Develop Reference

Statically defining options for a type of a derived class

I'm developing a framework where a class inheriting from an abstract class of the framework needs to be able to specify the schema for the options it can accept when it is called to DoStuff().
I started out with an abstract GetOptionsSchema() method like this:
public abstract class Widget
{
public abstract OptionsSchema GetOptionsSchema();
public abstract void DoStuff(Options options);
}
Other developers would then extend on my framework by creating custom Widget types:
public abstract class FooWidget: Widget
{
public overide DoStuff(Options options)
{
//Do some FooWidget stuff
}
public overide OptionsSchema GetOptionsSchema()
{
//Return options for FooWidget
}
}
This works but requires the framework to create an instance of every Widget type to determine options schema they accept, even if it has no need to actually DoStuff() with any of these types.
Ultimately, I'd like to be able to determine the options schema for a specific Widget type directly from a System.Type. I would create a custom OptionsSchema attribute, but constructing these schemas is more complicated then would make sense to do in the constructor of an attribute. It needs to happen in a method.
I've seen other frameworks solve similar problems by creating a custom attribute that identifies a static method or property by name. For example the TestCaseSource attribute in NUnit.
Here's what this option might look like:
public abstract class Widget
{
public abstract void DoStuff(Options options);
}
[OptionsSchemaSource(nameof(GetOptionsSchema))]
public abstract class FooWidget: Widget
{
public overide DoStuff(Options options)
{
//Do some FooWidget stuff
}
public static OptionSchema GetOptionsSchema()
{
//Return options for FooWidget
}
}
I like how the OptionsSchemaSource attribute makes it possible to get the options schema directly from a System.Type, but this also seem much less discoverable to other developers creating custom Widget types.
With the abstract method another Widget developer knows they must override GetOptionSchema() because their code would not compile otherwise. With the OptionsSchemaSource attribute the best I could do would be to hope people read my documentation and have the framework throw an exception at run-time if it encounters a Widget with out an OptionsSchemaSource attribute.
Is there an alternative/better/recommended approach to this?

You pretty much already know everything of interest to judge what's the best approach.
As already mentioned, you cannot have static interfaces defined on your type, so there is no way you can ensure a new developer is enforced to add the attribute.
So, the two alternatives you identified are the only two I can think of.
Now, let's do a pros and cons and try to sharpen them.
Attribute
You can lessen the pain of ensuring devs put attributes on the classes with meaningful error messages. I would say that you should manage the discovery of the classes based exclusively on Attributes, not in inheritance.
If you manage everything with Attributes, you don't need to inherit from Widget.
This is a pro, because now everyone can inherit if it's desirable, and re-implement if it's preferred.
The con is that the implementation of discoverability will be more complex: you will need to use reflection at start up, get a MethodInfo, check that the method has the correct signature, give proper errors in case and invoke the method unboxing the result as needed.
Think about it: you would like a static method because you don't need to instantiate a single typed Widget instance, but actually instantiating a new Widget could very well be not a big deal.
Abstract class
Well, you enforce an inheritance chain over you developers, which could be ok, necessary or entirely optional (you judge), but you get a self documenting experience.
The apparent con is that at startup you need to instantiate a Widget for every derived type you discover, but that could very well be peanuts compared to assembly scanning and type checking and methodinfo discovery and method calls through reflection.
Ugly? Kind of. Inefficient? Not so much. And it's code that is invisible to your end user.
IMHO
I find quite a good tradeoff, when designing a framework, to put some "ugly" code inside the framework, if it means that every single implementation using the library is going to be even a little bit better.
All in all, if you're designing a library that you want to be flexible and discoverable, you should expect a developer to read at least a quick start guide. If they can read in 5 minutes a single bit of information (either "extend a base class" or "add a single or a couple attributes") and that single bit gives them an direction into discovering every aspect of widget registration, I would be ok: you can't really get much better than this.
My call: I would go the abstract class route with a smallish caveat. I really don't like having an enforced base class. So I would organize discovery at startup based on interface, IWidget, containing the GetOptionsSchema method and everything is needed to use the widget (which could be the DoStuff method, but could very well be something else). At startup you search for implementations of the interface which are not abstract, and you're good to go.
If, and only if, the only bit you really need in advance is a string or other similarly simple type, I would require an additional attribute.
[OptionsSchemaName("http://something")]
public class MyWidget : WidgetBase
{
public overide DoStuff(Options options)
{
//Do some FooWidget stuff
}
public static OptionSchema GetOptionsSchema()
{
//Return options for FooWidget
}
}
Then, your type discovery infrastructure can search for non-abstract IWidgets and throw a meaningful error right at startup like the type MyWidget is lacking an OptionsSchemaName attribute. Every implementation of IWidget must define one. See http://mydocs for information.
Bang! Nailed it!

