C# in Depth

108 CHAPTER 3 Parameterized typing with generics3.6.3 Lack of generic properties, indexers, and other member typesWe’ve seen generic types (classes, structs, delegates, and interfaces) and we’ve seengeneric methods. There are plenty of other members that could be parameterized.However, there are no generic properties, indexers, operators, constructors, finalizers,or events. First let’s be clear about what we mean here: clearly an indexer can havea return type that is a type parameter—List is an obvious example. KeyValue-Pair provides similar examples for properties. What you can’t have isan indexer or property (or any of the other members in that list) with extra typeparameters. Leaving the possible syntax of declaration aside for the minute, let’s lookat how these members might have to be called:SomeClass instance = new SomeClass("x");int x = instance.SomeProperty;byte y = instance.SomeIndexer["key"];instance.Click += ByteHandler;instance = instance + instance;I hope you’ll agree that all of those look somewhat silly. Finalizers can’t even be calledexplicitly from C# code, which is why there isn’t a line for them. The fact that we can’tdo any of these isn’t going to cause significant problems anywhere, as far as I cansee—it’s just worth being aware of it as an academic limitation.The one exception to this is possibly the constructor. However, a static genericmethod in the class is a good workaround for this, and the syntax with two lists of typearguments is horrific.These are by no means the only limitations of C# generics, but I believe they’re theones that you’re most likely to run up against, either in your daily work, in communityconversations, or when idly considering the feature as a whole. In our next two sectionswe’ll see how some aspects of these aren’t issues in the two languages whose featuresare most commonly compared with C#’s generics: C++ (with templates) and Java(with generics as of Java 5). We’ll tackle C++ first.3.6.4 Comparison with C++ templatesC++ templates are a bit like macros taken to an extreme level. They’re incredibly powerful,but have costs associated with them both in terms of code bloat and ease ofunderstanding.When a template is used in C++, the code is compiled for that particular set of templatearguments, as if the template arguments were in the source code. This means thatthere’s not as much need for constraints, as the compiler will check whether you’reallowed to do everything you want to with the type anyway while it’s compiling the codefor this particular set of template arguments. The C++ standards committee has recognizedthat constraints are still useful, though, and they will be present in C++0x (thenext version of C++) under the name of concepts.The C++ compiler is smart enough to compile the code only once for any given setof template arguments, but it isn’t able to share code in the way that the CLR does withLicensed to Rhona Hadida