C# in Depth

94 CHAPTER 3 Parameterized typing with genericsgeneric type definitions—and there are new ones as well that let you inquire about thegeneric nature of the type.METHODS AND PROPERTIES OF SYSTEM.TYPEThere are far too many new methods and properties to look at them all in detail, butthere are two particularly important ones: GetGenericTypeDefinition and Make-GenericType. They are effectively opposites—the first acts on a constructed type,retrieving the generic type definition; the second acts on a generic type definition andreturns a constructed type. Arguably it would have been clearer if this method hadbeen called ConstructGenericType, MakeConstructedType, or some other name withconstruct or constructed in it, but we’re stuck with what we’ve got.Just like normal types, there is only one Type object for any particular type—so callingMakeGenericType twice with the same types as parameters will return the same referencetwice, and calling GetGenericTypeDefinition on two types constructed fromthe same generic type definition will likewise give the same result for both calls.Another method—this time one which already existed in .NET 1.1—that is worthexploring is Type.GetType, and its related Assembly.GetType method, both of whichprovide a dynamic equivalent to typeof. You might expect to be able to feed each lineof the output of listing 3.10 to the GetType method called on an appropriate assembly,but unfortunately life isn’t quite that straightforward. It’s fine for closed constructedtypes—the type arguments just go in square brackets. For generic type definitions,however, you need to remove the square brackets entirely—otherwise GetType thinksyou mean an array type. Listing 3.11 shows all of these methods in action.Listing 3.11Various ways of retrieving generic and constructed Type objectsstring listTypeName = "System.Collections.Generic.List`1";Type defByName = Type.GetType(listTypeName);Type closedByName = Type.GetType(listTypeName+"[System.String]");Type closedByMethod = defByName.MakeGenericType(typeof(string));Type closedByTypeof = typeof(List);Console.WriteLine (closedByMethod==closedByName);Console.WriteLine (closedByName==closedByTypeof);Type defByTypeof = typeof(List);Type defByMethod = closedByName.GetGenericTypeDefinition();Console.WriteLine (defByMethod==defByName);Console.WriteLine (defByName==defByTypeof);The output of listing 3.11 is just True four times, validating that however you obtain areference to a particular type object, there is only one such object involved.As I mentioned earlier, there are many new methods and properties on Type,such as GetGenericArguments, IsGenericTypeDefinition, and IsGenericType. Thedocumentation for IsGenericType is probably the best starting point for furtherexploration.Licensed to Rhona Hadida