL code. Much prevision is made for something that may or may not happen in the future ( the natural PK changed so we will have to use the RDBMS cascade update funtionality ) at the expense of day-to-day task like having to join more tables in every query and having to write code for importing data between databases, an otherwise very strightfoward procedure (due to the need to avoid PK colisions and having to create stage/equivalence tables beforehand ). Other argument is that indexes based on integers are faster, but that has to be supported with benchmarks. Obviously, long, varying varchars are not good for PK. But indexes based on short, fix-length varchar are almost as fast as integers. The questions - Is there any canonical source that supports the "all-PK-must-be-surrogates" approach ? - Has Codd's relational model been superceded by a newer relational model ?

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  • Web Services and code lists

    - by 0x0me
    Our team heavily discuss the issues how to handle code list in a web service definition. The design goal is to describe a provider API to query a system using various values. Some of them are catalogs resp. code lists. A catalog or code list is a set of key value pairs. There are different systems (at least 3) maintaining possibly different code lists. Each system should implement the provider API, whereas each system might have different code list for the same business entity eg. think of colors. One system know [(1,'red'),(2,'green')] and another one knows [(1,'lightgreen'),(2,'darkgreen'),(3,'red')] etc. The access to the different provider API implementations will be encapsulated by a query service, but there is already one candidate which might use at least one provider API directly. The current options to design the API discussed are: use an abstract code list in the interface definition: the web service interface defines a well known set of code list which are expected to be used for querying and returning data. Each API provider implementation has to mapped the request and response values from those abstract codelist to the system specific one. let the query component handle the code list: the encapsulating query service knows the code list set of each provider API implementation and takes care of mapping the input and output to the system specific code lists of the queried system. do not use code lists in the query definition at all: Just query code lists by a plain string and let the provider API implementation figure out the right value. This might lead to a loose of information and possibly many false positives, due to the fact that the input string could not be canonical mapped to a code list value (eg. green - lightgreen or green - darkgreen or both) What are your experiences resp. solutions to such a problem? Could you give any recommendation?

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  • Looking for a better Factory pattern (Java)

    - by Sam Goldberg
    After doing a rough sketch of a high level object model, I am doing iterative TDD, and letting the other objects emerge as a refactoring of the code (as it increases in complexity). (That whole approach may be a discussion/argument for another day.) In any case, I am at the point where I am looking to refactor code blocks currently in an if-else blocks into separate objects. This is because there is another another value combination which creates new set of logical sub-branches. To be more specific, this is a trading system feature, where buy orders have different behavior than sell orders. Responses to the orders have a numeric indicator field which describes some event that occurred (e.g. fill, cancel). The combination of this numeric indicator field plus whether it is a buy or sell, require different processing buy the code. Creating a family of objects to separate the code for the unique handling each of the combinations of the 2 fields seems like a good choice at this point. The way I would normally do this, is to create some Factory object which when called with the 2 relevant parameters (indicator, buysell), would return the correct subclass of the object. Some times I do this pattern with a map, which allows to look up a live instance (or constructor to use via reflection), and sometimes I just hard code the cases in the Factory class. So - for some reason this feels like not good design (e.g. one object which knows all the subclasses of an interface or parent object), and a bit clumsy. Is there a better pattern for solving this kind of problem? And if this factory method approach makes sense, can anyone suggest a nicer design?

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  • Using visitor pattern with large object hierarchy