It's not currently possible to enforce the attribute at compile time; that would've been ideal for your use case. It's also not possible to have an abstract static method, or have a static method specified in an interface; so there is no way to ensure the method is actually there at compile time, except by enforcing an instance method via abstract class or interface (which will require an instance of the type to access).
I'd go with the attribute idea - it's not unreasonable to expect developers to read documentation; even with overriding an abstract method, the developer would need to know how to construct an OptionSchema in the overridden method - back to documentation!

c# practices for correct object oriented techniques

I have some c# code that has been working well for a while now.. and I have to say, whilst I understand the basics of OO principles, there is obviously more than one way to skin a cat (although I hate that phrase!).
So, I have a base abstract class that is acting as a basic data service class as follows (much simplified just for ease of reading):
public abstract class dataservice
{
public enum OutputType : int { XmlTOJson = 0, Xml = 1, Json=2 }
protected object SomeDBcall(string StoredProcedure)
{
// Just assume we are using SQLclient/DB access..
object SomeReturnObjValue = db.ExecuteScalar(cmd);
return SomeReturnObjValue;
{
}
.. so basically I might have a few basic DB retrieve/update/delete calls in the abstract class.. mainly as there are the basis of any DB operation I have in my app.
So now we have a class that implements the base class, say in my case a customer class:
public class Customer : dataservice
{
Public String CustomerDoSomething(string SomeDataEtc)
{
// Ok, so again for simplicity sake, we are going to use the base class to
// call a DB retrieve
object ReturningObj = SomeDBcall("my stored procedure");
return ReturningObj.ToString();
}
}
So I guess my question is this: Is the above method "ok" to use? considering a virtual method could be over-ridden if required, however in this case I only want the base class to use those methods which are protected as the means to call the DB operations.
Any clarity/guidance very appreciated!

Sure, it's "ok", though I see no reason for the base class to be abstract. abstract classes are great for implementing some common logic and leaving the rest up to derived classes to implement. However, you have no abstract/virtual methods, so I don't see the point here.

Perhaps you can let your abstract class be concrete and use it as some kind of helper class which handles the database related stuff you need. As far as the example code shows, there is no need to have multiple database accessing classes, just different parameters.

Overview
Many times, your "development itself will guide you".
Practical answer.
(1) You define a base class "dataservice", and from that class, several other classes will be based upon. You marked as "abstract", thats good. It's not mean to have variables by itself.
Some developers won't mark that class as "abstract", its not obligatory, but, its a not a bad idea, but, its a "good practice", to marked "abstract".
And, other methods will be added, used by the subclasses, maybe overriden, maybe not.
For know, those methods are protected, and anot mean to be used outside the object, but, by other methods. That's ok.
Maybe, later, a method may be required to be used outside the class, and may have to change to public.
(2) You add a subclass "Customer" that is a descendant from "DataService" You add a method that has to be used outside the class, and marked as "public", good.
It's only meant to be used by this class, not the parent class. So, no "virtual" or "override" required. Good.
(3) Your example its very simple. Most things you did, seems fine to me.
Eventually, when you add more code, things may change, example a method in the base class that was private may become public, or you may "rename" or "refactor" a method, like "dosomething", and found out that its better to be in the base class, or maybe not.
Summary
There are other answers, that mention, rules, or concepts. Seems to me that they are OK, but, skip the fact that you are learning to use O.O.P. better. Some people just try to "eat the cake in one wingle big bite", and that's not a good idea.
P.D. "can ur skin ur rabbit", sounds better to me.
Cheers.

