How do I detect when a directory or file changes without constant scanning

Question

Other than reading all the files and comparing them with a previous snapshot, is there a way to detect when a directory changes in C# with Windows? I don't mind PInvoke if that's what it takes.

EDIT The FileSystemWatcher class is great, but one gotcha is that you must kick off any work in the callback notification to a background task/thread to avoid blocking which will cause dropped events.

Answer 1

Use the FileSystemWatcher class - it does what you want. It won't tell you which bytes in the file changed, but it will tell you which files have changes.

From the doc:

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.