    - by T. Fabre
    Context I've been using with a hierarchy of objects (an expression tree) a "pseudo" visitor pattern (pseudo, as in it does not use double dispatch) : public interface MyInterface { void Accept(SomeClass operationClass); } public class MyImpl : MyInterface { public void Accept(SomeClass operationClass) { operationClass.DoSomething(); operationClass.DoSomethingElse(); // ... and so on ... } } This design was, however questionnable, pretty comfortable since the number of implementations of MyInterface is significant (~50 or more) and I didn't need to add extra operations. Each implementation is unique (it's a different expression or operator), and some are composites (ie, operator nodes that will contain other operator/leaf nodes). Traversal is currently performed by calling the Accept operation on the root node of the tree, which in turns calls Accept on each of its child nodes, which in turn... and so on... But the time has come where I need to add a new operation, such as pretty printing : public class MyImpl : MyInterface { // Property does not come from MyInterface public string SomeProperty { get; set; } public void Accept(SomeClass operationClass) { operationClass.DoSomething(); operationClass.DoSomethingElse(); // ... and so on ... } public void Accept(SomePrettyPrinter printer) { printer.PrettyPrint(this.SomeProperty); } } I basically see two options : Keep the same design, adding a new method for my operation to each derived class, at the expense of maintainibility (not an option, IMHO) Use the "true" Visitor pattern, at the expense of extensibility (not an option, as I expect to have more implementations coming along the way...), with about 50+ overloads of the Visit method, each one matching a specific implementation ? Question Would you recommand using the Visitor pattern ? Is there any other pattern that could help solve this issue ?

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  • Modular Architecture for Processing Pipeline

    - by anjruu
    I am trying to design the architecture of a system that I will be implementing in C++, and I was wondering if people could think of a good approach, or critique the approach that I have designed so far. First of all, the general problem is an image processing pipeline. It contains several stages, and the goal is to design a highly modular solution, so that any of the stages can be easily swapped out and replaced with a piece of custom code (so that the user can have a speed increase if s/he knows that a certain stage is constrained in a certain way in his or her problem). The current thinking is something like this: struct output; /*Contains the output values from the pipeline.*/ class input_routines{ public: virtual foo stage1(...){...} virtual bar stage2(...){...} virtual qux stage3(...){...} ... } output pipeline(input_routines stages); This would allow people to subclass input_routines and override whichever stage they wanted. That said, I've worked in systems like this before, and I find the subclassing and the default stuff tends to get messy, and can be difficult to use, so I'm not giddy about writing one myself. I was also thinking about a more STLish approach, where the different stages (there are 6 or 7) would be defaulted template parameters. Can anyone offer a critique of the pattern above, thoughts on the template approach, or any other architecture that comes to mind?

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  • ASP.NET Webforms developers and web designers: how to interact?

    - by just_name
    I'm an ASP.NET Webforms developer, and I face some problems when I deal with designers. Designers always complain about the asp.net server controls. They'd rather just have an html file and create css files along with the required images to go with those. Sometimes, if the design phase is done in advance, I get html files with related css files, but then we face many problems integrating the design with the aspx files (sever controls an telerik controls ... etc). What I want to ask about is: How do I overcome these problems? The designers prefer php- and mvc developers because of the problems with .net server controls. I need to know how to interact with the designers in the correct way. Are there any tools or applications to provide the designers with the rendered (html page) of the .aspx pages? By that I mean the page in runtime rather than the aspx in Visual Studio. They do use Web Expression but they want the rendered page in html as well.

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  • Should Android and iPhone UI be different?

    - by Phonon
    I'm not completely new to developing apps, but I'm at a point where I'm trying to develop something and deploy it on several mobile platforms. To only concentrate on two major ones, suppose I'm developing an app for Android and iPhone and designing UI and the general user interaction architecture. Both platforms give guidelines as to how their UIs should work. For example, most iPhone apps have the Navigation Bar (the one that says Testing 1 and has a Back button) and an Icon Bar for navigating a program, while Android uses an Options Menu fetched via a Menu button and the "back" navigation is handled with the physical Back button on the device. I've seen many apps that try to force the same UI on every platform. For example, custom-building an iPhone style Icon Bar and putting it in their Android apps, but it just doesn't quite look right to me and it feels like it violates UI design guidelines somewhat. Are there any good design patters for implementing something sufficiently similar on both platforms, yet still platform-specific enough so that the user would not feel out of their comfort zone? What do people usually do in these situations?

