C# in Depth

Partial types187code generator can own the file where it will work, and completely overwrite thewhole file every time it wants to.Some code generators may even choose not to generate a C# file at all until thebuild is well under way. For instance, the Windows Presentation Foundation version ofthe Snippy application has Extensible Application Markup Language (XAML) filesthat describe the user interface. When the project is built, each XAML file is convertedinto a C# file in the obj directory (the filenames end with “.g.cs” to show they’ve beengenerated) and compiled along with the partial class providing extra code for thattype (typically event handlers and extra construction code). This completely preventsdevelopers from tweaking the generated code, at least without going to extremelengths of hacking the build file.I’ve been careful to use the phrase code generator instead of just designer becausethere are plenty of code generators around besides designers. For instance, in VisualStudio 2005 web service proxies are generated as partial classes, and you may wellhave your own tools that generate code based on other data sources. One reasonablycommon example of this is Object Relational Mapping (ORM)—some ORM tools usedatabase entity descriptions from a configuration file (or straight from the database)and generate partial classes representing those entities.This makes it very straightforward to add behavior to the type: overriding virtualmethods of the base class, adding new members with business logic, and so forth. It’s agreat way of letting the developer and the tool work together, rather than constantlysquabbling about who’s in charge.One scenario that is occasionally useful is for one file to be generated containingmultiple partial types, and then some of those types are enhanced in other files, onemanually generated file per type. To return to the ORM example, the tool could generatea single file containing all the entity definitions, and some of those entities couldhave extra code provided by the developer, using one file per entity. This keeps thenumber of automatically generated files low, but still provides good visibility of themanual code involved.Figure 7.2 shows how the uses of partial types for XAML and entities are similar, butwith slightly different timing involved when it comes to creating the autogenerated C#code.A somewhat different use of partial types is as an aid to refactoring. Sometimes atype gets too big and assumes too many responsibilities. One first step to dividing thebloated type into smaller, more coherent ones can be to first split it into a partial typeover two or more files. This can be done with no risk and in an experimental manner,moving methods between files until each file only addresses a particular concern.Although the next step of splitting the type up is still far from automatic at that stage,it should be a lot easier to see the end goal.When partial types first appeared in C# 2, no one knew exactly how they’d be used.One feature that was almost immediately requested was a way to provide optional“extra” code for generated methods to call. This need has been addressed by C# 3 withpartial methods.Licensed to Rhona Hadida