I have seen this statement in many of the documention samples, like here
This class is the default implementation of the "ISomeInterface" interface
what exactly this means ? Thanks
This is somewhat misleading, since an interface, by definition, provides no implementation.
However, many portions of the framework try to make life easier - so they provide a method which takes an interface, but also provides an overload with no parameters. A good example is List<T>.Sort.
The documentation here is suggesting that, if you use a method that would normally require an IComparer<T>, but use it via some overload that doesn't, you'll get the referenced "default implementation" used instead.
However, this is really an "implementation detail" of classes unrelated to the interface itself. I personally think this is a poor choice of words in the documentation, and should be something more like:
Many types in the framework rely on a common implementation of this interface provided by
the Comparer class.
This would, in my opinion, provide a more clear meaning to this...
It means that when you call the Sort method without argument (without explicitly specifying a comparer) it will use this default implementation.
This means this class is the one implementing the interface. It points to the object that implements the interface itself without any derived or inherited members, but plainly the interface. This is the class that cirectly corresponds to this interface.
public interface IComparer {
// Some members to implement here.
}
public class Comparer : IComparer {
// IComparer members implementation
}
Related
I am looking at nServiceBus and came over this interface
namespace NServiceBus
{
public interface IMessage
{
}
}
What is the use of an empty interface?
Usually it's to signal usage of a class. You can implement IMessage to signal that your class is a message. Other code can then use reflection to see if your objects are meant to be used as messages and act accordingly.
This is something that was used in Java a lot before they had annotations. In .Net it's cleaner to use attributes for this.
#Stimpy77 Thanks! I hadn't thought of it that way.
I hope you'll allow me to rephrase your comment in a more general way.
Annotations and attributes have to be checked at runtime using reflection. Empty interfaces can be checked at compile-time using the type-system in the compiler. This brings no overhead at runtime at all so it is faster.
Also known as a Marker Interface:
http://en.wikipedia.org/wiki/Marker_interface_pattern
In java Serializable is the perfect example for this. It defines no methods but every class that "implements" it has to make sure, that it is really serializable and holds no reference to things that cannot be serialized, like database connections, open files etc.
In Java, empty interfaces were usually used for "tagging" classes - these days annotations would normally be used.
It's just a way of adding a bit of metadata to a class saying, "This class is suitable for <this> kind of use" even when no common members will be involved.
Normally it's similar to attributes. Using attributes is a preferred to empty interfaces (at least as much as FxCop is aware). However .NET itself uses some of these interfaces like IRequiresSessionState and IReadOnlySessionState. I think there is performance loss in metadata lookup when you use attributes that made them use interfaces instead.
An empty interface acts simply as a placeholder for a data type no better specified in its interface behaviour.
In Java, the mechanism of the interface extension represents a good example of use. For example, let's say that we've the following
interface one {}
interface two {}
interface three extends one, two {}
Interface three will inherit the behaviour of 'one' and 'two', and so
class four implements three { ... }
has to specify the two methods, being of type 'three'.
As you can see, from the above example, empty interface can be seen also as a point of multiple inheritance (not allowed in Java).
Hoping this helps to clarify with a further viewpoint.
They're called "Mark Interfaces" and are meant to signal instances of the marked classes.
For example... in C++ is a common practice to mark as "ICollectible" objects so they can be stored in generic non typed collections.
So like someone over says, they're to signal some object supported behavior, like ability to be collected, serialized, etc.
Been working with NServiceBus for the past year. While I wouldn't speak for Udi Dahan my understanding is that this interface is indeed used as a marker primarily.
Though I'd suggest you ask the man himself if he'd had thoughts of leaving this for future extension. My bet is no, as the mantra seems to be to keep messages very simple or at least practically platform agnostic.
Others answer well on the more general reasons for empty interfaces.
I'd say its used for "future" reference or if you want to share some objects, meaning you could have 10 classes each implementing this interface.
And have them sent to a function for work on them, but if the interface is empty, I'd say its just "pre"-work.
