Querying Active Directory

Question

I have a piece of code that I am using to retrieve a first name from Active Directory based on a logon id (called a COMITID) that is also in Active Directory.
I am using a filter

The code is as follow:

1.    WindowsIdentity currentIdentity = WindowsIdentity.GetCurrent();
2.    //code above gets the Windows logon ID of the current user
3.    string _userID = System.Security.Principal.WindowsIdentity.GetCurrent().Name.Split('\\').Last();
4.    //all the code below is to strip out everything so I am left with just the domain (string domain)
5.    string s = currentIdentity.Name;
6.    int stop = s.IndexOf("\\");
7.    string domain = (stop > -1) ? s.Substring(0, stop) : string.Empty;

8.    //code below setting the DirectoryEntry and DirectorySearcher variables
9.    DirectoryEntry entry = new DirectoryEntry("LDAP://" + domain);
10.    DirectorySearcher dSearch = new DirectorySearcher(entry);

11.    //code below is the filter
12.    dSearch.Filter = "(objectClass=user)" ;

13.    //code below sets the properties I want to retrieve to the givenName (Fisrt name), sn (the surname) and samAccountName (the logon id)
14.    var propertiesToLoad = new[]
15.    {
16.    "givenName",
17.    "sn",
18.    "samAccountName>"
19.    };

20.    //adding the properties
21.    dSearch.PropertiesToLoad.AddRange(propertiesToLoad);

22.    //looping through Active Directory
23.    foreach (SearchResult searchEntry in dSearch.FindAll())
24.    {
25.    //getting the entry I want
26.    var userEntry = searchEntry.GetDirectoryEntry();
27.    //in reality "XBBLDZQ" will not be hardcoded in; this example is just for testing
28.    if (userEntry.Properties["samAccountName"].Value.ToString().Trim().Contains("XBBLDZQ"))

29.    {
30.    //shows the first name in a messagebox
31.    MessageBox.Show(userEntry.Properties["givenName"].Value.ToString());
32.    }
33.    }



This does not give me back the first name (line 31). However when I change the filter (line 12) from what is there now:
 dSearch.Filter = "(objectClass=user)";
to

dSearch.Filter = "(&(objectClass=user)(samAccountName=XBBLDZQ))";

then it works.
This is obviously not the way I want it to work as I don't want to filter by a single COMITID (XBBLDZQ), I want to loop through them all and pick out the one with the COMITID I want (XBBLDZQ). This is what line 28 should do.
This makes no sense to me as if I change line 12 in the way I described above, it is setting the filter to say "only give me the users with the COMITID XBBLDZQ". If I remove the filter it should give me all COMITID's (so line 31 should still execute).
Am I right or maybe I don't understand the way filters work. If I remove the filter completely then it doesn't work either. The only way it works is if I add the COMITID as a filter like below;

dSearch.Filter = "(&(objectClass=user)(samAccountName=XBBLDZQ))";


But that kind of defeats the purpose of what I want to do in the first place.