9.4. Path Strings

When a program is running, there is alway a current working directory. Files in the current working directory can to referred to by their simple names, e.g., sample.txt, so with our conventions, project files can be referred to by their simple names.

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

/Users/anh

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

C:\Windows\System32

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.

When you run a project through Xamarin Studio with the default setup, the current directory is the directory two levels below the project directory, in a folder created by the system, bin\Debug or bin/Debug on a Mac. We choose to modify and simplify this in our projects working with files, so the Output Folder is just the project folder.

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.

For example, suppose you solve Safe Sum File Exercise by puting your new project safe_sum_file in the same solution as the original sum_file. That means the parent folder for both projects is the solution folder. If you want to run your new safe_sum_file.cs program (assuming you made the Output Path be the project folder) and want tp open the numbers.txt file in the sum_file project, then, when prompted in the program, you would refer to the file to read as ..\sum_file\numbers.txt in Windows or ../sum_file/numbers.txt on a Mac. Follow this one step at a time: Starting from the safe_sum_file project folder, where the program is running, go up one folder (..) to the solution folder, then down into the sum_file project folder, and refer to the numbers.txt file in that folder.

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("..", "sum_file", "numbers.txt") will return a string with characters ..\sum_file\numbers.txt or ../sum_file/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 "..\\sum_file\\numbers.txt", or the raw string prefix, @ can come to the rescue: @"..\sum_file\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.

9.4.1.1. Path String Exercise

In the path string illustration above to open numbers.txt, we assumed for simplicity that the sum_file and safe_sum_file projects were in the same Xamarin solution. Imagine the following alternate assumptions, more like the way we suggested you actually set up your projects:

  • You have your own solution including the safe_sum_file project.
  • Your solution’s folder and the examples solution folder are both subfolder of the same parent folder.
  • You are running the safe_sum_file.cs program from your safe_sum_file project folder.
  • You want the user to reference the numbers.txt in the sum_file project inside our examples project.

What path string would you enter to be able to open that file?

9.4.1.2. File Line Removal Exercise

Complete the function described below, and make a Main program and sample file to test it. Modify the Xamarin defaults so the Output Path is the project folder.

/// Take all lines from reader that do not start with startToRemove
/// and copy them to writer.
static void FileLineRemoval(StreamReader reader, StreamWriter writer
                            char startToRemove)

For example, in Unix/Mac scripts lines starting with '#' are comment lines. Making startToRemove be '#' would write only non-comment lines to the writer.