Why are extension methods only allowed in non-nested, non-generic static class?
Is it useless to consider extension methods in nested, generic static class?
Why are extension methods only allowed in non-nested, non-generic static class?
As Pratik points out, the question we face is not "why are extension methods not allowed in nested or generic classes?" The question we face as language designers is "why should extension methods be allowed in nested or generic classes?"
Unless the feature is justified by some real-world user need, we're not going to take on the considerable costs of designing, implementing, testing, documenting and maintaining the feature.
Basically, extension methods were designed to make LINQ work. Anything that didn't contribute to making LINQ work was cut. LINQ only needs extension methods in static, non-generic, non-nested classes to work, so that's what we designed and implemented.
If you have a scenario where extension methods would be useful in non-static, generic, or nested classes then I'm happy to take a look at the scenario. The more real-world scenarios we get, the more likely it is that we'll make a feature in some hypothetical future language that benefits those scenarios.
Is it useless to consider extension methods in nested, generic static class?
No, it is a great idea to consider it. We would be remiss in our duties if we did not consider it. We considered it carefully for a long time and decided that on the basis of that consideration, the costs of doing the feature were not justified by the benefits accrued.
As Eric Lippert has written many times in his blog, each change to C# is carefully evaluated against a set of criteria to justify it and not based on technology sake alone. In this case, what was not required to enable LINQ was cut off to reduce risk. Check this blog post from Eric for a similar question.
I believe that this was done so that compiler can locate/search the extension method in reasonable amount of time. Further, note that complier search extension methods only in set of namespaces that are in file scope - this is also done for similar reasons.
If you think about generic static class scenario, compiler must try to instantiate the concrete types for all possible type combinations in order to match the extension method. Even if complier can be smart in doing this, instantiating concrete type for calling extension methods may have side effect that developer may not be aware of.
Related
We are debating at work the best way to define methods for an entity class - as extensions methods or using partial classes. The kind of methods we're talking about don't modify the state of the entity, they are purely "helper" methods that interrogate the state and return a value.
The main benefit to both approaches is to keep the entity class clean, while still providing intellisense support to client code.
I don't have a strong preference either way but am curious to know if others have a preference (or know of documented guidelines) towards one or the other.
I started writing the list of merits for each approach that I could think of, but in the end all I've come up with is:
Partial Classes
The method definition resides within the class (even if it's another file) so Visual Studio tool support for "find method" (e.g. ALT-\ in resharper) will locate the method
The existence of the other file containing helper methods is obvious as soon as the entity class is opened due to use of the partial keyword
Extension Methods
The naming of the file ("entityNameExtension") and its whereabouts in the project (in an "Extensions" sub-folder) are intuitive and easy to search for
Can anyone else add their opinion to this?
PS I don't feel this is a duplicate of the following question, as the asker of that question was content to mark a response which outlined the functional differences as the correct answer, which doesn't answer the question about which approach is best practice in this scenario:
Partial Class vs Extension Method
EDIT - I'm seeking people's preference towards one approach or the other, as there are no documented guidelines that we can find for this particular scenario. Both approaches are possible and neither violates any design principles, so it is a matter of preference and I'd like to know yours.
In my opinion, extension methods are good for two things. First, when you apply them to interfaces, it gives you the illusion of writing an abstract base class that lets you define some common methods, but it's more flexible because a class can only have one base class but can implement multiple interfaces. Second, if you apply it to regular classes, then I tend to look at it as some kind of hacking. When the original class lacks some methods, and you really feel like they should have those methods, but they don't, and they are out of your reach, so you are forced to implement them somewhere else, as utility methods, and it gives you an illusion that it's actually there.
Both cases are syntactic sugar only in the end, but extending interfaces makes much more sense to me, if I just look at LINQ's Enumerable class for example. I've used those extension methods on dozens of completely different classes, so it really paid off. An example of a class extension method is when I made my own string.IsNullOrWhitespace before it was added to the framework.
Extending an interface seems right because the interface defines a contract, and you can rely on that contract in your extension method, but when you extend a regular class, it may change and break your extension method. Of course, interfaces may change, too, but they tend to be more thoroughly designed I think, but I don't have any statistics.
Then there's the case of object-oriented programming. Where do you feel like your method should go, who uses those additional methods, where are the boundaries. If you think a method belongs inside a class, then put it in the class. It makes sense, it's simple. People wrote really good classes before extension methods were invented, they put everything where it belonged and life was good, haha.
