C# in Depth

Usage ideas and guidelines273those who are going to use and maintain your code. So, you need to consult with therelevant people as far as you can: this depends on your type of project and its audience,of course, but it’s nice to present different options and get appropriate feedback.Extension methods make this particularly easy in many cases, as you candemonstrate both options in working code simultaneously—turning a method into anextension method doesn’t stop you from calling it explicitly in the same way as before.The main question to ask is the one I referred to at the start of this section: is the“what does it do” of the code more important than the “how does it do it?” That variesby person and situation, but here are some guidelines to bear in mind:■■■■■■■■■Everyone on the development team should be aware of extension methods andwhere they might be used. Where possible, avoid surprising code maintainers.By putting extensions in their own namespace, you make it hard to use them accidentally.Even if it’s not obvious when reading the code, the developer writing itshould at least be aware of what she’s doing. Use a projectwide or companywideconvention for naming the namespace. You may choose to take this one step furtherand use a single namespace for each extended type. For instance, you couldcreate a TypeExtensions namespace for classes that extend System.Type.The decision to write an extension method should always be a conscious one. Itshouldn’t become habitual—certainly not every static method deserves to be anextension method.An extension method is reasonably valid if it’s applicable to all instances of theextended type. If it’s only appropriate in certain situations, I’d make it clearthat the method is not part of the type by leaving it as a “normal” static method.Document whether or not the first parameter (the value your method appearsto be called on) is allowed to be null—if it’s not, check the value in the methodand throw an exception if necessary.Be careful not to use a method name that already has a meaning in theextended type. If the extended type is a framework type or comes from a thirdpartylibrary, check all your extended method names whenever you change versionsof the library.Question your instincts, but acknowledge that they affect your productivity. Justlike with implicit typing, there’s little point in forcing yourself to use a featureyou instinctively dislike.Try to group extension methods into static classes dealing with the sameextended type. Sometimes related classes (such as DateTime and TimeSpan) canbe sensibly grouped together, but avoid grouping extension methods targetingdisparate types such as Stream and string within the same class.Think really carefully before adding extension methods with the same extendedtype and same name in two different namespaces, particularly if there are situationswhere the different methods may both be applicable (they have the samenumber of parameters). It’s reasonable for adding or removing a using directiveto make a program fail to build, but it’s nasty if it still builds but changesthe behavior.Licensed to Rhona Hadida