无法获取c#中ldap用户的主组角色，即“域用户”

using System.DirectoryServices;

string ldapPath = "LDAP://ldap.example.com";
string username = "username";
string password = "password";

DirectoryEntry entry = new DirectoryEntry(ldapPath, username, password);

string filter = "(sAMAccountName=username)";
string[] propertiesToLoad = { "memberOf" };

DirectorySearcher searcher = new DirectorySearcher(entry, filter, propertiesToLoad);

SearchResult result = searcher.FindOne();

if (result != null)
{
    DirectoryEntry userEntry = result.GetDirectoryEntry();
    PropertyValueCollection groups = userEntry.Properties["memberOf"];

    foreach (string group in groups)
    {
        Console.WriteLine("Group: " + group);
    }
}