Filewatcher

If you want to check folder is empty or not, you can use this.

private static bool IsFolderEmpty(string path)
{
    return !Directory.EnumerateFileSystemEntries(path).Any();
}

string path = @"D:\Publish";
if (IsFolderEmpty(path))
    Console.WriteLine($"Folder : {path} is empty");
else
    Console.WriteLine($"Folder : {path} is not empty");

If you want to do this using FileSystemWatcher

public class FileWatcherController : ApiController
{
    private FileSystemWatcher watcher;

    public FileWatcherController()
    {
        // Set the directory to watch
        string path = @"C:\temp\";
        watcher = new FileSystemWatcher();
        watcher.Path = path;

        // Set the filter to only pick up changes to all files 
        watcher.Filter = "*.*";

        // Enable raising events
        watcher.EnableRaisingEvents = true;

        // Add event handlers
        watcher.Changed += OnChanged;
        watcher.Created += OnChanged;
        watcher.Deleted += OnChanged;
    }

    private void OnChanged(object source, FileSystemEventArgs e)
    {
        // check if folder is empty or not
        if (IsFolderEmpty(watcher.Path))
        {
            Console.WriteLine($"Folder : {watcher.Path} is empty");
        }
        else
        {
            Console.WriteLine($"Folder : {watcher.Path} is not empty");
        }
    }
    private bool IsFolderEmpty(string path)
    {
        return !Directory.EnumerateFileSystemEntries(path).Any();
    }
}