Extension method: Difference between revisions

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In [[object-oriented computer programming]], an '''extension method ''' is a [[Method (computer programming)|method]] added to an object after the original object was [[Compiler|compiled]]. The modified object is often a class, a prototype or a type. Extension methods are features of some object-oriented programming languages. There is no syntactic difference between calling an extension method and calling a method declared in the type definition.<ref name="ms_ext">{{cite web|url=http://msdn.microsoft.com/en-us/library/bb383977.aspx|title=Extension Methods|publisher=Microsoft|accessdate=2008-11-23}}</ref>

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Extension methods are features of numerous languages including [[C Sharp (programming language)|C#]], [[Java (programming language)|Java]] via [http://manifold.systems/docs.html#the-extension-manifold Manifold] or [https://projectlombok.org/features/experimental/ExtensionMethod Lombok] or [https://github.com/rogerkeays/fluent Fluent], [[Gosu (programming language)|Gosu]], [[JavaScript]], [[Oxygene (programming language)|Oxygene]], [[Ruby (programming language)|Ruby]], [[Smalltalk]], [[Kotlin (programming language)|Kotlin]], [[Dart (programming language)|Dart]], [[VB.NET|Visual Basic.NET]] and [[Xojo]]. In dynamic languages like [[Python (programming language)|Python]], the concept of an extension method is unnecessary because non-builtin classes can be extended without any special syntax (an approach known as "[[Monkey patch|monkey-patching]]", employed in libraries such as [[gevent]]).

In VB.NET and Oxygene, they are recognized by the presence of the "<code>extension</code>" keyword or attribute. In Xojo, the "<code>Extends</code>" keyword is used with global methods.

In C#, they're are implemented as static methods in static classes, with the first argument being of extended class and preceded by "<code>this</code>" keyword.

In Java you add, extension methods are added via [http://manifold.systems/ Manifold], a jar file you addadded to yourthe project's classpath. Similar to C#, a Java extension method is declared static in an [http://manifold.systems/docs.html#extension-classes @Extension] class where the first argument has the same type as the extended class and is annotated with <code>@This</code>. Alternatively, the [https://github.com/rogerkeays/fluent Fluent] plugin allows you to callcalling any static method as an extension method without using annotations, as long as the method signature matches.

In Smalltalk, any code can add a method to any class at any time, by sending a method creation message (such as <code>methodsFor:</code>) to the class the user wants to extend. The Smalltalk method category is conventionally named after the package that provides the extension, surrounded by asterisks. For example, when Etoys application code extends classes in the core library, the added methods are put in the <code>*etoys*</code> category.

In Ruby, like Smalltalk, there is no special language feature for extension, as Ruby allows classes to be re-opened at any time with the <code>class</code> keyword, in this case, to add new methods. The Ruby community often describes an extension method as a kind of [[monkey patch]]. There is also a newer feature for adding safe/local extensions to the objects, called [https://docs.ruby-lang.org/en/2.5.0/syntax/refinements_rdoc.html Refinements], but it is known to be less used.

In Swift, the <code>extension</code> keyword marks a class-like construct that allows the addition of methods, constructors, and fields to an existing class, including the ability to implement a new interface/protocol to the existing class.<ref>{{Cite web |title=Extensions — The Swift Programming Language (Swift 5.7) |url=https://docs.swift.org/swift-book/LanguageGuide/Extensions.html |access-date=2022-06-12 |website=docs.swift.org}}</ref>