You might want to look to the Template pattern to define the interface in the base (abstract or not) class with defined protected virtual hooks that can be overridden in the concrete subclasses. As mentioned by another poster, if you just intend to add DB services to each of your domain areas you might look to encapsulate the basic database service methods into a helper class rather than deriving from the database service.
Thanks #jgauffin for questioning my LSP violation statement. It was not correct and has been removed. There are lots of cases where extending the public interface of the base class by subclasses is warranted. Of course, by doing that one needs to be careful that you have an instance of a Y and not an X or a Z when performing a Y-specific operation A(), assuming that both Y and Z derive from X where Y adds the new public method A() and Z does not.
An example of the Template pattern in the OP's context would allow better encapsulation of custom functionality within subclasses without extending the public interface. However, this only works if there is not external influence exerted on the subclass instance, such as the OP's SomeDataEtc parameter. This works best when the instance is immutable.
public abstract class DataService
{
protected object myWidget = new Widget();
public object SomeDataBaseCall(string storedProcedure)
{
DoSomeCustomThing();
//do db stuff
object SomeReturnObjValue = db.ExecuteScalar(storedProcedure);
return SomeReturnObjValue;
}
protected void DoSomeCustomThing() {}
}
public class Customer : DataService
{
override protected void DoSomeCustomThing()
{
// do your custom thing here
}
}
Additionally, in the OP's example, it would seem prudent to use delegation within the derived class's new public method to call the base class's SomeDBCall method to execute the stored procedure. If you are redundantly coding the db access methods then there is no benefit to the proposed inheritance.
As was also mentioned elsewhere, you might be better off altogether by using composition rather than inheritance for the data service functionality.

No. Guess your following data access object pattern (DAO). Either way your Customer is not your data access class. It uses a class for data access. What I mean is that your DAO should favor composition over inheritance.
Something like:
public class Customer : IDataAccessObject
{
public Customer()
{
_dataAccess = new DataAccess();
}
public string CustomerDoSomething(string SomeDataEtc)
{
object ReturningObj = _dataAccess.SomeDBcall("my stored procedure");
return ReturningObj.ToString();
}
}
Why? Your objects get's a single responsibility which means that it's easier to extend and refactor them.
You can read up about SOLID which is some fundamental programming principles.
Since you are a .NET developer I also recommend that you embrace the naming guidelines.

Interfaces vs. inheritance: which is better in this case?

Let's suppose I have a widget class:
struct Widget {
public Color Color { get; set; }
public int Frobbles { get; set; }
}
Now, I need to make a factory to create these widgets, so I build a WidgetFactory:
abstract class WidgetFactory {
public virtual Widget GetWidget();
}
As it turns out, you can make widgets out of several different materials, but the resulting widgets are pretty much the same. So, I have a few implementations of WidgetFactory:
class GoldWidgetFactory : WidgetFactory {
public GoldWidgetFactory(GoldMine goldmine) {
//...
}
public Widget GetWidget() {
Gold g = goldmine.getGold();
//...
}
}
class XMLWidgetFactory : WidgetFactory {
public XMLWidgetFactory(XmlDocument xmlsource) {
//...
}
public Widget GetWidget() {
XmlNode node = //whatever
//...
}
}
class MagicWidgetFactory : WidgetFactory {
public Widget GetWidget() {
//creates widget from nothing
}
}
My question is this: Should WidgetFactory be an abstract class, or an interface? I can see arguments in both directions:
Base class:
The implementations ARE WidgetFactories
They might be able to share functionality, (say, a List<Widget> WidgetFactory.GetAllWidgets() method)
Interface:
The implementations do not inherit any data or functionality from the parent
Their internal workings are completely different
Only one method is defined
To those answering, this does not (currently) parallel to any real-world problem, but if/when I need to implement this pattern, it would be good to know. Also, "it doesn't matter" is a valid answer.
Edit: I should point out why go through this in the first place. The hypothetical usage of this class hierarchy would be something like:
//create a widget factory
WidgetFactory factory = new GoldWidgetFactory(myGoldMine);
//get a widget for our own nefarious purposes
Widget widget = factory.GetWidget();
//this method needs a few widgets
ConsumeWidgets(factory);
So, having a GetGoldWidget() method in WidgetFactory is not a very good idea. Plus, perhaps advents in Widget technology allow us to add different and more exotic types of widgets in the future? It's easier and cleaner to add a new class to handle them than shoehorn a method into an existing class.