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  • Object desing problem for simple school application

    - by Aragornx
    I want to create simple school application that provides grades,notes,presence,etc. for students,teachers and parents. I'm trying to design objects for this problem and I'm little bit confused - because I'm not very experienced in class designing. Some of my present objects are : class PersonalData() { private String name; private String surename; private Calendar dateOfBirth; [...] } class Person { private PersonalData personalData; } class User extends Person { private String login; private char[] password; } class Student extends Person { private ArrayList<Counselor> counselors = new ArrayList<>(); } class Counselor extends Person { private ArrayList<Student> children = new ArrayList<>(); } class Teacher extends Person { private ArrayList<ChoolClass> schoolClasses = new ArrayList<>(); private ArrayList<Subject> subjects = new ArrayList<>(); } This is of course a general idea. But I'm sure it's not the best way. For example I want that one person could be a Teacher and also a Parent(Counselor) and present approach makes me to have two Person objects. I want that user after successful logging in get all roles that it has (Student or Teacher or (Teacher & Parent) ). I think I should make and use some interfaces but I'm not sure how to do this right. Maybe like this: interface Role { } interface TeacherRole implements Role { void addGrade( Student student, Grade grade, [...] ); } class Teacher implements TeacherRole { private Person person; [...] } class User extends Person{ ArrayList<Role> roles = new ArrayList<>(); } Please if anyone could help me to make this right or maybe just point me to some literature/article that covers practical objects design.

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  • How to correctly write an installation or setup document

    - by UmNyobe
    I just joined a small start-up as a software engineer after graduation. The start-up is 4 year old, and I am working with the CEO and the COO, even if there are some people abroad. Basically they both used to do almost everything. I am currently on some kind of training phase. I have at my disposition architecture, setup and installation internal documentation. Architecture documentation is like a bible and should contain complete information. The rest are used to give directions in different processes. The issue is that these documents are more or less dated, as they just didn't have the time to change them. I will be in charge of training the next hires, and updating these documents is part of my training. In some there is a lot of hard-coded information like: Install this_module_which_still_exists cd this_dir_name_changed cp this_file_name_changed other_dir_name_changed ./config_script.sh ./execute_script.sh The issues i have faced : Either the module installation is completely different (for instance now there is an rpm, or a different OS) Either names changed, and i need to switch old names by new names Description of the purpose of the current step missing. Information about a whole topic is missing Fortunately these guys are around and I get all the information I want and all the explanations I need. I want to bring a design to the next documents so in the future people don't feel like they are completely rewriting a document each time they are updating it. Do you have suggestions? If there is a lightweight design methodology available online you can point me to it's nice too. One thing I will do for sure is set up a versioning repository for the documents alone. There is already one for the source code so I don't know why internal documents deserve a different treatment.

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  • How should I structure the implementation of turn-based board game rules?

    - by Setzer22
    I'm trying to create a turn-based strategy game on a tilemap. I'm using design by component so far, but I can't find a nice way to fit components into the part I want to ask. I'm struggling with the "game rules" logic. That is, the code that displays the menu, allows the player to select units, and command them, then tells the unit game objects what to do given the player input. The best way I could thing of handling this was using a big state machine, so everything that could be done in a "turn" is handled by this state machine, and the update code of this state machine does different things depending on the state. However, this approach leads to a large amount of code (anything not model-related) going into a big class. Of course I can subdivide this big class into more classes, but it doesn't feel modular and upgradable enough. I'd like to know of better systems to handle this in order to be able to upgrade the game with new rules without having a monstruous if/else chain (or switch / case, for that matter). Any ideas? What specific design pattern other than MVC should I be using?

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  • Help with Abstract Factory Pattern

    - by brazc0re
    I need help with a abstract factory pattern design. This question is a continuation of: Design help with parallel process I am really confused where I should be initializing all of the settings for each type of medium (ex: RS232, TCP/IP, etc). Attached is the drawing on how I am setting up the pattern: As shown, when a medium is created, each medium imposes a ICreateMedium interface. I would assume that the Create() method also create the proper object, such as SerialPort serialPort = new SerialPort("COM1", baud); however, TCPIPMedium would have an issue with the interface because it wouldn't need to initialize a serial port object. I know I am doing something majorly wrong here. I just can't figure it out and have been stuck for a while. What I also get confused on show the interface IMedium will get access to the communication object once it is created so it can write out the appropriate byte[] packet. Any guidance would be greatly appreciated. My main goal is to have the Communicator class spit a packet out without caring which type of medium is active.

