I would think that adding that attribute to an interface would be helpful make sure you do not create classes that use the interface and forget to make them serializable.
This could be a very fundamental question, but I wanted to ask the experts.
Interfaces define a contract and do not have any state of their own.
Serialization is about saving and loading state into and out of an object model.
Not much point to serializing something that holds no state.
To answer the practical question of forcing an implementation of an interface to be Serializable - this is why the ISerializable interface exists.
In .NET you can declare an interface that should implement other interfaces:
interface MustBeSerializable : ISerializable {}
See some more information here.
If you want to force classes that implement your custom interface IMyInterface to be serializable you can define it has:
interface IMyInterface : ISerializable
{
// Custom interface definition
}
This more clearly indicates that the implementing class should support serialization. This does not remove the need to mark the class with the [Serializable] attribute.
IIRC, you can also create a FxCop custom rule that checks that classes that inherit from IMyInterface are marked with the respective [Serializable] attribute and this way removing the need to classes implement custom serialization.
There are some good albeit esoteric reasons behind what an interface is and isn't which keeps this from being possible. That said however: I agree with you. There are many things that would be useful if we could incorporate them into interfaces. [Serializable] and statics come to mind.
Although they do not fit into the philosophy of what an interface is, they seem to incorporate this vacant grey area in single-inheritance OOP. There are of course work arounds but they feel very forced compared to the original intent.
Well, there is a reason that new classes are not marked as serializable by default: By adding the Serializable attribute, you acknowledge that you have ensured that serialization in your class works, by choosing proper data types for your instance fields and by adding serialization logic, if necessary.
So, if you "forgot" to add the Serializable attribute to your class, you most probably also forgot to check whether serialization really works on your class. Granted, in many cases it will work "out of the box", so adding the attribute is all that remains, but you are supposed to double-check and explicitly acknowledge that fact (by manually adding the attribute).
Related
In order to maintain binary backwards compatibility in .NET, you generally can't add new abstract methods to public classes and interfaces. If you do, then code built against the old version of the assembly that extends/implements your class/interface will fail at runtime because it fails to fully extend/implement the new version. For classes, however, there is a handy workaround:
public abstract class Foo {
internal Foo() { }
}
Because Foo's constructor is internal, no-one outside of my assembly can extend Foo. Thus, I can add new abstract methods to Foo without worrying about backward compatibility since I know that no class in another assembly can extend Foo.
My question is, is there a similar trick for interfaces? Can I create a public interface and somehow guarantee that no one outside of my assembly will be able to create an implementation of it?
No, you can't do that. But then, considering that the point of an interface is to define the behavior of an implementation by defining a contract, that makes sense.
What you can do, however, is create an internal interface that inherits from your public interface:
public interface IPublicInterface {
/* set-in-stone method definitions here */
}
internal interface IChildInterface : IPublicInterface {
/* add away! */
}
This should prevent any backwards compatibility issues with other assemblies while still allowing you to hide additional methods.
The downside, of course, is that you would have to remember to cast as IChildInterface when you need those, rather than simply being able to use it as an IPublicInterface
In all honesty, though, if you really wanted to define some assembly-only functionality while still requiring that the end user define their own implementations for some methods, then your best bet is probably an abstract class.
No, you can't.
But since in IL an interface is essentially just a pure abstract class (i.e. one without any implementation at all), you can use the technique you've already described and it will be practically the same.
As noted, keep in mind that this approach does restrict your type to inheriting just the fake "abstract class" interface. It can implement other interfaces, but won't be able to inherit any other type. This may or may not be a problem, depending on the scenario.
If it makes you feel better about the design, name your pure abstract class following the .NET convention for interfaces. E.g. IFoo instead of Foo.
Of course, it does imply the question: why do you want to do this? If you have no implementation at all, what harm could come from allowing other code to implement your interface?
But from a practical point of view, it's possible to enforce your rules the way you want.
This is not properly a question but something more like a thought I had recently.
I'm taking XmlAttribute to XmlSerialize a class as an example: you can set attributes to a class to choose which properties should be serialized, but the same thing can be done quite easy by implementing a teorical interface IXmlSerializable (it does exist something similar, I don't remember) and by overloading a method "Serialize" for that class which just call Serialize on properties you want to serialize (this.myProp1.Serialize()), same for Deserialize
So what I'm basically saying: isn't Attribute method a bit redundant? (I like it actually, but I don't find it logically different from an interface)
Thanks for any answer, as I've said this is just a thought... hopefully someone will find it interesting
Update 1: Well I explained myself in a wrong way, what I'm asking is "why should I choose attribute instead of an Interface (or opposite)", not exactly this specific case (I took serialization because was the first thing that pop out in my mind), by the way thanks for your answer because they are very interesting
From the comments and downvote, maybe I should highlight my main point here: something that can save me hours of work (per type) and horrible code complexity is very much not redundant, but very, very welcome.
"quite easy"? OK; I'm pretty experienced at serialization, but the implementation for that is not what I call easy. Quite the contrary, in fact.
If you don't want to use attributes, there is an overload for XmlSerializer that allows you to configure it at runtime.
