user authentication form AD

public static bool IsADSUser(string userAlias, string pwd)
{
    #region Source
    bool isADSUser = false;
    if (userAlias.Trim().Length == 0 || pwd.Length == 0)
    {
        isADSUser = false;
    }
    else
    {
        //call ADQuery to get UserInformation
        string filter = String.Format("(&(objectCategory=organizationalPerson)(samaccountname={0}))", userAlias);
        DirectoryEntry de = new DirectoryEntry(ADPath, userAlias, pwd, AuthenticationTypes.Secure);
        DirectorySearcher ds = new DirectorySearcher(de);
        ds.ReferralChasing = ReferralChasingOption.All;
        ds.Filter = filter;
        SearchResult result = null;
        using (de)
        {
            using (ds)
            {
                try
                {
                    result = ds.FindOne();
                }
                catch (Exception ex)
                {
                    if (ex.Message.Contains("Logon failure:"))
                    {
                        result = null;
                    }
                    else
                    {
                        throw ex;
                    }
                }
            }
        }
        if (result != null)
        {
            isADSUser = true;
        }
        else
        {
            isADSUser = false;
        }
    }
    return isADSUser;
    #endregion
}