Sample code to query OpenLDAP directory via .NET System.DirectoryServices.Protocols

 This posting is provided "AS IS" with no warranties, and confers no rights. 

 Use of included script samples are subject to the terms specified at https://www.microsoft.com/info/cpyright.htm 

 The sample code below provides a helper class, which performs a paged search against an LDAP directory.

 I tested this code against OpenLDAP 2.4.31. The code is based on the samples provided by Ethan Wilansky, 

 see link at the end of the post. 

 using System.Collections.Generic;
using System.DirectoryServices.Protocols;
using System.Globalization;
using System.Net;
using System.Security;

namespace OpenLDAPNextUID
{
    public class LDAPHelper
    {
        private readonly LdapConnection ldapConnection;
        private readonly string searchBaseDN;
        private readonly int pageSize;

        public LDAPHelper(
            string searchBaseDN,
            string hostName,
            int portNumber,
            AuthType authType,
            string connectionAccountName,
            SecureString connectionAccountPassword,
            int pageSize)
        {
            
            var ldapDirectoryIdentifier = new LdapDirectoryIdentifier(
                hostName,
                portNumber,
                true,
                false);
            
            var networkCredential = new NetworkCredential(
                connectionAccountName,
                connectionAccountPassword);

            ldapConnection = new LdapConnection(
                ldapDirectoryIdentifier,
                networkCredential) 
                {AuthType = authType};

            ldapConnection.SessionOptions.ProtocolVersion = 3;
            
            this.searchBaseDN = searchBaseDN;
            this.pageSize = pageSize;
        }

        public IEnumerable<SearchResultEntryCollection> PagedSearch(
            string searchFilter,
            string[] attributesToLoad)
        {

            var pagedResults = new List<SearchResultEntryCollection>();

            var searchRequest = new SearchRequest
                    (searchBaseDN,
                     searchFilter,
                     SearchScope.Subtree,
                     attributesToLoad);


            var searchOptions = new SearchOptionsControl(SearchOption.DomainScope);
            searchRequest.Controls.Add(searchOptions);

            var pageResultRequestControl = new PageResultRequestControl(pageSize);
            searchRequest.Controls.Add(pageResultRequestControl);

            while (true)
            {
                var searchResponse = (SearchResponse)ldapConnection.SendRequest(searchRequest);
                var pageResponse = (PageResultResponseControl)searchResponse.Controls[0];

                yield return searchResponse.Entries;
                if (pageResponse.Cookie.Length == 0)
                    break;

                pageResultRequestControl.Cookie = pageResponse.Cookie;
            }

            
        }
    }
}

Example of using the helper class

 using System;
using System.DirectoryServices.Protocols;
using System.Security;

namespace OpenLDAP
{
    class Program
    {
        static void Main(string[] args)
        {
            var password = new[]{'P','a','s','s','w','@','r','d'};
            var secureString = new SecureString();
            foreach (var character in password)
                secureString.AppendChar(character);

            var baseOfSearch = "dc=fabrikam,dc=com";
            var ldapHost = "ubuntu.fabrikam.com";
            var ldapPort = 636; //SSL
            var connectAsDN = "cn=admin,dc=fabrikam,dc=com";
            var pageSize = 1000;

            var openLDAPHelper = new LDAPHelper(
                baseOfSearch,
                ldapHost,
                ldapPort,
                AuthType.Basic, 
                connectAsDN,
                secureString,
                pageSize);

            var searchFilter = "nextUID=*";
            var attributesToLoad = new[] {"nextUID"};
            var pagedSearchResults = openLDAPHelper.PagedSearch(
                searchFilter,
                attributesToLoad);

            foreach (var searchResultEntryCollection in pagedSearchResults)
                foreach (SearchResultEntry searchResultEntry in searchResultEntryCollection)
                    Console.WriteLine(searchResultEntry.Attributes["nextUID"][0]);

            Console.Read();

        }
    }
}

Links

Introduction to System.DirectoryServices.Protocols (S.DS.P) by Ethan Wilansky