PrincipalContext principalContext =
    new PrincipalContext(ContextType.Domain,
                            domainName.Trim(),
                            domainContainer.Trim());

PrincipalContext principalContext = new PrincipalContext(ContextType.Domain);

// define a "query-by-example" principal - here, we search for all users
UserPrincipal qbeUser = new UserPrincipal(principalContext);

// create your principal searcher passing in the QBE principal
PrincipalSearcher srch = new PrincipalSearcher(qbeUser);

// find all matches
foreach (var found in srch.FindAll())
{
    UserPrincipal user = found as UserPrincipal;
    if (user != null)
    {
        Console.WriteLine(user.SamAccountName);
    }
}

var context = new PrincipalContext(ContextType.Domain);

var context = new PrincipalContext(ContextType.Domain, domainName, domainContainer);

new DirectoryEntry("LDAP://domain_name/container")

new DirectoryEntry("LDAP://domain_name/rootDse")