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• Recommended integration mechanism for bi-directional, authenticated, encrypted connection in C clien

  - by rcampbell

  Let me first give an example. Imagine you have a single server running a JVM application. This server keeps a collection of N equations, once for each client: Client #1: 2x Client #2: 1 + y Client #3: z/4 This server includes an HTTP interface so that random visitors can type https://www.acme.com/client/3 int their browsers and see the latest evaluated result of z/4. The tricky part is that either the client or the server may change the variable value at any time, informing the other party immediately. More specifically, Client #3 - a C app - can initially tell the server that z = 20. An hour later that same client informs the server that z = 23. Likewise the server can later inform the client that z = 28. As caf pointed out in the comments, there can be a race condition when values are changed by the client and server simultaneously. The solution would be for both client and server to send the operation performed in their message, which would need to be executed by the other party. To keep things simple, let's limit the operations to (commutative) addition, allowing us to disregard message ordering. For example, the client seeds the server with z = 20: server:z=20, client:z=20 server sends {+3} message (so z=23 locally) & client sends {-2} message (so z=18 locally) at the exact same time server receives {-2} message at some point, adds to his local copy so z=21 client receives {+3} message at some point, adds to his local copy so z=21 As long as all messages are eventually evaluated by both parties, the correct answer will eventually be given to the users of the client and server since we limited ourselves to commutative operations (addition of 3 and -2). This does mean that both client and server can be returning incorrect answers in the time it takes for messages to be exchanged and processed. While undesirable, I believe this is unavoidable. Some possible implementations of this idea include: Open an encrypted, always on TCP socket connection for communication Pros: no additional infrastructure needed, client and server know immediately if there is a problem (disconnect) with the other party, fairly straightforward (except the the encryption), native support from both JVM and C platforms Cons: pretty low-level so you end up writing a lot yourself (protocol, delivery verification, retry-on-failure logic), probably have a lot of firewall headaches during client app installation Asynchronous messaging (ex: ActiveMQ) Pros: transactional, both C & Java integration, free up the client and server apps from needing retry logic or delivery verification, pretty straightforward encryption, easy extensibility via message filters/routers/etc Cons: need additional infrastructure (message server) which must never fail, Database or file system as asynchronous integration point Same pros/cons as above but messier RESTful Web Service Pros: simple, possible reuse of the server's existing REST API, SSL figures out the encryption problem for you (maybe use RSA key a la GitHub for authentication?) Cons: Client now needs to run a C HTTP REST server w/SSL, client and server need retry logic. Axis2 has both a Java and C version, but you may be limited to SOAP. What other techniques should I be evaluating? What real world experiences have you had with these mechanisms? Which do you recommend for this problem and why?

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• Prefer extension methods for encapsulation and reusability?

  - by tzaman

  edit4: wikified, since this seems to have morphed more into a discussion than a specific question. 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. (edit: Even in the current .NET library, I can see places where it would've been useful to have extensions instead of instance methods - for example, all of the utility functions of List<T> (Sort, BinarySearch, FindIndex, etc.) would be incredibly useful if they were lifted up to IList<T> - getting free bonus functionality like that adds a lot more benefit to implementing the interface.) So what's the verdict? Are extension methods the acme of encapsulation and code reuse, or am I just deluding myself? (edit2: In response to Tomas - while C# did start out with Java's (overly, imo) OO mentality, it seems to be embracing more multi-paradigm programming with every new release; the main thrust of this question is whether using extension methods to drive a style change (towards more generic / functional C#) is useful or worthwhile..) edit3: overridable extension methods The only real problem identified so far with this approach, is that you can't specialize extension methods if you need to. I've been thinking about the issue, and I think I've come up with a solution. Suppose I have an interface MyInterface, which I want to extend - I define my extension methods in a MyExtension static class, and pair it with another interface, call it MyExtensionOverrider. MyExtension methods are defined according to this pattern: public static int MyMethod(this MyInterface obj, int arg, bool attemptCast=true) { if (attemptCast && obj is MyExtensionOverrider) { return ((MyExtensionOverrider)obj).MyMethod(arg); } // regular implementation here } The override interface mirrors all of the methods defined in MyExtension, except without the this or attemptCast parameters: public interface MyExtensionOverrider { int MyMethod(int arg); string MyOtherMethod(); } Now, any class can implement the interface and get the default extension functionality: public class MyClass : MyInterface { ... } Anyone that wants to override it with specific implementations can additionally implement the override interface: public class MySpecializedClass : MyInterface, MyExtensionOverrider { public int MyMethod(int arg) { //specialized implementation for one method } public string MyOtherMethod() { // fallback to default for others MyExtension.MyOtherMethod(this, attemptCast: false); } } And there we go: extension methods provided on an interface, with the option of complete extensibility if needed. Fully general too, the interface itself doesn't need to know about the extension / override, and multiple extension / override pairs can be implemented without interfering with each other. I can see three problems with this approach - It's a little bit fragile - the extension methods and override interface have to be kept synchronized manually. It's a little bit ugly - implementing the override interface involves boilerplate for every function you don't want to specialize. It's a little bit slow - there's an extra bool comparison and cast attempt added to the mainline of every method. Still, all those notwithstanding, I think this is the best we can get until there's language support for interface functions. Thoughts?

