You can just make a System.Uri object, and use IsFile to verify it’s a file, then Uri.LocalPath to extract the filename.
This is much safer, as it provides you a means to check the validity of the URI as well.
Edit in response to comment:
To get just the full filename, I’d use:
Uri uri = new Uri(hreflink);
if (uri.IsFile) {
string filename = System.IO.Path.GetFileName(uri.LocalPath);
}
This does all of the error checking for you, and is platform-neutral. All of the special cases get handled for you quickly and easily.