C# in Depth

68 CHAPTER 3 Parameterized typing with genericsgeneric type) such that in some places where you’d expect to see a normal type, yousee a type parameter instead.A type parameter is a placeholder for a real type. Type parameters appear in anglebrackets within a generic declaration, using commas to separate them. So in Dictionary the type parameters are TKey and TValue. When you use a generic typeor method, you specify the real types you want to use. These are called the typearguments—in listing 3.1, for example, the type arguments were string (for TKey) andint (for TValue).NOTE Jargon alert! There’s a lot of detailed terminology involved in generics.I’ve included it for reference—and because very occasionally it makes iteasier to talk about topics in a precise manner. It could well be useful ifyou ever need to consult the language specification, but you’re unlikelyto need to use this terminology in day-to-day life. Just grin and bear it forthe moment.The form where none of the type parameters have been provided with type argumentsis called an unbound generic type. When type arguments are specified, the type is said tobe a constructed type. Unbound generic types are effectively blueprints for constructedtypes, in a way similar to how types (generic or not) can be regarded as blueprints forobjects. It’s a sort of extra layer of abstraction. Figure 3.1 shows this graphically.As a further complication, constructed types can be open or closed. An open type isone that involves a type parameter from elsewhere (the enclosing generic method orDictionary(unbound generic type)Specification oftype argumentsHashtableDictionary(constructed type)Dictionary(constructed type)(etc)InstantiationInstantiationInstantiationInstance ofHashtableInstance ofDictionaryInstance ofDictionaryNongenericblueprintsGenericblueprintsFigure 3.1 Unbound generic types act as blueprints for constructed types, which then act as blueprintsfor actual objects, just as nongeneric types do.Licensed to Rhona Hadida