Empty interfaces are used to document that the classes that implement a given interface have a certain behaviour
For example in java the Cloneable interface in Java is an empty interface. When a class implements the Cloneable interface you know that you can call run the clone() on it.
Empty interfaces are used to mark the class, at run time type check can be performed using the interfaces.
For example
An application of marker interfaces from the Java programming language is the Serializable interface. A class implements this interface to indicate that its non-transient data members can be written to an ObjectOutputStream. The ObjectOutputStream private method writeObject() contains a series of instanceof tests to determine writeability, one of which looks for the Serializable interface. If any of these tests fails, the method throws a NotSerializableException.
An empty interface can be used to classify classes under a specific purpose. (Marker Interface)
Example : Database Entities
public interface IEntity {
}
public class Question implements IEntity {
// Implementation Goes Here
}
public class Answer implements IEntity {
// Implementation Goes Here
}
For Instance, If you will be using Generic Repository(ex. IEntityRepository), using generic constraints, you can prevent the classes that do not implement the IEntity interface from being sent by the developers.
I have 2 objects that inherit from an interface i created which worked nicely. The Objects are injected into another object calls the methods of both the object. The methods of the objects perform some simple XML manipulation which is then returned to the worker object.
I now have a change request which affects one object that inherits from the interface but not the other and I'm at a loss as to how I should handle this. I've created a couple of new methods and I simply throw a not implemented exception if its not used. This doesn't seem "Best Practice" to me. What is the best way to handle this scenario?
I think that this is a situation where the Interface Segregation Principle comes in place.
If you find yourself having two objects for which it does not make sense to expose the same set of public members, then probably they should not implement the same interface. Or at least not only the same interface. You have two options here, depending on your application's logic:
Leave the original interface as is, and the first class (the one not needing extra methods) unmodified. Define a new interface only for the new methods, and make the second class implement both interfaces.
Define a new interface that inherits from the old one and contains the new methods. Leave your first class unmodified, and have your second class implement the new interface.
Implementing an interface and doing nothing more than throwing an exception in some methods is indeed a bad practice, as it breaks the Liskov substitution principle.
An interface doesn't "need" to be fully implemented... Even in .NET there are classes that implement partially an interface (and that throw NotSupportedException() when used in an "illegal" way)... For example arrays are IList<> that don't support Add() or Remove()...
Or the Stream abstract class, that has an additional "pattern": CanRead, CanWrite, CanSeek, ..., so there are methods, and properties that tell if it is legal to use those methods.
Another way is to use an additional interface, and try casting it with as operator. The Entity Framework for example "returns" IQueryable<T>, that "are" IEnumerable<T>... But those objects even support the IDbAsyncEnumerable<T> interface... but not all the IQueryable<T> are IDbAsyncEnumerable<T>. You have to do a cast and see if the interface is present or not.
You could extend the interface like this:
public interface SimpleInterface
{
void SimpleMethod();
void OtherMethod();
}
public interface ExpandedInterface : SimpleInterface
{
void ExpandedMethod();
void OtherExpandedMethod();
}
That way you could declare, on your client code, if you really need an expanded interface implementer (in which case you should pass only an instance of the subset of concrete classes that implement ExpandedInterface) or it is enough to use a SimpleInterface implementer (in which case you can pass either).
The situation you presented (needing to add funcion to one object, but not other) has more to do with the client code than the interface implementers themselves. You have to think: "in this client class, what do I really need: an instance of SimpleInterface, or an instance of ExtendedInterface?"
Is it a pretty safe assumption that the following class is an odd representation of "downgrading" (for lack of a better word) the private class field?
public class AggregatedClass : ICollection<SingleClass>
{
List<SingleClass> _singleClassList;
// ...rest of code
}
I recently saw a "working" example of this, and it threw me for a bit of a loop. What is the point of the above? If List<T> implements ICollection<T>, then isn't the above class a reversal? You're having a private class field that's type class is an extension of it's parent's class implementation (ICollection<T>).