Partial classes are cool because they are not that big of a hack as extension methods. They are not syntactic sugar, not magic. It is merely the best and easiest way to deal with auto-generated classes, so I don't think too much of it. I've written some code generators, and they emit regions where humans can write their own stuff and it is not overwritten in subsequent code generations. It is more comfortable that way, but that's all. I can't change how .NET tools generate code, and they don't do it this way, so partial classes are the next best thing.
To sum it up, my opinion is to only use extension methods when you really have to, and go with partial classes whenever possible.
I dont know why you would create a partial class uless your original class has grown out of its purpose. Take a look at your classes you would like to extend, are they really doing one thing, or are they doing many things. Take a look at at the Single Responsibility Principle (http://en.wikipedia.org/wiki/Single_responsibility_principle).
If you can create methods that OTHER CLASSES can take advantage of, I would recommend creating an extension class. It will extend the capability of other classes, making your toolbox more flexible.
Do extension methods follow the object-oriented paradigm in C#?
Is it a good practice to use extension methods?
In the software development lifecycle how should we consider this question in the design phase?
Eric Lippert has blogged about this and I suspect I can't do much better than to quote him:
So, yes, the oft-heard criticism that
"extension methods are not
object-oriented" is entirely correct,
but also rather irrelevant. Extension
methods certainly are not
object-oriented. They put the code
that manipulates the data far away
from the code that declares the data,
they cannot break encapsulation and
talk to the private state of the
objects they appear to be methods on,
they do not play well with
inheritance, and so on. They're
procedural programming in a convenient
object-oriented dress.
They're also incredibly convenient and
make LINQ possible, which is why we
added them. The fact that they do not
conform to some philosophical ideal of
what makes an object-oriented language
was not really much of a factor in
that decision.
I would add, however, that they're useful beyond just LINQ - for the same reason that they're useful in LINQ. It's really nice to be able to express algorithms which work on arbitrary implementations of a particular interface (such as IEnumerable<T> in LINQ to Obhects). Such algorithms typically don't have any context beyond the interfaces you're working on, so they're often naturally static.
If you accept that you've got some static utility method, which syntax would you rather use?
// Traditional
CollectionUtils.Sort(collection);
// Extension methods
collection.Sort();
The latter is simply more readable in my opinion. It concisely expresses what you want to do. It doesn't make it clear how you want to do it, but that's less important for most of the time - and more important when you're debugging that particular line, of course.
Extension methods are not an object oriented language feature. (compared to: classes, inheritance, polymorphism etc).
Like every language feature, it should be used where it is appropriate and for what it is designed for. There are already dozens of questions about when and how to use Extension methods.
What are the best practices for using Extension Methods in .Net?
Possible overuses of Extension Methods
Do Extension Methods Hide Dependencies?
There are two parts to it.
Is it OO when we use it
No; it makes you feel that you are calling method on the particular type
Is it OO based on how it is compiled/built
Yes; Compiled code has a static method using the object on which extension method was invoked
Extension methods are just a language feature. They work on object instances and are very nice tool.
Consider them as a different way to extend class functionality. You can add new functionality to a class:
By adding a partial class declaration. The class then instantly gets a bunch of new methods and properties.
By including a namespace with your extension methods holder class. The class then gets a bunch of new methods again.
Rather an organizational / language feature. Does not break object-oriented concept in any way. Just as header/source file division in C/C++ has nothing to do with object-orientation, just a language/framework feature.
It depends. Extension methods are just a tool. They can be very useful when used appropriately. But if you use them too much, it can obscure your code.
Extension Methods are just static methods that work with a specific Class or Class Hierarchy. Python is OO but has modules, Ruby has mixins. I see it more as a language feature. I am pretty sure its still OO friendly
Long explanation aside, I have a situation where I need to basically re-implement a .NET framework class in order to extend the behavior in a manner that is not compatible with an inheritance or composition/delegation strategy. The question is not a matter of whether the course of action I am to take is what you would do, or recommend, it is instead a question of naming/coding-style.
Is there a paradigm for naming classes and methods that have the same functionality as an existing class or method ala the convention of ClassEx/MethodEx that exists in C++?
[edit]
I understand that choosing good names for this is important... I haven't written a line of code yet, and am instead taking the time to think through the ramifications of what I am about to undertake, and that includes searching for a clear, descriptive, name while trying to be concise. The issue is that the name I have in mind is not terribly concise.
[/edit]
Here are the ways I've seen in the .NET Framework itself:
Call it something slightly different, but don't use any specific suffix. For example, System.TimeZoneInfo was introduced to supersede System.TimeZone.
Put it in another namespace. For example, the WPF Button is in System.Windows instead of System.Windows.Forms.
