C#: How to check if a open file has been updated

You know the feature for example, you opened C:\test.txt if you also have the same file in another editor, and you edit it there, when you return, the app will prompt that the file has changed, whether you want to update it. How do I check if the file has been updated?

UPDATE

Asked a sister question "Using FileSystemWatcher to watch for ch开发者_运维问答anges to files"

You could use a FileSystemWatcher to get notifications from the file system.

You can either use a FileSystemWatcher, or you can poll for changes at opportune moments.

Note that the FileSystemWatcher may miss changes if under heavy load and is IDisposable. Failure to dispose it properly can cause stability issues (which I've had happen, personally). If you opt for polling, note that FileInfo caches some metadata so you'll need to call the FileInfo.Refresh method if you reuse FileInfo objects. Alternatively, use the File API.

For only a few files, polling is easier and safer to get right since it avoids the OS callback issues of FileSystemWatcher and never misses any events. For large number of files, the FileSystemWatcher is a must to achieve reasonable performance.

Either use FileSystemWatcher (preferred) or compare the last modified date periodically.