C# in Depth

258 CHAPTER 10 Extension methodsListing 10.2WebRequest request = WebRequest.Create("http://manning.com");using (WebResponse response = request.GetResponse())using (Stream responseStream = response.GetResponseStream())using (FileStream output = File.Create("response.dat")){StreamUtil.Copy(responseStream, output);}Listing 10.2 is quite compact, and the StreamUtil class has taken care of looping andasking the response stream for more data until it’s all been received. It’s done its jobas a utility class perfectly reasonably. Even so, it doesn’t feel very object-oriented. We’dreally like to ask the response stream to copy itself to the output stream, just like theMemoryStream class has a WriteTo method. It’s not a big problem, but it’s just a littleugly as it is.Inheritance wouldn’t help us in this situation (we want this behavior to be availablefor all streams, not just ones we’re responsible for) and we can’t go changing theStream class itself—so what can we do? With C# 2, we were out of options—we had tostick with the static methods and live with the clumsiness. C# 3 allows us to change ourstatic class to expose its members as extension methods, so we can pretend that themethods have been part of Stream all along. Let’s see what changes are required.10.2 Extension method syntaxUsing StreamUtil to copy a web response stream to a fileExtension methods are almost embarrassingly easy to create, and simple to use too.The considerations around when and how to use them are significantly deeper thanthe difficulties involved in learning how to write them in the first place. Let’s start offby converting our StreamUtil class to have a couple of extension methods.10.2.1 Declaring extension methodsYou can’t use just any method as an extension method—it has to have the followingcharacteristics:■ It has to be in a non-nested, nongeneric static class (and therefore has to be astatic method).■ It has to have at least one parameter.■ The first parameter has to be prefixed with the this keyword.■ The first parameter can’t have any other modifiers (such as out or ref).■ The type of the first parameter must not be a pointer type.That’s it—the method can be generic, return a value, have ref/out parameters otherthan the first one, be implemented with an iterator block, be part of a partial class, usenullable types—anything, as long as the above constraints are met.We will call the type of the first parameter the extended type of the method. It’s notofficial specification terminology, but it’s a useful piece of shorthand.Not only does the previous list provide all the restrictions, but it also gives thedetails of what you need to do to turn a “normal” static method in a static class into anLicensed to Rhona Hadida