C# in Depth

Advanced generics93Listing 3.10Using the typeof operator with type parametersstatic void DemonstrateTypeof(){Console.WriteLine(typeof(X));Console.WriteLine(typeof(List));Console.WriteLine(typeof(Dictionary));Displays method’stype parameterConsole.WriteLine(typeof(List));Console.WriteLine(typeof(Dictionary));DisplaysgenerictypesConsole.WriteLine(typeof(List));Console.WriteLine(typeof(Dictionary));}...DemonstrateTypeof();Displays closedtypes (despiteusing typeparameter)Most of listing 3.10 is as you might naturally expect, but it’s worth pointing out twothings. First, look at the syntax for obtaining the generic type definition of Dictionary. The comma in the angle brackets is required to effectively tell the compilerto look for the type with two type parameters: remember that there can be severalgeneric types with the same name, as long as they vary by the number of type parametersthey have. Similarly, you’d retrieve the generic type definition for MyClassusing typeof(MyClass). The number of type parameters is specified in IL (and infull type names as far as the framework is concerned) by putting a back tick after the firstpart of the type name and then the number. The type parameters are then indicated insquare brackets instead of the angle brackets we’re used to. For instance, the second lineprinted ends with List`1[T], showing that there is one type parameter, and the thirdline includes Dictionary`2[TKey,TValue].Second, note that wherever the method’s type parameter is used, the actual valueof the type argument is used at execution time. So the first line B prints List`1 rather than List`1, which you might have expected. In otherwords, a type that is open at compile time may be closed at execution time. This is veryconfusing. You should be aware of it in case you don’t get the results you expect, but otherwisedon’t worry. To retrieve a truly open constructed type at execution time, you need towork a bit harder. See the MSDN documentation for Type.IsGenericType for a suitablyconvoluted example.For reference, here’s the output of listing 3.10:System.Int32System.Collections.Generic.List`1[T]System.Collections.Generic.Dictionary`2[TKey,TValue]System.Collections.Generic.List`1[System.Int32]System.Collections.Generic.Dictionary`2[System.String,System.Int32]System.Collections.Generic.List`1[System.Int64]System.Collections.Generic.Dictionary`2[System.Int64,System.Guid]Having retrieved an object representing a generic type, there are many “next steps”you can take. All the previously available ones (finding the members of the type, creatingan instance, and so on) are still present—although some are not applicable forBDisplaysclosed typesLicensed to Rhona Hadida