Suffix it with a number. For example X509Certificate2 versus X509Certificate. (This practice was common with COM interfaces but has fallen out of favor in .NET.)
Note that the naming of TimeZoneInfo is a publicized case of Microsoft tackling this convtrovertial naming issue head on. See and http://blogs.msdn.com/kathykam/archive/2007/03/28/bye-bye-system-timezone2-hello-system-timezoneinfo.aspx and http://blogs.msdn.com/kcwalina/archive/2006/10/06/TimeZone2Naming.aspx for excellent information.
Try name your classes/methods with real meaning.
For example, if you extending the Random functionality to create random strings, name the class StringRandom or StringRandomizer and such.
If you writing class with general purpose extension methods that applying to specific class/interface, for example IList, name it ListExtensions.
If you writing random.Next method that returns random number between minValue and maxValue including maxValue, name the method NextIncludingMaxValue.
If you writing queue.Dequeue method that is thread safe, name if DequeueThreadSafe.
If you writing queue.Dequeue method that blocking until other thread enqueueing an item, name it DequeueBlocking.
And such...
C#, for the most part, avoids these situations entirely due to the ease in which you can extend a class with new methods without breaking binary compatibility (you can add methods, at will, to a class, just not an interface), and through the use of Extension methods.
There are few reasons to ever do this in C#, unlike C++. In C++, adding a method breaks compatibility, so "Ex" becomes a much more common scenario.
I give all my methods (and properties) camelCase names: so for example Invalidate is a framework method name, and invalidate is the name of one of my methods.
This (using camelCase names) is unconventional, so some people object to it, but I find it convenient.
No such problem with class names (for which I use the conventional UpperCase), because for class names there are their namespaces to distinguish them from the framework classes.
Closed. This question is opinion-based. It is not currently accepting answers.
Want to improve this question? Update the question so it can be answered with facts and citations by editing this post.
Closed 6 years ago.
Improve this question
In what circumstances (usage scenarios) would you choose to write an extension rather than sub-classing an object ?
< full disclosure : I am not an MS employee; I do not know Mitsu Furota personally; I do know the author of the open-source Componax library mentioned here, but I have no business dealings with him whatsoever; I am not creating, or planning to create any commercial product using extensions : in sum : this post is from pure intellectal curiousity related to my trying to (continually) become aware of "best practices" >
I find the idea of extension methods "cool," and obviously you can do "far-out" things with them as in the many examples you can in Mitsu Furota's (MS) blog postslink text.
A personal friend wrote the open-source Componax librarylink text, and there's some remarkable facilities in there; but he is in complete command of his small company with total control over code guidelines, and every line of code "passes through his hands."
While this is speculation on my part : I think/guess other issues might come into play in a medium-to-large software team situation re use of Extensions.
Looking at MS's guidelines at link text, you find :
In general, you will probably be
calling extension methods far more
often than implementing your own. ...
In general, we recommend that you
implement extension methods sparingly
and only when you have to. Whenever
possible, client code that must extend
an existing type should do so by
creating a new type derived from the
existing type. For more information,
see Inheritance (C# Programming
Guide). ... When the compiler
encounters a method invocation, it
first looks for a match in the type's
instance methods. If no match is
found, it will search for any
extension methods that are defined for
the type, and bind to the first
extension method that it finds.
And at Ms's link text :
Extension methods present no specific
security vulnerabilities. They can
never be used to impersonate existing
methods on a type, because all name
collisions are resolved in favor of
the instance or static method defined
by the type itself. Extension methods
cannot access any private data in the
extended class.
Factors that seem obvious to me would include :
I assume you would not write an extension unless you expected it be used very generally and very frequently. On the other hand : couldn't you say the same thing about sub-classing ?
Knowing we can compile them into a seperate dll, and add the compiled dll, and reference it, and then use the extensions : is "cool," but does that "balance out" the cost inherent in the compiler first having to check to see if instance methods are defined as described above. Or the cost, in case of a "name clash," of using the Static invocation methods to make sure your extension is invoked rather than the instance definition ?
How frequent use of Extensions would affect run-time performance or memory use : I have no idea.
So, I'd appreciate your thoughts, or knowing about how/when you do, or don't do, use Extensions, compared to sub-classing.
thanks, Bill
My greatest usage for them is to extend closed-off 3rd party APIs.
Most of the time, when a software developer is offering an API on Windows these days, they are leaning more and more toward .NET for that extensibility. I like to do this because I prefer to depend on my own methods that I can modify in the future and serve as a global entry point to their API, in the case that they change it.
