public bool DirectoryExists(string directory)
{
    if (String.IsNullOrEmpty(directory))
        throw new ArgumentException("No directory was specified to check for");

    // Ensure directory is ended with / to avoid false positives
    if (!directory.EndsWith("/"))
        directory += "/";

    try
    {
        var request = (FtpWebRequest)WebRequest.Create(directory);
        request.Method = WebRequestMethods.Ftp.ListDirectory;
        request.Credentials = new NetworkCredential(Username, Password);

        using (request.GetResponse())
        {
            return true;
        }
    }
    catch (WebException)
    {
        return false;
    }
}