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.
来源：https://stackoverflow.com/questions/1644743/evaluating-cost-benefits-of-using-extension-methods-in-c-sharp-3-0