Previously, when having to do this, and I couldn't inherit the API object because it was sealed or something, I would rely on the Adapter pattern to make my own classes that wrapped up their objects. This is a functional, but rather inelegant solution. Extension methods give you a beautiful way to add more functionality to something that you don't control.
Many other peoples' greatest usage for them is LINQ!
LINQ would not be possible without the extension methods provided to IEnumerable.
The reason why people love them is because they make code more readable.
I have noticed another MAJOR usage of extension methods (myself included) is to make code more readable, and make it appear as if the code to do something belongs where it is supposed to. It also gets rid of the dreaded "Util" static-god-class that I have seen many times over. What looks better... Util.DecimalToFraction(decimal value); or value.ToFraction();? If you're like me, the latter.
Finally, there are those who deem the "static method" as EVIL!
Many 'good programmers' will tell you that you should try to avoid static methods, especially those who use extensive unit testing. Static methods are difficult to test in some cases, but they are not evil if used properly. While extension methods ARE static... they don't look or act like it. This allows you to get those static methods out of your classes, and onto the objects that they really should be attached to.
Regarding performance..
Extension methods are no different than calling a static method, passing the object being extended as a parameter... because that is what the compiler turns it into. The great thing about that is that your code looks clean, it does what you want, and the compiler handles the dirty work for you.
I use extension methods as a way to improve the functionality for classes without increasing the complexity of the class. You can keep your classes simple, and then add your repetitive work later on as an extension.
The Min() and Max() extension methods are great examples of this. You could just as easily declare a private method that would calculate these, but an extension method provides better readability, makes the functionality available to your entire project, and didn't require making an array any more complex of an object.
Taking the sub-classing approach vs. extension methods requires a couple of things to be true
The type must be extendable (not-sealed)
All places the type is created must support a factory pattern of sorts or the other code will just create the base type.
Adding an extension method requires really nothing other than using a C# 3.0+ compiler.
But most importantly, an inheritance hierarchy should represent an is-a relationship. I don't feel that adding 1 or 2 new methods / behaviors to a class truly expressing this type of relationship. It is instead augmenting existing behavior. A wrapper class or extension method much better fits the scenario.
In some cases you can't use a subclass: string for instance is sealed. You can however still add extension methods.
I was wondering what is the design motive behind extension methods in C#
It allows you to create new functionality to an existing code base without editing the original code.
http://weblogs.asp.net/scottgu/archive/2007/03/13/new-orcas-language-feature-extension-methods.aspx
"Extension methods allow developers to add new methods to the public contract of an existing CLR type, without having to sub-class it or recompile the original type. Extension Methods help blend the flexibility of "duck typing" support popular within dynamic languages today with the performance and compile-time validation of strongly-typed languages.
Extension Methods enable a variety of useful scenarios, and help make possible the really powerful LINQ query framework that is being introduced with .NET as part of the "Orcas" release."
The primary reason for their existence is being able to somehow add features to a type without inheriting from it.
This was required to provide Where, Select, ... methods for use in LINQ for collections that didn't have one.
It provides multiple inheritance via the back door.
It languages such as C++, which support inheriting from many classes, extension methods aren't required. However multiple inheritance has lots of problems with it, and so modern languages have dropped it. However extension methods are a use-case where multiple inheritance is useful. Rather than re-introduce multiple inheritance though, the C# designers created extension methods (which is a very neat solution to the problem).
Quite often we end up writing for ourselves usefull little utility static classes that perform a common function on a type. I know there have been a number of times I wished I could simply inherit a class to add a feature but that class is sealed. I'm glad though they were sealed, unwarranted inheriting is a bad thing.
Extension methods make code look more intuitive by allowing those static methods to appear to be new instance methods of the type.
They also have the advantage of not polluting the actual member namespace of the type and allowing you to opt into them by means of the using keyword.
The existence of Extension Methods is very likely due to the need for Microsoft to add functionality to IEnumerable without changing the interface. If they added methods to the interface, then every existing implementation of IEnumerable (including Microsoft's) would no longer compile.
The alternative to changing IEnumerable is to create a utility class (called Enumerable) which has methods that perform transformations on instances of IEnumerable. This works, except the user experience is not the same as calling a method from an existing IEnumerable instance. For instance, compare the following equivalent statements.
IEnumerable<string> strings = myIntList.Select(num => num.ToString())
.Where(num => num.StartsWith('T'));
IEnumerable<string> strings =
Enumerable.Where(
Enumerable.Select(myIntList, num => num.ToString()),
num => num.StartsWith('T'));
To achieve the best of both worlds, the C# compiler team added support for extension methods, allowing the creation of the Enumerable class with special syntax and maintaining the same user experience of having added the methods to IEnumerable to begin with.