C# in Depth

Partial types185C# 2 allows more than one file to contribute to a type, and indeed IDEs can extendthis notion so that some of the code that is used for a type may not even be visible asC# source code at all. Types built from multiple source files are called partial types.In this section we’ll also learn about partial methods, which are only relevant in partialtypes and allow a rich but efficient way of adding manually written hooks intoautogenerated code. This is actually a C# 3 feature (this time based on feedback aboutC# 2), but it’s far more logical to discuss it when we examine partial types than to waituntil the next part of the book.7.1.1 Creating a type with multiple filesCreating a partial type is a cinch—you just need to include the partial contextualkeyword in the declaration for the type in each file it occurs in. A partial type can bedeclared within as many files as you like, although all the examples in this sectionuse two.The compiler effectively combines all the source files together before compiling.This means that code in one file can call code in another and vice versa, as shown infigure 7.1—there’s no need for “forward references” or other tricks.You can’t write half of a member in one file and half of it in another—each individualmember has to be complete within its own file. 1 There are a few obvious restrictionsabout the declarations of the type—the declarations have to be compatible. Anyfile can specify interfaces to be implemented (and they don’t have to be implementedin that file); any file can specify the base type; any file can specify a type parameterconstraint. However, if multiple files specify a base type, those base types have to bethe same, and if multiple files specify type parameter constraints, the constraints haveto be identical. Listing 7.1 gives an example of the flexibility afforded (while notdoing anything even remotely useful).Example1.cspartial class Example{void FirstMethod(){SecondMethod();}}void ThirdMethod(){}Example2.cspartial class Example{void SecondMethod(){ThirdMethod();}}Figure 7.1 Code in partial types is able to “see” all of the members of the type,regardless of which file each member is in.1There’s an exception here: partial types can contain nested partial types spread across the same set of files.Licensed to Rhona Hadida