Is it accurate to say the above example is not really a great design?
I would say there is nothing wrong with that. The AggregatedClass provides a contract that is ICollection<SingleClass> and how that contract is implemented is an implementation detail, no pun intended.
If for the example the class was defined like:
public class AggregatedClass : List<SingleClass>
then I would have to say that yes, there is a design problem with that. It is recommended to not expose List<T> in public API and instead expose Collection<T> since collection provides virtual methods like InsertItem and ClearItems and the list counterpart does not provide any virtual methods that can later be used to override behavior in a base class.
If however you don't want to specifically inherit from Collection<T> then implementing the ICollection<T> like in the example you provided is another suitable alternative, that also allows for the class to inherit from other base class.
This abstracts the List<T> as an implementation detail, and ifappropriate / needed the class can now provide additional logic in the various methods of the list (Add/Remove etc; noting that these are non-virtual on List-of-T).
If you mean the field should be ICollection - well, that is up to the class! It might be making use of the additional List-of-T methods, or it could just be an irrelevant implementation detail, that was handy for whoever wrote the class, and does no harm since it is private anyway.
It almost depends on what AggregatedClass what will do in the near future: by implementing ICollection using a private field leave the designer is free to make AggregatedClass derive from another one.
Since C# doesn't allow multiple inheritance, it is a precious thing.
In my opinion, if the class is "mainly" a List I would derive it directly. In the case I need to inherit from another class, it's always possible to refactor class in the way you have shown in your question.
I use interfaces for decoupling my code. I am curious, is the usage of explicit interface implementation meant for hiding functionality?
Example:
public class MyClass : IInterface
{
void IInterface.NoneWillCall(int ragh) { }
}
What is the benefit and specific use case of making this available only explicitly via the interface?
There are two main uses for it in my experience:
It allows you to overload methods by return value. For example, IEnumerable<T> and IEnumerable both declare GetEnumerator() methods, but with different return types - so to implement both, you have to implement at least one of them explicitly. Of course in this question both methods are provided by interfaces, but sometimes you just want to give a "normal" method with a different type (usually a more specific one) to the one from the interface method.
It allows you to implement part of an interface in a "discouraging" way - for example, ReadOnlyCollection<T> implements IList<T>, but "discourages" the mutating calls using explicit interface implementation. This will discourage callers who know about an object by its concrete type from calling inappropriate methods. This smells somewhat of interfaces being too broad, or inappropriately implemented - why would you implement an interface if you couldn't fulfil all its contracts? - but in a pragmatic sense, it can be useful.
One example is ICloneable. By implementing it explicitly, you can have still have a strongly typed version:
public class MyClass : ICloneable {
object ICloneable.Clone() {
return this.Clone();
}
public MyClass Clone() {
return new MyClass() { ... };
}
}
It is not meant for hiding methods but to make it possible to implement two methods with the same signature/name from different interface in to different ways.
If both IA and IB have the operation F you can only implement a different method for each F by explicitly implementing the interfaces.
It can be used for hiding. For example, some classess that implement IDisposable do so explicitly because they also have a Close() method which does the same thing.
You can also use the explicit interface definitions for when you are implementing two interfaces on one class and there is a signature clash and the functionality differs depending on the interface. However, if that happens it is usually a sign that your class is doing too much and you should look at splitting the functionality out a bit.
I have a project where quite a few functions and variable getters will be defined, abstractly. My question is should I use an abstract class for this(with each function throwing NotImplementedException), or should I just use an interface? Or should I use both, making both an interface and then an abstract class implementing the interface?
Note, even though all of these functions and such may be defined, it does not mean they will all be used in all use cases. For instance, AddUser in an authentication class may be defined in an interface, but not ever used in a website due to closed user sign up.
In general, the answer to the question of whether or not to use inheritance or an interface can be answered by thinking about it this way:
When thinking about hypothetical
implementing classes, is it a case
where these types are what I'm
describing, or is it a case where
these types can be or can do what I'm
describing?
