Recipe 2.13. Accessing Environment Variables


Problem

Your program relies on data stored in DOS-style environment variables, and you're ready to retrieve some of those values.

Solution

Use the My.Application. GetEnvironmentVariable() method to retrieve specific environment variable values.

Discussion

Microsoft's MS-DOS operating system predated Windows, and when Windows was first released, it needed to use and support many of the existing MS-DOS features. One such feature involved environment variables, a collection of name/value pairs that served as a set of global constants programs could read and use. For instance, the PATH variable stored a list of directories Windows used to locate programs. Other applications could read the PATH variable for their own use.

To retrieve the PATH environment variable from Visual Basic, use this statement:

 Dim thePath As String = _    My.Application.GetEnvironmentVariable("PATH") 

An error occurs if you supply a variable name that does not exist. If it does exist, the method returns just the value of the variable, not its name.

Visual Basic also includes a built-in Environ() function that provides similar functionality:

 Dim thePath As String = Environ("PATH") 

If the supplied variable name cannot be found, Environ() returns an empty string without raising an error.

Environ() also retrieves environment variables by numeric position. The following code scans through the set of environment variables until it hits a blank result, indicating the end of the set of variables:

 Dim counter As Integer Dim fullVariable As String Dim namePart As String Dim valuePart As String Dim equalsPosition As Integer For counter = 1 To 255     fullVariable = Environ(counter)     If (fullVariable = "") Then Exit For     equalsPosition = InStr(fullVariable, "=")     If (equalsPosition > 0) Then         namePart = Left(fullVariable, equalsPosition - 1)         valuePart = Mid(fullVariable, equalsPosition + 1)         ' ----- Use these values as needed.     End If Next counter 

See Also

For additional information on environment variables, see the online help included with Microsoft Windows. On Windows XP, access help from the Start button (Start Help), and search for "environment variables."




Visual Basic 2005 Cookbook(c) Solutions for VB 2005 Programmers
Visual Basic 2005 Cookbook: Solutions for VB 2005 Programmers (Cookbooks (OReilly))
ISBN: 0596101775
EAN: 2147483647
Year: 2006
Pages: 400

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