public static string GetSAMAccountName(string firstName, string lastName, string domain) {
    string result = string.Empty;
    string connectionPrefix = "LDAP://" + domain;
    using (DirectoryEntry entry = new DirectoryEntry(connectionPrefix))
    using (DirectorySearcher searcher = new DirectorySearcher(entry)) {

        searcher.Filter = string.Format("(&(objectClass=user)(&(sn={1})(givenName={0})))", firstName, lastName);
        SearchResult searchResult = searcher.FindOne();

        if (searchResult == null) {
            throw new NullReferenceException(string.Format("unable to locate the user {0} {1} in the {2} domain", firstName, lastName, domain));
        }
        DirectoryEntry directoryObject = searchResult.GetDirectoryEntry();
        result = directoryObject.Properties["sAMAccountName"].Value as string;
        entry.Close();
    }
    return result;
}