FileSystemWatcher Class

Listens to the file system change notifications and raises events when a directory, or file in a directory, changes.

Namespace: System.IO
Assembly: System (in system.dll)

public ref class FileSystemWatcher : public Component, ISupportInitialize
public class FileSystemWatcher extends Component implements ISupportInitialize
public class FileSystemWatcher extends Component implements ISupportInitialize
Not applicable.

Use FileSystemWatcher to watch for changes in a specified directory. You can watch for changes in files and subdirectories of the specified directory. You can create a component to watch files on a local computer, a network drive, or a remote computer.

To watch for changes in all files, set the Filter property to an empty string ("") or use wildcards ("*.*"). To watch a specific file, set the Filter property to the file name. For example, to watch for changes in the file MyDoc.txt, set the Filter property to "MyDoc.txt". You can also watch for changes in a certain type of file. For example, to watch for changes in text files, set the Filter property to "*.txt".

There are several types of changes you can watch for in a directory or file. For example, you can watch for changes in Attributes, the LastWrite date and time, or the Size of files or directories. This is done by setting the NotifyFilter property to one of the NotifyFilters values. For more information on the type of changes you can watch, see NotifyFilters.

You can watch for renaming, deletion, or creation of files or directories. For example, to watch for renaming of text files, set the Filter property to "*.txt" and call the WaitForChanged method with a Renamed specified for its parameter.

The Windows operating system notifies your component of file changes in a buffer created by the FileSystemWatcher. If there are many changes in a short time, the buffer can overflow. This causes the component to lose track of changes in the directory, and it will only provide blanket notification. Increasing the size of the buffer with the InternalBufferSize property is expensive, as it comes from non-paged memory that cannot be swapped out to disk, so keep the buffer as small yet large enough to not miss any file change events. To avoid a buffer overflow, use the NotifyFilter and IncludeSubdirectories properties so you can filter out unwanted change notifications.

For a list of initial property values for an instance of FileSystemWatcher, see the FileSystemWatcher constructor.

Please note the following when using the FileSystemWatcher class.

  • Hidden files are not ignored.

  • In some systems, FileSystemWatcher reports changes to files using the short 8.3 file name format. For example, a change to "LongFileName.LongExtension" could be reported as "LongFi~.Lon".

  • This class contains a link demand and an inheritance demand at the class level that applies to all members. A SecurityException is thrown when either the immediate caller or the derived class does not have full-trust permission. For details about security demands, see Link Demands.

Copying and moving folders

The operating system and FileSystemWatcher object interpret a cut-and-paste action or a move action as a rename action for a folder and its contents. If you cut and paste a folder with files into a folder being watched, the FileSystemWatcher object reports only the folder as new, but not its contents because they are essentially only renamed.

To be notified that the contents of folders have been moved or copied into a watched folder, provide OnChanged and OnRenamed event handler methods as suggested in the following table.

Event Handler

Events Handled



Changed, Created, Deleted

Report changes in file attributes, created files, and deleted files.



List the old and new paths of renamed files and folders, expanding recursively if needed.

Events and Buffer Sizes

Note that several factors can affect which file system change events are raised, as described by the following:

  • Common file system operations might raise more than one event. For example, when a file is moved from one directory to another, several OnChanged and some OnCreated and OnDeleted events might be raised. Moving a file is a complex operation that consists of multiple simple operations, therefore raising multiple events. Likewise, some applications (for example, antivirus software) might cause additional file system events that are detected by FileSystemWatcher.

  • The FileSystemWatcher can watch disks as long as they are not switched or removed. The FileSystemWatcher does not raise events for CDs and DVDs, because time stamps and properties cannot change. Remote computers must have one of the required platforms installed for the component to function properly.

  • If multiple FileSystemWatcher objects are watching the same UNC path in Windows XP prior to Service Pack 1, or Windows 2000 SP2 or earlier, then only one of the objects will raise an event. On machines running Windows XP SP1 and newer, Windows 2000 SP3 or newer or Windows Server 2003, all FileSystemWatcher objects will raise the appropriate events.

  • Setting the Filter does not decrease what goes into the buffer.

Note that a FileSystemWatcher does not raise an Error event when an event is missed or when the buffer size is exceeded, due to dependencies with the Windows operating system. To keep from missing events, follow these guidelines:

  • Increasing the buffer size with the InternalBufferSize property can prevent missing file system change events.

  • Avoid watching files with long file names. Consider renaming using shorter names.

  • Keep your event handling code as short as possible.

