Classes and Inheritance - Illustrated C# 2005 - page 168

Sealed Classes
In the previous section, you saw that an abstract class must be used as a base class—it cannot
be instantiated as a stand-alone class. The opposite is true of a sealed class.

￿ A sealed class can be used only as a stand-alone class—it cannot be used as a base class.

A sealed class is labeled with the sealed modifier. ￿

For example, the following class is a sealed class. Any attempt to use it as the base class of
another class will produce a compile error.

Keyword
↓
sealed class MyClass
{
...
}

External Methods
An external method is a method that does not have an implementation in the declaration.
Often the implementation is in a language other than C#.

External methods are marked with the extern modifier and do not have an implementa-
tion in the class declaration. The implementation is replaced by a semicolon.

￿

Keyword
↓
public static extern int GetCurrentDirectory(int size, StringBuilder buf);
↑
No implementation

￿ Connecting the declaration with the implementation is implementation-dependent,

but is often done using the DLLImport attribute. Attributes are covered in detail in
Chapter 21.