But I shudder whenever I hear "implement IXmlSerializable". The attribute approach is very quick and easy:
[XmlRoot("foo"), XmlType("foo")]
[XmlInclude(typeof(SuperFoo))]
public class Foo {
public string X {get;set;}
[XmlAttribute("y")]
public int? Y {get;set;}
[XmlElement("item")]
public List<string> Items {get;set;}
}
public class SuperFoo : Foo {}
I challenge you to write a robust implementation of IXmlSerializable for that very simple example in under 2 hours... and remember that every line you write is a line you have to maintain.
Well, from the best I can tell, they are logically different.
Implementing IXmlSerializable directly impacts the class itself, because you are adding an interface and one or more methods into the implementation of the class. In essence, You are making your own class directly responsibly for the it's serialization.
However, adding the XmlAttribute attributes does not directly impact the functionality of the class, instead you are just decorating it with attributes so that XmlSerializer can carry out the actual serialization functiohality. In this case, you are deferring the serialization to the XmlSerializer class, and providing just enough metadata about your class for XmlSerializer to do it's work.
This is why I prefer the latter attribute approach. When I'm writing a class, I want it to be serializable, but the last thing I care about is the specifics of the implementation, so I always start with thaqt approach and 99% of the time it works fine with very little work. However, if you did need more fine-grain control over the serialization, the implement the IXmlSerializable interface and write your own serialization code.
The programmatic method of implementing the interface may give a bit more control (and likely more speed), but is harder to create and maintain than the attribute method. I mostly use attributes.
You can select properties to (not) serialize with attributes. Implementation of interface is serialization by code.
Is there an easy way to enforce a derived class must be serialiable?
Suppose I define a interface that needs the derived classes to be serializable. According to this post, I cannot just specify the serializable attribute in the interface, because derived classes don't need to respect that.
I believe I could have the interface inherit from the ISerializable interface, but does that mean that the derived class couldn't use the attribute to specify serialization (as opposed to actually implementing the methods for ISerializable)?
We cannot use [Serializable] as
gives error with an interface.
We may use [Serializable] attribute
with base class but even then this
attribute is not inherited. This does
not seem possible.
Have a look a this link as well.
Enforcing serializable from an
interface without forcing classes to
custom serialize in C#.
I would think that adding that attribute to an interface would be helpful make sure you do not create classes that use the interface and forget to make them serializable.
This could be a very fundamental question, but I wanted to ask the experts.
Interfaces define a contract and do not have any state of their own.
Serialization is about saving and loading state into and out of an object model.
Not much point to serializing something that holds no state.
To answer the practical question of forcing an implementation of an interface to be Serializable - this is why the ISerializable interface exists.
In .NET you can declare an interface that should implement other interfaces:
interface MustBeSerializable : ISerializable {}
See some more information here.
If you want to force classes that implement your custom interface IMyInterface to be serializable you can define it has:
interface IMyInterface : ISerializable
{
// Custom interface definition
}
This more clearly indicates that the implementing class should support serialization. This does not remove the need to mark the class with the [Serializable] attribute.
IIRC, you can also create a FxCop custom rule that checks that classes that inherit from IMyInterface are marked with the respective [Serializable] attribute and this way removing the need to classes implement custom serialization.
There are some good albeit esoteric reasons behind what an interface is and isn't which keeps this from being possible. That said however: I agree with you. There are many things that would be useful if we could incorporate them into interfaces. [Serializable] and statics come to mind.
Although they do not fit into the philosophy of what an interface is, they seem to incorporate this vacant grey area in single-inheritance OOP. There are of course work arounds but they feel very forced compared to the original intent.
Well, there is a reason that new classes are not marked as serializable by default: By adding the Serializable attribute, you acknowledge that you have ensured that serialization in your class works, by choosing proper data types for your instance fields and by adding serialization logic, if necessary.
So, if you "forgot" to add the Serializable attribute to your class, you most probably also forgot to check whether serialization really works on your class. Granted, in many cases it will work "out of the box", so adding the attribute is all that remains, but you are supposed to double-check and explicitly acknowledge that fact (by manually adding the attribute).
When employing custom attributes to store meta-data, is it best to decorate the interface, or the class that implements the interface, assuming that any class that implements the interface would have the same data in the attribute?
Update: Basically i'm writing a custom data storage mechanism for a project, and the objects represent the various tables being stored. The custom attribute is used to designate which table in the dataset is used to store the objects of that class, and also to identify which tables are involved in a n:m relationship.
So if i put the attributes on the interface, is this a clearer approach, or does it clutter the interface and make accessing the data itself more cumbersome?
It depends on the scenario. WCF, for example, decorates interfaces for the operation contracts.
However, if you are going to be talking about objects (rather than the interface itself), note that it can be painful for calling code to get hold of interface metadata, especially if the class uses explicit interface implementation.
It would be more common to decorate the class, but that isn't quite the same question ;-p
If the attribute really is specific to the interface (not the instances), then fine - decorate the interface and talk about typeof(IFoo) etc. But if you expect code to be able to set per-type values for the attributes, it will have to be at the class level.
What is the scenario?
Well it depends on whether or not the interface has anything to do with the metadata.
interface IRunnable
{
void Run();
}
class Test : IRunnable
{
public void Run() { }
}
In this example it would make sense to put the attributes on the interface if they pertain to the intent of the interface. If the attributes are applicable across all implementations then put the attributes on the interface.
However if the attributes have nothing to do with the "runnability" (with "runnability" pertaining to IRunnable not the CLR) of the class then put the attributes on the class.