The following example creates a FileSystemWatcher to watch the directory specified at run time. The component is set to watch for changes in LastWrite and LastAccess time, the creation, deletion, or renaming of text files in the directory. If a file is changed, created, or deleted, the path to the file prints to the console. When a file is renamed, the old and new paths print to the console.

Use the System.Diagnostics and System.IO namespaces for this example.

public ref class Watcher
   // Define the event handlers.
   static void OnChanged( Object^ /*source*/, FileSystemEventArgs^ e )
      // Specify what is done when a file is changed, created, or deleted.
      Console::WriteLine( "File: {0} {1}", e->FullPath, e->ChangeType );

   static void OnRenamed( Object^ /*source*/, RenamedEventArgs^ e )
      // Specify what is done when a file is renamed.
      Console::WriteLine( "File: {0} renamed to {1}", e->OldFullPath, e->FullPath );

   [PermissionSet(SecurityAction::Demand, Name="FullTrust")]
   int static run()
      array<String^>^args = System::Environment::GetCommandLineArgs();

      // If a directory is not specified, exit program.
      if ( args->Length != 2 )
         // Display the proper way to call the program.
         Console::WriteLine( "Usage: Watcher.exe (directory)" );
         return 0;

      // Create a new FileSystemWatcher and set its properties.
      FileSystemWatcher^ watcher = gcnew FileSystemWatcher;
      watcher->Path = args[ 1 ];

      /* Watch for changes in LastAccess and LastWrite times, and 
          the renaming of files or directories. */
      watcher->NotifyFilter = static_cast<NotifyFilters>(NotifyFilters::LastAccess |
            NotifyFilters::LastWrite | NotifyFilters::FileName | NotifyFilters::DirectoryName);

      // Only watch text files.
      watcher->Filter = "*.txt";

      // Add event handlers.
      watcher->Changed += gcnew FileSystemEventHandler( Watcher::OnChanged );
      watcher->Created += gcnew FileSystemEventHandler( Watcher::OnChanged );
      watcher->Deleted += gcnew FileSystemEventHandler( Watcher::OnChanged );
      watcher->Renamed += gcnew RenamedEventHandler( Watcher::OnRenamed );

      // Begin watching.
      watcher->EnableRaisingEvents = true;

      // Wait for the user to quit the program.
      Console::WriteLine( "Press \'q\' to quit the sample." );
      while ( Console::Read() != 'q' )

int main() {

public class Watcher
    public static void main(String[] args1)

    /** @attribute PermissionSet(SecurityAction.Demand, Name="FullTrust")
    public static void Run()
        String args[] = System.Environment.GetCommandLineArgs();

        // If a directory is not specified, exit program.
        if (args.length != 2) {

            // Display the proper way to call the program.
            Console.WriteLine("Usage: Watcher.exe (directory)");

        // Create a new FileSystemWatcher and set its properties.
        FileSystemWatcher watcher = new FileSystemWatcher();

        /* Watch for changes in LastAccess and LastWrite times, and 
           the renaming of files or directories.
            (NotifyFilters.LastAccess |NotifyFilters.LastWrite |
            NotifyFilters.FileName | NotifyFilters.DirectoryName);

        // Only watch text files.

        // Add event handlers.
        watcher.add_Changed(new FileSystemEventHandler(OnChanged));
        watcher.add_Created(new FileSystemEventHandler(OnChanged));
        watcher.add_Deleted(new FileSystemEventHandler(OnChanged));
        watcher.add_Renamed(new RenamedEventHandler(OnRenamed));

        // Begin watching.

        // Wait for the user to quit the program.
        Console.WriteLine("Press \'q\' to quit the sample.");
        while ((Console.Read() != 'q')) {


    // Define the event handlers.
    private static void OnChanged(Object source,FileSystemEventArgs e)
        // Specify what is done when a file is changed, created, or deleted.
        Console.WriteLine(("File: " + e.get_FullPath() + " " 
            + e.get_ChangeType()));
    } //OnChanged

    private static void OnRenamed(Object source, RenamedEventArgs e)
        // Specify what is done when a file is renamed.
        Console.WriteLine("File: {0} renamed to {1}",
    } //OnRenamed
} //Watcher

  • SecurityPermission  for calling members of ProcessStartInfo. Demand value: LinkDemand; Named Permission Sets: FullTrust.
  • SecurityPermission  for deriving from the ProcessStartInfo class. Demand value: InheritanceDemand; Named Permission Sets: FullTrust.

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

Windows 98, Windows Server 2000 SP4, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

The Microsoft .NET Framework 3.0 is supported on Windows Vista, Microsoft Windows XP SP2, and Windows Server 2003 SP1.

.NET Framework

Supported in: 3.0, 2.0, 1.1, 1.0