FileSystemWatcher

This class monitors a directory for changes. A program may need to process new files (often written to the directory by other programs).

Many features

FileSystemWatcher provides a way to monitor a directory for file changes, creations, deletions and renames. We do not need Windows Forms to use it.

A console program

This program uses a FileSystemWatcher static field. We initialize it with the new keyword in Init(). We add its Changed event handler.

using System;
using System.IO;

class Program
{
    static void Main()
    {
        Console.WriteLine("MAIN");
        Init();
        // Run an infinite loop.
        while (true)
        {
            Console.WriteLine("TYPE SOMETHING");
            string line = Console.ReadLine();
            Console.WriteLine("TYPED: " + line);
        }
    }

    /// <summary>
    /// Watcher.
    /// </summary>
    static FileSystemWatcher _watcher;

    /// <summary>
    /// Init.
    /// </summary>
    static void Init()
    {
        Console.WriteLine("INIT");
        string directory = @"C:\programs\";
        Program._watcher = new FileSystemWatcher(directory);
        Program._watcher.Changed +=
            new FileSystemEventHandler(Program._watcher_Changed);
        Program._watcher.EnableRaisingEvents = true;
        Program._watcher.IncludeSubdirectories = true;
    }

    /// <summary>
    /// Handler.
    /// </summary>
    static void _watcher_Changed(object sender, FileSystemEventArgs e)
    {
        Console.WriteLine("CHANGED, NAME: " + e.Name);
        Console.WriteLine("CHANGED, FULLPATH: " + e.FullPath);
        // Can change program state (set invalid state) in this method.
        // ... Better to use insensitive compares for file names.
    }
}
MAIN
INIT
TYPE SOMETHING
test
TYPED: test
TYPE SOMETHING
CHANGED, NAME: file.txt
CHANGED, FULLPATH: C:\programs\file.txt
CHANGED, NAME: file.txt
CHANGED, FULLPATH: C:\programs\file.txt
hello
TYPED: hello
TYPE SOMETHING
CHANGED, NAME: file.txt
CHANGED, FULLPATH: C:\programs\file.txt
CHANGED, NAME: file.txt
CHANGED, FULLPATH: C:\programs\file.txt
HELLO WORLD!
X2
X3

Windows Forms tutorial

Before you can write any code, you must set up your Windows Forms program properly. Open the Toolbox and double-click on the FileSystemWatcher icon.

using System.IO;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        void fileSystemWatcher1_Changed(object sender, FileSystemEventArgs e)
        {
            // Occurs when the contents of the file change.
            MessageBox.Show(string.Format("Changed: {0} {1}", e.FullPath, e.ChangeType));
        }

        void fileSystemWatcher1_Created(object sender, FileSystemEventArgs e)
        {
            // FullPath is the new file's path.
            MessageBox.Show(string.Format("Created: {0} {1}", e.FullPath, e.ChangeType));
        }

        void fileSystemWatcher1_Deleted(object sender, FileSystemEventArgs e)
        {
            // FullPath is the location of where the file used to be.
            MessageBox.Show(string.Format("Deleted: {0} {1}", e.FullPath, e.ChangeType));
        }

        void fileSystemWatcher1_Renamed(object sender, RenamedEventArgs e)
        {
            // FullPath is the new file name.
            MessageBox.Show(string.Format("Renamed: {0} {1}", e.FullPath, e.ChangeType));
        }
    }
}

A problem

You may have a problem with receiving multiple Changed events for a single file change. This must be due to a technical detail about how some programs write files.

Timer

One of my favorite techniques for monitoring a program for changes is using a Timer instance and simply doing a scan on a regular interval.

A summary

We examined the FileSystemWatcher class. This type presents an excellent way to persistently monitor a directory for changes, without lots of custom code.