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• Usercontrol databinding within a databound datagridview

  - by user328259

  Good day. I'm developing a Windows application and working with Windows Forms; .Net 2.0. I have an issue databinding a generic List of Car Rental Companies to a DataGridView when that list contains (one of its properties) anoother generic List of Car Makes. I also have a UserControl that I need to bind to this [inner] generic list ... Class CarRentalCompany contains: string Name, string Location, List CarMakes Class CarMake contains: string Name, bool isFord, bool isChevy, bool isOther The UserControl has a label for CarMake.Name and 3 checkboxes for each of the booleans of the class. HOw do I make this user control bindable for the class? In my form, I have a DataGridView binded to the CarRentalCompany object. The list CarMakes could be 0 or more items and I can add these as necessary. How do I establish the binding of CarRentalCompanies properly so CarMakes will bind accordingly?? For example, I have: List CarRentalCompanies = new List(); CarRentalCompany company1 = new CarRentalCompany(); company1.Name = "Acme Rentals"; company1.Location = "New York, NY"; company1.CarMakes = new List<CarMake>(); CarMake car1 = new CarMake(); car1.Name = "The Yellow Car"; car1.isFord = true; car1.isChevy = false; car1.isOther = false; company1.CarMakes.Add(car1); CarMake car2 = new CarMake(); car2.Name = "The Blue Car"; car2.isFord = false; car2.isChevy = true; car2.isOther = false; company1.CarMakes.Add(car2); CarMake car3 = new CarMake(); car3.Name = "The Purple Car"; car3.isFord = false; car3.isChevy = false; car3.isOther = true; company1.CarMakes.Add(car3); CarRentalCompanies.Add(company1); CarRentalCompany company2 = new CarRentalCompany(); company1.Name = "Z-Auto Rentals"; company1.Location = "Phoenix, AZ"; company1.CarMakes = new List<CarMake>(); CarMake car4 = new CarMake(); car4.Name = "The OrangeCar"; car4.isFord = true; car4.isChevy = false; car4.isOther = false; company2.CarMakes.Add(car4); CarMake car5 = new CarMake(); car5.Name = "The Red Car"; car5.isFord = true; car5.isChevy = false; car5.isOther = false; company2.CarMakes.Add(car5); CarMake car6 = new CarMake(); car6.Name = "The Green Car"; car6.isFord = true; car6.isChevy = false; car6.isOther = false; company2.CarMakes.Add(car6); CarRentalCompanies.Add(company2); I load my form and in my load form I have the following: Note: CarDataGrid is a DataGridView BindingSource bsTheRentals = new BindingSource(); DataGridViewTextBoxColumn companyName = new DataGridViewTextBoxColumn(); companyName.DataPropertyName = "Name"; companyName.HeaderText = "Company Name"; companyName.Name = "CompanyName"; companyName.AutoSizeMode = DataGridViewAutoSizeColumnMode.AllCells; DataGridViewTextBoxColumn companyLocation = new DataGridViewTextBoxColumn(); companyLocation.DataPropertyName = "Location"; companyLocation.HeaderText = "Company Location"; companyLocation.Name = "CompanyLocation"; companyLocation.AutoSizeMode = DataGridViewAutoSizeColumnMode.AllCells; ArrayList carMakeColumnsToAdd = new ArrayList(); // Loop through the CarMakes list to add each custom column for (int intX=0; intX < CarRentalCompanies.CarMakes[0].Count; intX++) { // Custom column for user control string carMakeColumnName = "carColumn" + intX; CarMakeListColumn carMakeColumn = new DataGridViewComboBoxColumn(); carMakeColumn.Name = carMakeColumnName; carMakeColumn.DisplayMember = CarRentalCompanies.CarMakes[intX].Name; carMakeColumn.DataSource = CarRentalCompanies.CarMakes; // this is the CarMAkes List carMakeColumn.AutoSizeMode = DataGridViewAutoSizeColumnMode.Fill; carMakeColumnsToAdd.Add(carMakeColumn); } CarDataGrid.DataSource = bsTheRentals; CarDataGrid.Columns.AddRange(new DataGridViewColumn[] { companyName, companylocation, carMakeColumnsToAdd }); CarDataGrid.AutoGenerateColumns = false; The code I provided does not work because I am unfamiliar with UserControls and custom DataGridViewColumns and DataGridViewCells - I know I must derive from these classes in order to use my User Control properly. I appreciate any advice/assistance/help in this. Thank you. Lawrence

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