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  • .net developers and web designers: how to interact?

    - by just_name
    I'm an asp.net developer, and I face some problems when I deal with designers. The designer always complains about the asp.net server controls. They rather just have an html file and create css files along with the required images to go with those. Sometimes if the design phase is done in advance I get html files with related css files, but then we face many problems integrating the design with the aspx files (sever controls an telerik controls ... etc). What I want to ask about is: How to overcome these problems? The designers prefer php- and mvc developers because of the problems with .net server controls. I need to know how to interact with the designers in a correct way. Are there any tools or applications to provide the designers with the rendered (html page) of the .aspx pages? By that I mean the page in runtime rather than the aspx in visual studio. They do use Web Expression but they want the rendered page in html as well.

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  • Solid principles vs YAGNI

    - by KeesDijk
    When do the SOLID principles become YAGNI. As programmers we make trade-off's all the time, between complexity, maintainabillity, time to build and so forth.Amongst others. two of the smartest guidelines for making choices are in my mind the SOLID principles and YAGNI. If you don't need it, dont build it and keep it clean. Now for example when i watch the dimecast series on SOLID I see it start out as a fairly simple programm and ending up prettty complex (end yes complexity is also in the eye of the beholder) but it still makes me wonder, when do SOLID principles turn into something you don't need. All solid principles are ways of working that enable use te make changes at a later stage. But what if the problem to solve is a pretty simple one and it's a through away application, than what ? Or are the SOLID principles something that apply always ?

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  • Open source level editor for HTML5 platform game?

    - by Lai Yu-Hsuan
    A natty GUI editor is very helpful to create level map. I want to use some open-source choices rather than build my own from scratch. I found Tiled Map Editor but it doesn't work for what I want. Though I'm building HTML5 game, I don't have to use a HTML5 level editor as long as it can output well-formatted map files which my javascript can read. Edit: Sorry for the confusion. Tiled does not work for me because to make the player perform a 'tricky' jump, sometimes I want to set the distance between two platforms to, say, 7/3 or 8/3 tiles. But in Tiled I get only 2 or 3. If Tiled can do this, please teach me.

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  • NoSQL as file meta database

    - by fga
    I am trying to implement a virtual file system structure in front of an object storage (Openstack). For availability reasons we initially chose Cassandra, however while designing file system data model, it looked like a tree structure similar to a relational model. Here is the dilemma for availability and partition tolerance we need NoSQL, but our data model is relational. The intended file system must be able to handle filtered search based on date, name etc. as fast as possible. So what path should i take? Stick to relational with some indexing mechanism backed by 3 rd tools like Apache Solr or dig deeper into NoSQL and find a suitable model and database satisfying the model? P.S: Currently from NoSQL Cassandra or MongoDB are choices proposed by my colleagues.

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  • Solid principles vs YAGNI

    - by KeesDijk
    When do the SOLID principles become YAGNI? As programmers we make trade-offs all the time, between complexity, maintainability, time to build and so forth. Amongst others, two of the smartest guidelines for making choices are in my mind the SOLID principles and YAGNI. If you don't need it; don't build it, and keep it clean. Now for example, when I watch the dimecast series on SOLID, I see it starts out as a fairly simple program, and ends up as a pretty complex one (end yes complexity is also in the eye of the beholder), but it still makes me wonder: when do SOLID principles turn into something you don't need? All solid principles are ways of working that enable use to make changes at a later stage. But what if the problem to solve is a pretty simple one and it's a throwaway application, then what? Or are the SOLID principles something that always apply? As asked in the comments: Solid Principles YAGNI

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  • Is there any danger in committing to a component library such as SmartGwt or Swing?

