C# 5.0 Programmer's Reference

394 ❘ CHAPTER 17 Configuration and Resources
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Configuration and resource files store information in files with specific formats.
This chapter doesn’t discuss more general uses of files. Reading and writing files,
manipulating the filesystem, working with databases (which may be stored in files),
and working with XML files are all topics covered by later chapters.
Environment Variables
Environment variables give information about the operating system environment in which the program
runs. They hold information such as the computer’s name, the user’s login name, the location
of the system’s temporary directory, the number of processors the system has, and the program’s
current working directory. You can also store configuration information in environment variables
for your programs to use.
There are three types of environment variables that apply at the system, user, and process levels.
System-level variables apply to all processes started on the system; user-level variables apply to
processes started by a particular user; and process-level variables apply to a particular process
and any other processes that it starts.
Environment variables are loaded when a process starts, and they are inherited by any process launched
by the initial process. During program development, variables are loaded when you start Visual Studio.
Their values are inherited by the program you are working on when you start it. If you make changes
to the system’s environment variables, you need to close and reopen Visual Studio before your program
can see the changes.
A program can also create temporary process-level variables that are inherited by any processes you
launch. Those values disappear when the original process ends.
A C# program can use the System.Environment class to read and write environment values. Before
you learn how to use that class, however, you should learn how the operating system sets environment
variables’ values.
Setting Environment Variables
Environment variables are normally set on a systemwide basis before the program begins. In older
operating systems, batch files such as autoexec.bat set these values. More recent systems provide
Control Panel tools to set environment variables.
Newer systems also use an autoexec.nt file to set environment variables that apply only to commandline
(console) applications, so they don’t affect GUI applications. Sometimes, you can use this fact to
your advantage by giving different kinds of applications different environment settings.
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