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.NET Forum / .NET Framework / General / January 2005Need help: about OOP inheritance/abstract class
Hi, I have a base usercontrol with a method (blank method, no code), I have another few usercontrols that will inherit this base usercontrol, but I want to force all the usercontrol that inheriting this base usercontrol to override the method with its own code. How do I do it? I have tried to make the base usercontrol an abstract class (mustinherits + mustoverrides), this works, but all the usercontrols that inherit it will not able to open in VS designer because of inherting an abstract class. I have also tried to use interface, this works as well, but I can't force those usercontrols that inherit base usercontrol MUST have the interface. Anyone has a solution for this? Thanks, Tee > Hi, > > I have a base usercontrol with a method (blank method, no code), I have > another few usercontrols that will inherit this base usercontrol, but I > want to force all the usercontrol that inheriting this base usercontrol to > override the method with its own code. How do I do it? If you are going to /force/ them to override it ( polymorphism ) why have it in the first place? You should define an *interface* with a stub method and force them to implement it. I do this in one of my web service applications. There are two interfaces and a base abstract class. The interface accounts for methods that must be implemented in the derived classes, but which have no default method in the abstract class. So each derived class has one abstract class and two interfaces to inherit from.  Signaturehttp://texeme.com Textcasting Technology Incognito Blog http://incognito.texeme.com Hi, > You should define an *interface* with a stub method and force them to implement it. Is stub method a keyword? > I do this in one of my web service applications. There are two ........... What I am doing is a usercontrol, what I meant "I can't open the usercontrol in VS designer" is because when it opens up the form/usercontrol, it actually called it as "new", and abstract class can't be declared as new" Thanks. > > Hi, > > [quoted text clipped - 14 lines] > the abstract class. So each derived class has one abstract class and two > interfaces to inherit from. > Is stub method a keyword? Sorry, bad choice of words. Have you ever used the /interface/ keyword though? You define methods, but just in terms of the input parameters. Then, its up to the inheriting classes to implement. Similar to, but not exactly, to an abstract class. And there's no *default* implementation. > What I am doing is a usercontrol, what I meant "I can't open the > usercontrol in VS designer" is because when it opens up the > form/usercontrol, it actually called it as "new", and abstract class can't > be declared as new" No, but you can create a class that instantiates the user control in the constructor, or that inherits it, and inherit the interfaces. > Thanks. > [quoted text clipped - 26 lines] >> Incognito Blog >> http://incognito.texeme.com Signaturehttp://texeme.com Textcasting Technology Incognito Blog http://incognito.texeme.com > Have you ever used the /interface/ keyword though? Yes, I know what is it and used it before. Since I can't force a class to implements an interface, I don't think this is what I am looking for. > No, but you can create a class that instantiates the user control in the > constructor, or that inherits it, and inherit the interfaces. Sorry I am somehow a bit lost in this, I can't figure it out. Can you tell me how to instantiates user control in constructor? I think this should be what I need, greatly appreciate if you could elaborate on this. Thanks, Tee > > Is stub method a keyword? > [quoted text clipped - 47 lines] > >> Incognito Blog > >> http://incognito.texeme.com >> Have you ever used the /interface/ keyword though? > Yes, I know what is it and used it before. Since I can't force a class to [quoted text clipped - 4 lines] > Sorry I am somehow a bit lost in this, I can't figure it out. Can you tell > me how to instantiates user control in constructor? I used this trick when I was trying to get an XmlDocument to load at runtime (static), by instantiating it. Normally, I had to run the .Load method -- but I can't run a method when I declare, I have to declare, then .Load. So, I created a new class with a constructor that did the load. In my calling class CustomXmlDocClass myXml = new CustomXmlDocClass() Then in the constructor of the instantiated class: CustomXmlDocClass { public XmlDocument xd; CustomXmlDocCalss() //the constructor { XmlDocument _xd = new XmlDocument(); _xd.Load("whatever.xml"); xd = _xd; } } Now myXml is loaded with whatever.xml at run time. > I think this should be what I need, greatly appreciate if you could > elaborate on this. [quoted text clipped - 66 lines] >> Incognito Blog >> http://incognito.texeme.com SignatureTexeme http://texeme.com I haven't tried, but have you tried to use inheirtance AND interface together? i.e. * base class with stuff that should be inherited as-is * interface with the definition of the method that the inheriting classes should implement/override * inheriting/implementing classes: they inherit from the base class, AND they implement the interface. The children should then get all that the base class provides and Be Forced to implement the method(s) defined in the interface. HTH, F.O.R. I have tried it. > The children should then get all that the base class provides and Be > Forced to implement the method(s) defined in the interface. > HTH, > F.O.R. The base class that implements the interface will automatically has the method in the class (just an empty method, no code). And yes, the children get all the base class provided, and they inherit the empty method as well ... the problem is it won't force the children to overrides it. Thanks. > I haven't tried, but have you tried to use inheirtance AND interface > together? [quoted text clipped - 10 lines] > HTH, > F.O.R. This is not just a problem with user controls; it is also a problem with inheriting Forms. You cannot use an abstract base class for the reasons you mentioned. There is no way around it at present. The best you can do is write the methods in the base class to throw a NotImplementedException. Then if the child class does not override them it will blow up at run time. There is no way to enforce the override via a compile-time check. > This is not just a problem with user controls; it is also a problem > with inheriting Forms. [quoted text clipped - 6 lines] > it will blow up at run time. There is no way to enforce the override > via a compile-time check. Well there is one workaround which I've used. You can use use compile time constants to declare your class such that the MustInherit attribute is only enforced in Release builds e.g. #If Debug then Public MyClass #Else Public MustInherit MyClass #End if ... Any methods that should be overridden can be given a default implementation in the debug build (to write a message to the console or throw an exception) e.g. ... #If Debug then Public Sub MethodThatMustBeOverriden() Debug.Fail("Method not overridden") End Sub #Else Public MustOverride Sub MethodThatMustBeOverriden() #End if This has worked OK for me in the past. Hope this helps, Nick Hall Very clever. Perhaps I'll try that on my projects next time. Very great idea, thanks! > > This is not just a problem with user controls; it is also a problem > > with inheriting Forms. [quoted text clipped - 36 lines] > > Nick Hall > > The children should then get all that the base class provides and Be > > Forced to implement the method(s) defined in the interface. [quoted text clipped - 6 lines] > empty method as well ... the problem is it won't force the children to > overrides it. I didn't say that the base class should implement the interface ;-) ANd, yes I now realize that the solution I proposed would have the draw back of the base class not implementing the method(s) in your interface. SO it might or might not work for you. HTH, F.O.R. Why not just(shortened): abstract class Abstract { public abstract void MustOverrideThisAbstractMethod ( ) ; } class Concrete : Abstract { override public void MustOverrideThisAbstractMethod ( ) { //Implement something here. } } Declaring a method as abstract within an abstract class means the base class will demand an implementation of that method from the concrete derived class. SignatureRegards, Dennis JD Myr?n Oslo Kodebureau > Hi, > [quoted text clipped - 16 lines] > Thanks, > Tee Hi Dennis, See my previous post: >I have tried to make the base usercontrol an abstract class (mustinherits + mustoverrides), this works, but all the usercontrols that inherit it will not able to open in VS designer because of inherting an abstract class. This just won't work on forms or usercontrols ... it won't effect runtime, but it's trouble enough if you can't open it in VS Designer. Thanks, Tee > Why not just(shortened): > [quoted text clipped - 36 lines] > > Thanks, > > Tee Sorry, i should probably read the posts fully before replying them. I had a bunch of webforms once which all needed common extra features in order to provide the functionality of a step in a wizard. There was no way i could force those webforms to implement my custom interface in addition to inherit System.Web.UI.Page. But i myself though, knew which ones of them should provide that functionality, and had them implement that interface. class WizardStep1 : System.Web.UI.Page, IWizardStep SignatureRegards, Dennis JD Myr?n Oslo Kodebureau > Hi Dennis, > [quoted text clipped - 55 lines] >> > Thanks, >> > Tee Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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