In the example that you have given WidgetFactory has absolutely no reason to be an abstract class since there are not shared attributes or methods between different implementations of the factory.
Even if there was shared functionality, it would be more idiomatic to make an interface and pass it around to the users of WidgetFactory, to reduce the mount of knowledge those components need to have about the factory.
The overall implementation is fine and is really an abstract factory pattern, the only addition I would do is IWidgetFactory:
public interface IWidgetFactory {
Widget GetWidget();
}
abstract class WidgetFactory : IWidgetFactory {
//common attributes and methods
}
//Defferent implementations can still inherit from the base abstract class
class GoldWidgetFactory : WidgetFactory {
public GoldWidgetFactory(GoldMine goldmine) {
//...
}
public Widget GetWidget() {
Gold g = goldmine.getGold();
//...
}
}

In this case I see no benefit to using an abstract class instead of an interface.
I would generally favour interfaces over abstract classes:
They don't use up your one opportunity at class inheritance
They can be easier to mock
They feel "purer" somehow (it's clear just from the interface what the implementer needs to provide; you don't need to check each method to see whether or not it's concrete, abstract, or virtual)
In this case, however, you could easily use a delegate as there's only a single method... basically a Func<Widget>.
I disagree with Larry's idea of just using a single factory to directly create all the widgets with separate methods - as you may want to pass the WidgetFactory as a dependency to another class which doesn't need to know about the source, but needs to call CreateWidget either at a different time or possibly multiple times.
However, you could have a single widget factory with multiple methods each returning a Func<Widget>. That would give the benefits of having a single factory class while also allowing for dependency injection of the "factory" notion.

Honestly, what ever else, besides the Concrete Factory classes, do you expect to inherit from WidgetFactory? Anything?... ever?
If not it probably doesn't ever matter.
If down the road you want to add common code between them all than an abstract class would be your best bet.
Also I don't really see the need for your factory methods to implement any other interface except that of your creation method. So it doesn't matter whether it's abstract or interface. It all comes down to whether in the future you will want to add additional functionality in the future to the abstract class.

You don't need inheritance or an interface or even more than one class. The single factory should make all different kinds of widgets ; you can just pass in the materials as a parameter to the create method. The idea is to hide the aspects of different construction of objects from the caller - by making a bunch of different classes you are exposing this, not hiding it.

C# share code between classes

In Visual Studio 2008 using C#, what is the best way to share code across multiple classes and source files?
Inheritance is not the solution as the classes already have a meaningful hierarchy.
Is there some neat feature that's like a C include file that let's you insert code anywhere you want in another class?
EDIT:
ok, i guess we need a concrete example...
There are several hundred classes in the domain with a well thought out class heirarchy. Now, many of these classes need to print. There is a utility printer class that handles the printing. Let's say there are 3 different print methods that are dependent on the class that is being printed. The code that calls the print method (6 lines) is what I'm trying to avoid copying and pasting across all the different client class pages.
It'd be nice if people wouldn't assume they knew more about the domain that the op - especially when they specifically mention techniques that don't fit...

If you have functionality that you use frequently in classes that represent very different things, in my experience that should fall into just a few categories:
Utilities (e.g. string formatting, parsing, ...)
Cross-cutting concerns (logging, security enforcement, ...)
For utility-type functionality you should consider creating separate classes, and referencing the utility classes where needed in the business class.
public class Validator
{
public bool IsValidName(string name);
}
class Patient
{
private Validator validator = new Validator();
public string FirstName
{
set
{
if (validator.IsValidName(value)) ... else ...
}
}
}
For cross-cutting concerns such as logging or security, I suggest you investigate Aspect-Oriented Programming.
Regarding the PrintA vs. PrintB example discussed in other comments, it sounds like an excellent case for the Factory Pattern. You define an interface e.g. IPrint, classes PrintA and PrintB that both implement IPrint, and assign an instance of IPrint based on what the particular page needs.
// Simplified example to explain:
public interface IPrint
{
public void Print(string);
}
public class PrintA : IPrint
{
public void Print(string input)
{ ... format as desired for A ... }
}
public class PrintB : IPrint
{
public void Print(string input)
{ ... format as desired for B ... }
}
class MyPage
{
IPrint printer;
public class MyPage(bool usePrintA)
{
if (usePrintA) printer = new PrintA(); else printer = new PrintB();
}
public PrintThePage()
{
printer.Print(thePageText);
}
}

