9.4. Path Strings

When a program is running, there is alway a current working directory. When you run a project through Xamarin Studio, by default the current directory is the directory two levels below the project directory.

Files in the current working directory can to referred to by their simple names, e.g., sample.txt.

Referring to files not in the current directory is more complicated. You should be aware from using the Windows Explorer or the Finder that files and directories are located in a hierarchy of directories in the file system. On a Mac, the file system is unified in one hierarchy. On Windows, each drive has its own hierarchy.

Files are generally referred to by a chain of directories before the final name of the file desired. A path string is used to represent such a sequence of names. Elements of the directory chain are separated by operating system specific punctuation: In Windows the separator is backslash, \, and on a Mac it is (forward) slash, /. For example on a Mac the path


starts with a /, meaning the root or top directory in the hierarchy, and Users is a subdirectory, and anh is a subdirectory of Users (in this case the home directory for the user with login anh). It is similar with Windows, except there may be a drive in the beginning, and the separator is a \, so


is on C: drive; Windows is a subdirectory of the root directory \, and System32 is a subdirectory of Windows. Each drive in Windows has a separate file hierarchy underneath it.

Paths starting from the root of a file system, with \ or / are called absolute paths. Since there is always a current directory, it makes sense to allow a path to be relative to the current directory. In that case do not start with the slash that would indicate the root directory. For example, if the current directory is your home directory, you likely have a subdirectory Downloads, and the Downloads directory might contain examples.zip. From the home directory, this file could be referred to as Downloads\examples.zip or Downloads/examples.zip on a Mac.

Relative to a Xamarin Studio project directory, the current directory for execution of the program is bin\Debug or bin/Debug on a Mac.

Referring to files in the current directory just by their plain file name is actually an example of using relative paths.

With relative paths, you sometimes want to move up the directory hierarchy: .. (two periods) refers to the directory one level up the chain.

Next imagine reversing the relative path from a Xamarin Studio project directory to the current directory for execution: If the current directory is the execution directory, then .. refers to directory bin, and then ..\.. or ../.. refers to the project directory. Further, if the project directory contains the file numbers.txt, then it could be referred to relative to the execution directory as ..\..\numbers.txt or ../../numbers.txt.

Occasionally you need to refer explicitly to the current directory: It is referred to as ”.” (a single period).

9.4.1. Paths in C#

The differing versions of paths for Windows and a Mac are a pain to deal with. Luckily C# abstracts away the differences. It has a Path class in the System.IO namespace that provides many handy functions for dealing with paths in an operating system independent way:

For one thing, C# knows the path separator character for your operating system, Path.DirectorySeparatorChar.

More useful is the function Path.Combine, which takes any number of string parameters for sequential parts of a path, and creates a single string appropriate for the current operating system. For example, Path.Combine("bin", "Debug") will return "bin\Debug" or "bin/debug" as appropriate. Path.Combine("..", "..", "numbers.txt") will return a string with characters ..\..\numbers.txt or ../../numbers.txt.

Even if you know you are going to be on Windows, file paths are a problem because \ is the string escape character. To enter the Windows path above explicitly you would need to have "..\\..\\numbers.txt", or the raw string prefix, @ can come to the rescue: @"..\..\numbers.txt".

You can look at the Path class in the MSDN documentation for many other operations with path strings.

Path strings are used by the Directory Class and by the File Class.