Nbsp;   Partial Classes, Structures, and Interfaces

In this section, I discuss partial classes, structures, and interfaces. The partial keyword tells the C# compiler that the source code for a single class, structure, or interface definition may span one or more source code files. It should be noted that the compiler combines all of a type’s partials together at compile time; the CLR always works on complete type definitions. There are three main reasons why you might want to split the source code for a type across multiple files:

■ Source controlSuppose a type’s definition consists of a lot of source code, and a program- mer checks it out of source control to make changes. No other programmer will be able to modify the type at the same time without doing a merge later. Using the partial keyword allows you to split the code for the type across multiple source code files, each of which can be checked out individually so that multiple programmers can edit the type at the same time.

■ Splitting a class, structure, or interface into distinct logical units within a single fileI sometimes create a single type that provides multiple features so that the type can provide a complete solution. To simplify my implementation, I will sometimes declare the same partial type repeatedly within a single source code file. Then, in each part of the partial type, I imple- ment one feature with all its fields, methods, properties, events, and so on. This allows me to easily see all the members that provide a single feature grouped together, which simplifies my coding. Also, I can easily comment out a part of the partial type to remove a whole feature from the type and replace it with another implementation (via a new part of the partial type).

■ Code spittersIn Microsoft Visual Studio, when you create a new project, some source code files are created automatically as part of the project. These source code files contain templates that give you a head start at building these kinds of projects. When you use the Visual Studio designers and drag and drop controls onto the design surface, Visual Studio writes source code for you automatically and spits this code into the source code files. This really improves your productivity. Historically, the generated code was emitted into the same source code file

that you were working on. The problem with this is that you might edit the generated code accidentally and cause the designers to stop functioning correctly. Starting with Visual Studio 2005, when you create a new form, user control, and so on, Visual Studio creates two source code files: one for your code and the other for the code generated by the designer. Because the designer code is in a separate file, you’ll be far less likely to accidentally edit it.

The partial keyword is applied to the types in all files. When the files are compiled together, the compiler combines the code to produce one type that is in the resulting .exe or .dll assembly file (or

.netmodule module file). As I stated in the beginning of this section, the partial types feature is com- pletely implemented by the C# compiler; the CLR knows nothing about partial types at all. This is why all of the source code files for the type must use the same programming language, and they must all be compiled together as a single compilation unit.

Date: 2016-03-03; view: 681