How to display all users from AD via c#?

[Amrulloh]

I tried this code but it failed showing me the error "unknown error" and I think in this part findall() of my code.

 public void User()
        {
            try
            {
                DirectoryEntry entry = new DirectoryEntry("ldap://test.atb.dom");
                entry.AuthenticationType = AuthenticationTypes.Secure;

                DirectorySearcher desearch = new DirectorySearcher(entry);
                desearch.Filter = "(&(objectClass=user))";

                SearchResultCollection results = desearch.FindAll();

                foreach (SearchResult srUser in results)
                {
                    DirectoryEntry de = srUser.GetDirectoryEntry();
                    if (de.Username != null)
                        comboBox1.Items.Add(de.Username);

                }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.ToString());
            }
        }

Any idea how to display users into combobox or how to fix my problem?
thanks