C# in Depth

188 CHAPTER 7 Concluding C# 2: the final featuresGuiPage.xaml.cs(Handwritten C#)GuiPage.xaml(XAML)Schema/model(Database, XML, etc)XAML to C#converter(Build time)Code generator(Prebuild)GuiPage.g.cs(C#)Customer.cs(Handwritten C#)GeneratedEntities.cs(C# - includes partialCustomer class)C# compilationC# compilationGuiPage type(Part of an assembly)Customer type(Part of an assembly)Using XAML for declarative UI designPrebuilding partial classes for database entitiesFigure 7.2Comparison between XAML precompilation and autogenerated entity classes7.1.3 Partial methods—C# 3 only!Just to reiterate my previous explanation, I realize that the rest of this part of the bookhas just been dealing with C# 2 features—but partial methods don’t fit with any of theother C# 3 features and they do fit in very well when describing partial types. Apologiesfor any confusion this may cause.Back to the feature: sometimes we want to be able to specify behavior in a manuallycreated file and use that behavior from an automatically generated file. For instance,in a class that has lots of automatically generated properties, we might want to be ableto specify code to be executed as validation of a new value for some of those properties.Another common scenario is for a code-generation tool to include constructors—manuallywritten code often wants to hook into object construction to setdefault values, perform some logging, and so forth.In C# 2, these requirements could only be met either by using events that the manuallygenerated code could subscribe to, or by making the automatically generatedcode assume that the handwritten code will include methods of a particular name—making the whole code fail to compile unless the relevant methods are provided.Alternatively, the generated code can provide a base class with virtual methods that donothing by default. The manually generated code can then derive from the class andoverride some or all of the methods.All of these solutions are somewhat messy. C# 3’s partial methods effectively provideoptional hooks that have no cost whatsoever if they’re not implemented—any callsto the unimplemented partial methods are removed by the compiler. It’s easiest tounderstand this with an example. Listing 7.2 shows a partial type specified in two files,with the constructor in the automatically generated code calling two partial methods,one of which is implemented in the manually generated code.Licensed to Rhona Hadida