You can't just load in code that you'd like to have added into a class in C# via a preprocessor directive like you would in C.
You could, however, define an interface and declare extension methods for that interface. The interface could then be implemented by your classes, and you can call the extension methods on those classes. E.g.
public interface IShareFunctionality { }
public static class Extensions
{
public static bool DoSomething(this IShareFunctionality input)
{
return input == null;
}
}
public class MyClass : Object, IShareFunctionality
{
public void SomeMethod()
{
if(this.DoSomething())
throw new Exception("Impossible!");
}
}
This would allow you to reuse functionality, but you cannot access the private members of the class like you would be able to if you could, say, hash include a file.
We might need some more concrete examples of what you want to do though?

A C# utility class will work. It acts like a central registry for common code (or like the VB.NET Module construct) - it should contain code that's not specific to any class otherwise it should have been attached to the relevant class.
You don't want to start copying source code around if you don't have to because that would lead to code update problems considering the duplication.
As long as the source doesn't need to retain state, then use a static class with static method.
static public class MySharedMembers {
static public string ConvertToInvariantCase(string str) {
//...logic
}
// .... other members
}

If the classes are in the same namespace, there's no need for an include analog. Simply call the members of the class defined in the other function.
If they're not in the same namespace, add the namespace of the classes you want to use in the usings directives and it should work the same as above.
I'm confused by the question: it seems you need to work on your basic OO understanding.

Checkout extension methods: http://msdn.microsoft.com/en-us/library/bb383977.aspx

I don't know of a way to include portions of files but one thing we do frequently is to add an existing file and "link" it from its current location. For example, we have an assemblyInfo.cs file that every project refers to from a solution directory. We change it once and all the projects have the same info because they're referring to the same file.
Otherwise, suggestions about refactoring "common" routines in a common.dll are the best thing I've come up with in .Net.

I am not sure exactly what you mean by a "meaningful" structure already, but this sounds like a place where you could use base class implementation. Though not as "verbose" as C++ multiple inheritance, you might get some benefit out of using chained base class implementation to reuse common functions.
You can preserve class hierarchy, at least visually and override behavior as needed.

Pull out the repetitive code into services. The repetitive code is a clue that there might be some room for refactoring.
For example, create a "PrintingService" which contains the logic needed to print. You can then have the classes that need to print have a dependency on this service (either via the constructor or a parameter in a method which requires the service).
Another tip i have along these lines is to create interfaces for base functionality and then use the interfaces to code against. For example, i had bunch of report classes which the user could either fax, email, or print. Instead of creating methods for each, i created a service for each, had them implement an interface that had a single method of Output(). I could then pass each service to the same method depending on what kind of output the user wanted. When the customer wanted to use eFax instead of faxing through the modem, it was just a matter of writing a new service that implemented this same interface.

To be honest I can't think of anything like includes in Visual C#, nor why you would want that feature. That said, partial classes can do something like it sounds what you want, but using them maybe clashes against your "classes already have a meaningful hierarchy" requirement.

You have many options, TT, extension method, delegate, and lambda

Using explicit interfaces to ensure programming against an interface

I have seen arguments for using explicit interfaces as a method of locking a classes usage to that interface. The argument seems to be that by forcing others to program to the interface you can ensure better decoupling of the classes and allow easier testing.
Example:
public interface ICut
{
void Cut();
}
public class Knife : ICut
{
void ICut.Cut()
{
//Cut Something
}
}
And to use the Knife object:
ICut obj = new Knife();
obj.Cut();
Would you recommend this method of interface implementation? Why or why not?
EDIT:
Also, given that I am using an explicit interface the following would NOT work.
Knife obj = new Knife();
obj.Cut();

To quote GoF chapter 1:
"Program to an interface, not an implementation".
"Favor object composition over class inheritance".
As C# does not have multiple inheritance, object composition and programming to interfaces are the way to go.
ETA: And you should never use multiple inheritance anyway but that's another topic altogether.. :-)
ETA2: I'm not so sure about the explicit interface. That doesn't seem constructive. Why would I want to have a Knife that can only Cut() if instansiated as a ICut?

