C# in Depth

Extension method syntax26110.2.3 How extension methods are foundIt’s important to know how to call extension methods—but it’s also important to knowhow to not call them—how to not be presented with unwanted options. To achievethat, we need to know how the compiler decides which extension methods to use inthe first place.Extension methods are made available to the code in the same way that classes aremade available without qualification—with using directives. When the compiler seesan expression that looks like it’s trying to use an instance method but none of theinstance methods are compatible with the method call (if there’s no method with thatname, for instance, or no overload matches the arguments given), it then looks for anappropriate extension method. It considers all the extension methods in all theimported namespaces and the current namespaces, and matches ones where there’san implicit conversion from the expression type to the extended type.NOTE Implementation: How does the compiler spot an extension method in a library? Towork out whether or not to use an extension method, the compiler has tobe able to tell the difference between an extension method and othermethods within a static class that happen to have an appropriate signature.It does this by checking whether System.Runtime.CompilerServices.ExtensionAttribute has been applied to the method. This attribute is newto .NET 3.5, but the compiler doesn’t check which assembly the attributecomes from. This means that you can still use extension methods even ifyour project targets .NET 2.0—you just need to define your own attributewith the right name, in the right namespace.If multiple applicable extension methods are available for different extended types(using implicit conversions), the most appropriate one is chosen with the “better conversion”rules used in overloading. For instance, if IChild inherits from IParent, andthere’s an extension method with the same name for both, then the IChild extensionmethod is used in preference to the one on IParent. This is crucial to LINQ, as you’llsee in section 12.2, where we meet the IQueryable interface.It’s important to note that instance methods are always used beforeextension methods, but the compiler doesn’t warn of an extensionmethod that matches an existing instance method. If a new version of theframework were to introduce a CopyTo method in Stream that took thesame parameters as our extension method, recompiling our code againstthe new framework would silently change the meaning of the methodcall. (Indeed, that’s one reason for choosing CopyTo instead ofWriteTo—we wouldn’t want the meaning to change depending onwhether the compile-time type was Stream or MemoryStream.)One potential problem with the way that extension methods are made available tocode is that it’s very wide-ranging. If there are two classes in the same namespace containingmethods with the same extended type, there’s no way of only using the extensionmethods from one of the classes. Likewise, there’s no way of importing aWarning:subtleversioningissue!Licensed to Rhona Hadida