C# 5.0 Programmer's Reference

42 ❘ CHAPTER 3 Program and Code File Structure
If you change the project’s default namespace, any classes that you create in the future are placed in
the new namespace.
Class Definitions
The WindowsFormsApplication1 example program shown earlier included the following
class definition.
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
}
Chapter 12 has a lot more to say about classes. For now you should know that all classes (and
structures, enumerations, and other program elements) must be declared inside some namespace.
One last fact about classes is worth mentioning here during the discussion of program and code file
structure. If you look at the previous class definition, you see that it includes the partial keyword.
That keyword means that this is only part of the class’s definition and that other class statements
may contain more pieces of the class.
TIP Usually a code file includes a single namespace and that defines some or
all of a single primary class. The namespace may also include helper structures,
classes, and other elements used by the primary class. You can define as many
namespaces and classes as you like in a single file, but then managing the file
becomes harder.
If you use the Project menu’s Add Class and other commands to create new
classes, you’ll get one class per file.
In this example, the form’s controls are defined in the file Form1.Desginer.cs. If you look at that file,
you can see the following code (with some code omitted).
namespace WindowsFormsApplication1
{
partial class Form1
{
... Code omitted ...
}
}
This code begins with the same namespace statement used in the file Form1.cs. That means the code
in this file is in the same namespace as the code in the other file.
Inside the namespace statement is a partial class statement that adds more code to the Form1 class.
If a class is split into multiple pieces, then all the pieces must use the partial keyword. That means
if you see a class declared with that keyword, you know there are other pieces somewhere.
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