9.2. Writing FilesΒΆ

Thus far we have only used the Console class for input and output, so we have neither read nor written data from/to files.

By default Xamarin Studio places data files in a place that makes sense for advanced projects, but not for our usage. Thus far it has not mattered, and we have left the default directory structure alone. Now, however, it matters, and will be a lot simpler if we make a small change to the setup of all our projets using data files. This will also simplify command-line usage when we get to it.

Try the following:

  1. In Xamarin Studio go to the example project first_file. Double click on the project line in the Solution pad to open the Options dialog. (If this does not work for some reason you can also open the drop-down project menu and select Options.)

  2. In the left column of the dialog under Build, the last entry should be Output. Click on it. The output path entry should end with


    This is not the way Xamarin sets it up by default. Originally Output Path ended with


    This version has the extra /bin/$(Configuration).


When you create a new project to use data files, make sure the /bin/$(Configuration) is removed from the end of this Output Path field.

Now let us examine the files here.

  1. In the dropdown menu for the first_file project, select “Open Containing Folder” on Windows or “Reveal in Finder” on a Mac. In any event the selection opens the project folder in the operating system, showing all the files, not just the ones you see listed in Xamarin in the project.
  2. Look at the folder. You should not see a file sample.txt. Keep the folder handy.
  3. Now build (not run yet ) the first_file project.
  4. Look at the operating system folder again. You should now see build products, first_file.exe, the executable file, and first_file.exe.mdb, extra debugging information if there are errors in execution.
  5. Now run the program. This program does not produce output to the screen, so just close the execution window.
  6. The program did do something: Look at the operating system folder again. Now you should see a file sample.txt. This is a file created by the program you ran.

Here is the program:

using System;
using System.IO;

namespace IntroCS
   class FirstFile  // basics of file writing
      public static void Main()
         StreamWriter writer = new StreamWriter("sample.txt");
         writer.WriteLine("This program is writing");
         writer.WriteLine("our first file.");

Look at the code. Note the extra namespace being used at the top. You will always need to be using System.IO when working with files. Here is a slightly different use of a dot, ., to indicate a subsidiary namespace.

The first line of Main creates a StreamWriter object assigned to the variable writer. A StreamWriter links C# to your computer’s file system for writing, not reading. Files are objects, like a Random, and use the new syntax to create a new one. The parameter in the constructor gives the name of the file to connect to the program, sample.txt - the same as the file name we saw created by the program.


If the file already existed, the old contents are destroyed silently by creating a StreamWriter.

If you do not use any operating system directory separators in the name ('\' or '/', depending on your operating system), then the file will lie in the current directory, discussed more shortly. The Xamarin Studio default is for this current directory to be the bin/Debug subdirectory. Our change to the output path converts it so the current directory is the main project folder.

The second and third lines of Main write the specified strings to lines in the file. Note that the StreamWriter object writer, not Console, comes before the dot and WriteLine. This is yet another variation on the use of a dot, .: between an object and a function tied to this object. In this situation the function tied to an object is more specifically called a method, in object-oriented terminology. All the uses of a dot (except for a numerical literal value) share a common idea, indicating a named part or attribute of a larger thing.

The last line of Main is important for cleaning up. Until this line, this C# program controls the file, and nothing may be actually written to the operating system file yet: Since initiating a file operation is thousands of times slower than memory operations, C# buffers data, saving small amounts and writing a larger chunk all at once.


The call to the Close method is essential for C# to make sure everything is really written, and to relinquish control of the file for use by other programs.

It is a common bug to write a program where you have the code to add all the data you want to a file, but the program does not end up creating a file. Usually this means you forgot to close the file!

If you were to run the program from the command line instead of from Xamarin Studio, the file would appear in the current directory.

Just as you can use a String Format Operation with functions Write and WriteLine of the Console class, you can also use a format string with the corresponding methods of a StreamWriter, and embed fields by using braces in the format string.