FIO Helper Class

This is an optional section. It was much more important before we realized how easily we much simplified file manipulating projects by changing the Output Path setting.

Still there are a variety of situations where a program may want to access resources in the file system, without know ahead of time exactly which folder contains the file resource. The FIO library class that we display here illustrates how a program can search for the directory in which a file resides, given a list of folder options. This section also illustrates use of methods from the Path and File classes.

In this case we are specifically thinking of two possible uses of program source files: When executing, the project folder may be the current directory or, in the default setup for Xamarain, the current directory may be two folders down in bin/Debug.

We use one idea that is discussed more in the next chapter: We need a sequence of directory strings to look through. At this point we have only discussed sequences of individual characters. The variable paths contains a sequence of directory paths to check. (More on the syntax comes up shortly in Arrays.) In our case we make the sequence contain ".", the current directory, "..", the parent directory, and Path.Combine("..", ".."), the parent’s parent. We make paths a static variable, so it is visible in all the functions in the class.

Then the sequence paths can be used in the foreach loop:

/// Find a directory containing the filename
/// and return the full file path, if it exists.
/// Otherwise return null.
public static string GetPath(string filename)
   foreach (string dir in paths) {
      string filePath = Path.Combine(dir, filename);
      if (File.Exists(filePath))
         return filePath;
   return null;

For each directory path in paths, we create a filePath as if the file were in that directory. We return the first path that actually exists. We allow for the file to not be in any of the directories in paths. If we do not find it, we return null (no object).

For convenience, we have an elaboration, using GetPath, that directly opens the file to read:

/// Find a directory containing filename;
/// return a new StreamReader to the file
/// or null if the file does not exist.
public static StreamReader OpenReader(string filename)
   string filePath = GetPath(filename);
   if (filePath == null)
      return null;
      return new StreamReader( filePath);

We have a variation on GetPath that just return the path to the directory containing the file. Here is the heading:

/// Return a directory conaining the filename, if it exists.
/// Otherwise return null.
public static string GetLocation(string filename)

This is useful in case you want to later write into the same directory that you read from. You can get a location from GetLocation and then write to the same directory, creating a StreamWriter. You can use the convenience function:

/// Join the location directory and filename;
/// open and return a StreamWriter to the file.
public static StreamWriter OpenWriter(string location, string filename)
   string filePath = Path.Combine(location, filename);
   return new StreamWriter(filePath);

The entire FIO class is in fio/fio.cs

We illustrate the use of FIO functions in example file fio_usage/fio_usage.cs:

using System;
using System.IO;

namespace IntroCS
   class FIOTest
      public static void Main(string[] args)
         string sample = "sample.txt";
         string output = "output.txt";
         Console.WriteLine("Directory of {0}: {1}",
                           sample, FIO.GetLocation(sample));
         Console.WriteLine("Path to  {0}: {1}",
                           sample, FIO.GetPath(sample));
         StreamReader reader1 = FIO.OpenReader(sample);
         if (reader1 != null) {
            Console.WriteLine("First reader test passed.");

         StreamReader reader2 = FIO.OpenReader(FIO.GetLocation(sample),
         if (reader2 != null) {
            Console.WriteLine("Second reader test passed.");

         StreamWriter writer1 = FIO.OpenWriter(FIO.GetLocation(sample),
         writer1.WriteLine("File in the same directory as {0}.", sample);
         Console.WriteLine("Writer test passed; file written at \n {0}",

If you look at the fio_usage project in our examples solution, you see that sample.txt is a file in the project folder. The program ends up writing to a new file in the same (project) directory. Remember that even though the new file output.txt appears in the project directory, it does not appear in the Solution pad unless you add it to the project. You can see it in the file system, and open it if you like.

This project was created with the default set up: Output path two folders down. If you change to Output path to the main project folder, it should still work. If you open a terminal/console and go to the project directory, you can compile and run this program, and it will also work.