    - by Banang
    Since February this year I have been working on an app that's built using SmartGWT components. Generally, I find the components very nice to work with, and the fact that they're open source and free to use is just fantastic. However, I can't seem to shake the feeling that it's not a durable way of developing, but I can't quite explain why. Maybe it's because I know that any minute now the team developing it could decide to stop, which would leave me and my team in a bit of a pickle, but I'm sure there must be something more. I have been trying to find ways of explaining this feeling to myself, but to no avail. Therefore I turn to you, dear community, to ask if you can come up with a good reason why committing to building your app (that's supposed to be around for many more years to come) using a component library such as SmartGWT is a bad idea? Is there any reason I should just have developed the components myself? Or did I make the right choice when deciding not to reinvent the wheel and just go for what was readily available?

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  • How to solve cyclic dependencies in a visitor pattern

    - by Benjamin Rogge
    When programming at work we now and then face a problem with visitors and module/project dependencies. Say you have a class A in a module X. And there are subclasses B and C in module Y. That means that module Y is dependent on module X. If we want to implement a visitor pattern to the class hierarchy, thus introducing an interface with the handle Operations and an abstract accept method in A, we get a dependency from module Y to module X, which we cannot allow for architectural reasons. What we do is, use a direct comparison of the types (i.e. instanceof, since we program in Java), which is not satisfying. My question(s) would be: Do you encounter this kind of problem in your daily work (or do we make poor architectural choices) and if so, how is your approach to solve this?

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  • User Control as container

    - by Luca
    I'm designing a simple expander control. I've derived from UserControl, drawn inner controls, built, run; all ok. Since an inner Control is a Panel, I'd like to use it as container at design time. Indeed I've used the attributes: [Designer(typeof(ExpanderControlDesigner))] [Designer("System.Windows.Forms.Design.ParentControlDesigner, System.Design", typeof(IDesigner))] Great I say. But it isn't... The result is that I can use it as container at design time but: The added controls go back the inner controls already embedded in the user control Even if I push to top a control added at design time, at runtime it is back again on controls embedded to the user control I cannot restrict the container area at design time into a Panel area What am I missing? Here is the code for completeness... why this snippet of code is not working? [Designer(typeof(ExpanderControlDesigner))] [Designer("System.Windows.Forms.Design.ParentControlDesigner, System.Design", typeof(IDesigner))] public partial class ExpanderControl : UserControl { public ExpanderControl() { InitializeComponent(); .... [System.Security.Permissions.PermissionSet(System.Security.Permissions.SecurityAction.Demand, Name = "FullTrust")] internal class ExpanderControlDesigner : ControlDesigner { private ExpanderControl MyControl; public override void Initialize(IComponent component) { base.Initialize(component); MyControl = (ExpanderControl)component; // Hook up events ISelectionService s = (ISelectionService)GetService(typeof(ISelectionService)); IComponentChangeService c = (IComponentChangeService)GetService(typeof(IComponentChangeService)); s.SelectionChanged += new EventHandler(OnSelectionChanged); c.ComponentRemoving += new ComponentEventHandler(OnComponentRemoving); } private void OnSelectionChanged(object sender, System.EventArgs e) { } private void OnComponentRemoving(object sender, ComponentEventArgs e) { } protected override void Dispose(bool disposing) { ISelectionService s = (ISelectionService)GetService(typeof(ISelectionService)); IComponentChangeService c = (IComponentChangeService)GetService(typeof(IComponentChangeService)); // Unhook events s.SelectionChanged -= new EventHandler(OnSelectionChanged); c.ComponentRemoving -= new ComponentEventHandler(OnComponentRemoving); base.Dispose(disposing); } public override System.ComponentModel.Design.DesignerVerbCollection Verbs { get { DesignerVerbCollection v = new DesignerVerbCollection(); v.Add(new DesignerVerb("&asd", new EventHandler(null))); return v; } } } I've found many resources (Interaction, designed, limited area), but nothing was usefull for being operative...

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  • How to prevent users from inputting letters or numbers ?

