C# 5.0 Programmer's Reference

Classes ❘ 273
Extension Tension
You can add an extension method to a class even if it is declared sealed. If you add
features that don’t match the class’s original purpose, you can make the class more
confusing and harder to use correctly. Adding new features to a sealed class also
violates the idea that the class is in some sense “finished.” To avoid confusion, don’t
add extension methods to a sealed class unless you must.
For more information on extension methods, see the section “Extension Methods”
in Chapter 6, “Methods.”
partial
The partial keyword tells C# that the current declaration defines only part of the class. The following
code shows the Person class broken into two pieces.
partial class Person
{
public string FirstName, LastName;
}
partial class Person
{
public string Street, City, State, Zip;
}
You can break a class into any number of pieces. (If you do break a class into pieces, all of them
must include the partial keyword.) At compile time, C# finds the pieces and combines them to
define the class.
Normally, you wouldn’t break a class into pieces within a single module. In fact, it’s not a good idea to
break a class into pieces unless you must. Keeping all the class’s code in one piece makes it easier to find.
Picking Up the Pieces
The reason partial classes were introduced was to hide automatically generated
code from developers.
For example, when you create a form in a Windows Forms application, the project
contains a lot of code to create and initialize the form’s controls. That code is confusing,
and if you modify it, you can break the form so that it won’t work anymore.
To help protect that code from accidental breakage, Visual Studio places it in a
separate module with a name similar to Form1.Designer.cs. Any code you add to
the form goes into the similarly named file Form1.cs. The partial keyword lets
both of these pieces of code help define the Form1 class.
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