Querying Active Directory in C#

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 follows

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

[*]            //code below setting the DirectoryEntry and DirectorySearcher variables
[*]            DirectoryEntry entry = new DirectoryEntry("LDAP://" + domain);
[*]            DirectorySearcher dSearch = new DirectorySearcher(entry);
[*]           
[*]            //code below is the filter
[*]            dSearch.Filter = "(&(objectClass=user))";

[*]            //code below sets the properties I want to retrieve to the givenName (Fisrt name), sn (the surname) and samAccountName (the logon id)
[*]            var propertiesToLoad = new[] 
[*]            { 
[*]            "givenName",
[*]            "sn",
[*]            "samAccountName>"
[*]            };

[*]            //adding the properties
[*]            dSearch.PropertiesToLoad.AddRange(propertiesToLoad);

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

[*]                {
[*]                    //shows the first name in a messagebox
[*]                    MessageBox.Show(userEntry.Properties["givenName"].Value.ToString());
[*]                }
[*]            }
[/LIST]

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.

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

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