I've only used it in scenarios where I want to restrict access to certain methods.
public interface IWriter
{
void Write(string message);
}
public interface IReader
{
string Read();
}
public class MessageLog : IReader, IWriter
{
public string Read()
{
// Implementation
return "";
}
void IWriter.Write(string message)
{
// Implementation
}
}
public class Foo
{
readonly MessageLog _messageLog;
IWriter _messageWriter;
public Foo()
{
_messageLog = new MessageLog();
_messageWriter = _messageLog;
}
public IReader Messages
{
get { return _messageLog; }
}
}
Now Foo can write messages to it's message log using _messageWriter, but clients can only read. This is especially beneficial in a scenario where your classes are ComVisible. Your client can't cast to the Writer type and alter the information inside the message log.

Yes. And not just for testing. It makes sense to factor common behaviour into an interface (or abstract class); that way you can make use of polymorphism.
public class Sword: ICut
{
void ICut.Cut()
{
//Cut Something
}
}
Factory could return a type of sharp implement!:
ICut obj = SharpImplementFactory();
obj.Cut();

This is a bad idea because their usage breaks polymorphism. The type of the reference used should NOT vary the behavior of the object. If you want to ensure loose coupling, make the classes internal and use a DI technology (such as Spring.Net).

There are no doubt certain advantages to forcing the users of your code to cast your objects to the interface types you want them to be using.
But, on the whole, programming to an interface is a methodology or process issue. Programming to an interface is not going to be achieved merely by making your code annoying to the user.

Using interfaces in this method does not, in and of itself, lead to decoupled code. If this is all you do, it just adds another layer of obfuscation and probably makes this more confusing later on.
However, if you combine interface based programming with Inversion of Control and Dependency Injection, then you are really getting somewhere. You can also make use of Mock Objects for Unit Testing with this type of setup if you are into Test Driven Development.
However, IOC, DI and TDD are all major topics in and of themselves, and entire books have been written on each of those subjects. Hopefully this will give you a jumping off point of things you can research.

Well there is an organizational advantage. You can encapsulate your ICuttingSurface, ICut and related functionality into an Assembly that is self-contained and unit testable. Any implementations of the ICut interface are easily Mockable and can be made to be dependant upon only the ICut interface and not actual implementations which makes for a more modular and clean system.
Also this helps keep the inheritance more simplified and gives you more flexibility to use polymoprhism.

Allowing only callers expecting to explicit interface type ensures methods are only visible in the context they are needed in.
Consider a logical entity in a game and u decide that instead of a class responsibile for drawing/ticking the entities you want the code for tick/draw to be in the entity.
implement IDrawable.draw() and ITickable.tick() ensures an entity can only ever be drawn/ticked when the game expects it to. Otherwise these methods wont ever be visible.
Lesser bonus is when implementing multiple interfaces, explicit implementations let you work around cases where two interface method names collide.

Another potential scenario for explicitly implementing an interface is when dealing with an existing class that already implements the functionality, but uses a different method name. For example, if your Knife class already had a method called Slice, you could implement the interface this way:
public class Knife : ICut
{
public void Slice()
{
// slice something
}
void ICut.Cut()
{
Slice();
}
}

If the client code doesn't care about anything other than the fact that it can use the object to Cut() things, then use ICut.

Yes, but not necessarily for the given reasons.
An example:
On my current project, we are building a tool for data entry. We have certain functions that are used by all (or almost all) tabs, and we are coding a single page (the project is web-based) to contain all of the data entry controls.
This page has navigation on it, and buttons to interact with all the common actions.
By defining an interface (IDataEntry) that implements methods for each of the functions, and implementing that interface on each of the controls, we can have the aspx page fire public methods on the user controls which do the actual data entry.
By defining a strict set of interaction methods (such as your 'cut' method in the example) Interfaces allow you to take an object (be it a business object, a web control, or what have you) and work with it in a defined way.
For your example, you could call cut on any ICut object, be it a knife, a saw, a blowtorch, or mono filament wire.
For testing purposes, I think interfaces are also good. If you define tests based around the expected functionality of the interface, you can define objects as described and test them. This is a very high-level test, but it still ensures functionality. HOWEVER, this should not replace unit testing of the individual object methods...it does no good to know that 'obj.Cut' resulted in a cutting if it resulted in the wrong thing being cut, or in the wrong place.