C# FileSystemWatcher Examples

Use the FileSystemWatcher class. Specify the Changed event handler to monitor for file changes.

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.
Changed: This is triggered when a file in the directory is changed. It may execute twice per edit.
Note: To prevent duplicate work, it is a good idea to use a flag bool to indicate what happened—so we just set "needs update" to true.
C# program that uses FileSystemWatcher 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. } } Output 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 Contents, 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.
Then: In the bottom tray, select Properties on the fileSystemWatcher1 label. You can specify the directory you want to monitor for changes.
Example: We use a directory named C:\watch. Next in the Properties panel, click on the lightning bolt and add 4 event handlers.
Info: Once the program is run, you can change files in the C:\watch directory and dialog boxes will display the events.
C# program that uses FileSystemWatcher, Windows Forms 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.
Note: You may need special code to filter Changed events. This depends on your application.

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.
And: In this way, you could simply scan a directory every five seconds for changes.
Also: You can then call methods based on the changed detected. You could use the FileInfo type as well as the Timer type for this.
TimerFileInfoFileInfo Length

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.

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