Consider, for example, the IEnumerable<T> interface. The classes that implement IEnumerable<T> are all different classes. They can be an enumerable structure, but they're fundamentally something else (a List<T> or a Dictionary<TKey, TValue> or a query, etc.)
On the other hand, look at the System.IO.Stream class. While the classes that inherit from that abstract class are different (FileStream vs. NetworkStream, for example), they are both fundamentally streams--just different kinds. The stream functionality is at the core of what defines these types, versus just describing a portion of the type or a set of behaviors that they provide.
Often you'll find it beneficial to do both; define an interface that defines your behavior, then an abstract class that implements it and provides core functionality. This will allow you to, if appropriate, have the best of both worlds: an abstract class for inheriting from when the functionality is core, and an interface to implement when it isn't.
Also, bear in mind that it's still possible to provide some core functionality on an interface through the use of extension methods. While this doesn't, strictly speaking, put any actual instance code on the interface (since that's impossible), you can mimic it. This is how the LINQ-to-Objects query functions work on IEnumerable<T>, by way of the static Enumerable class that defines the extension methods used for querying generic IEnumerable<T> instances.
As a side note, you don't need to throw any NotImplementedExceptions. If you define a function or property as abstract, then you don't need to (and, in fact, cannot) provide a function body for it within the abstract class; the inheriting classes will be forced to provide a method body. They might throw such an exception, but that's not something you need to worry about (and is true of interfaces as well).
Personally, I think it depends on what the "type" is defining.
If you're defining a set of behaviors, I would recommend an interface.
If, on the other hand, the type really defines a "type", then I'd prefer an abstract class. I would recommend leaving the methods abstract instead of providing an empty behavior, though.
Note, even though all of these functions and such may be defined, it does not mean they will all be used in all use cases.
If this is true, you should consider breaking this up into multiple abstract classes or interfaces. Having "inappropriate" methods in the base class/interface really is a violation of the Liskov Substitution Principle, and a sign of a design flaw.
If you're not providing any implementation, then use an interface otherwise use an abstract class. If there are some methods that may not be implemented in subclasses, it might make sense to create an intermediate abstract class to do the legwork of throwing NotSupportedException or similar.
One advantage of abstract classes is that one can add to an abstract class new class members whose default implementation can be expressed in terms of existing class members, without breaking existing inheritors of that class. By contrast, if any new members are added to an interface, every implementation of that interface must be modified to add the necessary functionality.
It would be very nice if .net allowed for an interface to include default implementations for properties, methods, and events which did not make any use of object fields. From a technical standpoint, I would think such a thing could be accomplished without too much difficulty by having for each interface a list of default vtable entries which could be used with implementations that don't define all vtable slots. Unfortunately, nothing like that ability exists in .net.
Abstract classes should be used when you can provide a partial implementation. Use interfaces when you don't want to provide any implementation at all - just definition.
In your question, it sounds like there is no implementation, so go with an interface.
Also, rather than throwing NotImplementedException you should declare your method/property with the abstract keyword so that all inheritors have to provide an implementation.
#Earlz I think refering to this: Note, even though all of these functions and such may be defined, it does not mean they will all be used in all use cases. is directly related to the best way to 'attack' this problem.
What you should aim at is minimizing the number of such functions so that it becomes irrelavant (or at least not that important) if you use either or. So improve the design as much as you can and you will see that it really doesn't matter which way you go.
Better yet post a high level of what you are trying to do and let's see if we can come up together with something nice. More brains working towards a common goal will get a better answer/design.
Another pattern that works in some situations is to create a base class that is not abstract. Its has a set of public methods that define the API. Each of these calls a Protected method that is Overideable.
This allows the derived class to pick and choose what methods it needs to implement.
So for instance
public void AddUser(object user)
{
AddUserCore(user);
}
protected virtual void AddUserCore(object user)
{
//no implementation in base
}