DirectoryEntry de = new DirectoryEntry ("LDAP://novellBox.sample.com","admin","password",AuthenticationTypes.None);
DirectorySearcher ds = new DirectorySearcher(de);
var test = ds.FindAll();

LDAP://novellBox.sample.com/cn=Users,dc=YourCompany,dc=com

using (DirectoryEntry de = new DirectoryEntry ("LDAP://CN=server,DC=domain,DC=com","admin","password",AuthenticationTypes.Secure))
{
    ...
}

Domain domain = Domain.GetDomain(new DirectoryContext(DirectoryContextType.Domain, "novellBox.sample.com");
DirectorySearcher ds = new DirectorySearcher(domain.GetDirectoryEntry(), searchQuery);
using (SearchResultCollection src = ds.FindAll())
{....}

// Autheticate in external LDAP
string ldapserver = "10.1.1.1:389";
string ldapbasedn = "o=mycompany";
string ldapuser = "cn=Administrator,o=mycompany";
string ldappassword = "adminpassword";
string ldapfilter = "(&(objectclass=person)(cn={0}))";

string user = "usertest";
string password = "userpassword";
try
{
    string DN = "";
    using (DirectoryEntry entry = new DirectoryEntry("LDAP://" + ldapserver + "/" + ldapbasedn, ldapuser, ldappassword, AuthenticationTypes.None))
    {
        DirectorySearcher ds = new DirectorySearcher(entry);
        ds.SearchScope = SearchScope.Subtree;
        ds.Filter = string.Format(ldapfilter, user);
        SearchResult result = ds.FindOne();
        if (result != null )
        {
            DN = result.Path.Replace("LDAP://" + ldapserver + "/" , "");
        }
    }
    // try logon
    using (DirectoryEntry entry = new DirectoryEntry("LDAP://" + ldapserver + "/" + ldapbasedn, DN, password, AuthenticationTypes.None))
    {
        DirectorySearcher ds = new DirectorySearcher(entry);
        ds.SearchScope = SearchScope.Subtree;
        SearchResult result = ds.FindOne();
    }
} catch (Exception) { }