9.2. Writing FilesΒΆ

Try the following:

  1. In Xamarin Studio build, not run, the project first_file. Build is the first selection in the local popup menu for first_file in the Solution pad. Recall to get the local popup menu
    • go to the Solution pad
    • right click on the project (Mac control-click)
  2. Next open an operating system directory window for the project. With Xamarin Studio open, a quick way to do that is to go to the same popup window, and this time select “Open Containing Folder”.
  3. Besides the project files from the Solutions pad, in the directory window you should also see a folder bin. Change to that folder and then to its sub-folder Debug. This is where the build step put its result first_file.exe and debug information first_file.exe.mdb. You should see no other file. Leave this window open.
  4. Now, back in Xamarin Studio, run the project. Depending on your operating system, you may or may not see a Console Window pop up. If you do see one, you should not see any evidence of program results. If you got a window, close it.
  5. Look at the directory window again. 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.


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 this Debug directory. This will be inconvenient in many circumstances, and later in the chapter we will see how to minimize the issue.

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!

As discussed above, Xamarin Studio places sample.txt in the Debug sub-subfolder, a hard-to-guess place in the file system, that is not shown in the Solution pad, so do not look for it there! As you should have checked above, you can see it in an operating system file window. Do drill down to the Debug folder if you have not already; open the sample.txt file with your favorite text processor. It should contain just what was written!

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.