    - by ZaZu
    Hello, I have a simple problem; Here is the code : #include<stdio.h> main(){ int input; printf("Choose a numeric value"); scanf("%d",&input); } I want the user to only enter numbers ... So it has to be something like this : #include<stdio.h> main(){ int input; printf("Choose a numeric value"); do{ scanf("%d",&input); }while(input!= 'something'); } My problem is that I dont know what to replace in 'something' ... How can I prevent users from inputting alphabetic characters ? Thanks for your help ! }

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  • Plagued by multithreaded bugs

    - by koncurrency
    On my new team that I manage, the majority of our code is platform, TCP socket, and http networking code. All C++. Most of it originated from other developers that have left the team. The current developers on the team are very smart, but mostly junior in terms of experience. Our biggest problem: multi-threaded concurrency bugs. Most of our class libraries are written to be asynchronous by use of some thread pool classes. Methods on the class libraries often enqueue long running taks onto the thread pool from one thread and then the callback methods of that class get invoked on a different thread. As a result, we have a lot of edge case bugs involving incorrect threading assumptions. This results in subtle bugs that go beyond just having critical sections and locks to guard against concurrency issues. What makes these problems even harder is that the attempts to fix are often incorrect. Some mistakes I've observed the team attempting (or within the legacy code itself) includes something like the following: Common mistake #1 - Fixing concurrency issue by just put a lock around the shared data, but forgetting about what happens when methods don't get called in an expected order. Here's a very simple example: void Foo::OnHttpRequestComplete(statuscode status) { m_pBar->DoSomethingImportant(status); } void Foo::Shutdown() { m_pBar->Cleanup(); delete m_pBar; m_pBar=nullptr; } So now we have a bug in which Shutdown could get called while OnHttpNetworkRequestComplete is occuring on. A tester finds the bug, captures the crash dump, and assigns the bug to a developer. He in turn fixes the bug like this. void Foo::OnHttpRequestComplete(statuscode status) { AutoLock lock(m_cs); m_pBar->DoSomethingImportant(status); } void Foo::Shutdown() { AutoLock lock(m_cs); m_pBar->Cleanup(); delete m_pBar; m_pBar=nullptr; } The above fix looks good until you realize there's an even more subtle edge case. What happens if Shutdown gets called before OnHttpRequestComplete gets called back? The real world examples my team has are even more complex, and the edge cases are even harder to spot during the code review process. Common Mistake #2 - fixing deadlock issues by blindly exiting the lock, wait for the other thread to finish, then re-enter the lock - but without handling the case that the object just got updated by the other thread! Common Mistake #3 - Even though the objects are reference counted, the shutdown sequence "releases" it's pointer. But forgets to wait for the thread that is still running to release it's instance. As such, components are shutdown cleanly, then spurious or late callbacks are invoked on an object in an state not expecting any more calls. There are other edge cases, but the bottom line is this: Multithreaded programming is just plain hard, even for smart people. As I catch these mistakes, I spend time discussing the errors with each developer on developing a more appropriate fix. But I suspect they are often confused on how to solve each issue because of the enormous amount of legacy code that the "right" fix will involve touching. We're going to be shipping soon, and I'm sure the patches we're applying will hold for the upcoming release. Afterwards, we're going to have some time to improve the code base and refactor where needed. We won't have time to just re-write everything. And the majority of the code isn't all that bad. But I'm looking to refactor code such that threading issues can be avoided altogether. One approach I am considering is this. For each significant platform feature, have a dedicated single thread where all events and network callbacks get marshalled onto. Similar to COM apartment threading in Windows with use of a message loop. Long blocking operations could still get dispatched to a work pool thread, but the completion callback is invoked on on the component's thread. Components could possibly even share the same thread. Then all the class libraries running inside the thread can be written under the assumption of a single threaded world. Before I go down that path, I am also very interested if there are other standard techniques or design patterns for dealing with multithreaded issues. And I have to emphasize - something beyond a book that describes the basics of mutexes and semaphores. What do you think? I am also interested in any other approaches to take towards a refactoring process. Including any of the following: Literature or papers on design patterns around threads. Something beyond an introduction to mutexes and semaphores. We don't need massive parallelism either, just ways to design an object model so as to handle asynchronous events from other threads correctly. Ways to diagram the threading of various components, so that it will be easy to study and evolve solutions for. (That is, a UML equivalent for discussing threads across objects and classes) Educating your development team on the issues with multithreaded code. What would you do?

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