c# - app - AD Group membership search crashes app for all users but me!

Are you able to discover which line caused the failure?
What was the exception?

I got "object reference not set to an instance of an object" right around:
int propertyCount = result.Properties["memberOf"].Count;

(I did notice and remember to change the "mydomain" to my actual domain)

You could make a special "debug" version or your program with Console.Writeline()s (or MessageBox.Show()s) before every line of the function, the last one to show will tell you what line it crashed on?

I also did this (had it return a datatable of all the properties it found)

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1. private DataTable authenticateUser()
2. {
3.     DataTable dt = new DataTable();
4.     dt.Columns.Add("Name");
5.     dt.Columns.Add("Value");
6.   string mydomain = "mydomain";
7.   int _SupervisorLevel = 0;
8.   string _Teller = "";
9.   string _Client = "";
10.  
11.   DirectoryEntry de = new DirectoryEntry();
12.   de.Path = "WinNT://"+mydomain+"/" + SystemInformation.UserName.ToString() + ",user";
13.   _Teller = de.Properties["FullName"].Value.ToString();
14.   _Client = SystemInformation.ComputerName.ToString();
15.   //Program._Teller = _Teller;
16.  
17.   DirectoryEntry DE = new DirectoryEntry("LDAP://" + mydomain);
18.   DirectorySearcher search = new DirectorySearcher();
19.   search.SearchRoot = DE;
20.   //search.Filter = "(givenName=" + SystemInformation.UserName.ToString() + ")";
21.   //search.PropertiesToLoad.Add("memberOf");
22.  
23.   try
24.   {
25.       SearchResult result = search.FindOne();
26.       foreach (string PropertyName in result.Properties.PropertyNames)
27.       {
28.         DataRow dr = dt.NewRow();
29.         dr["Name"] = PropertyName;
30.         string valstring = "";
31.         ResultPropertyValueCollection rvpc = result.Properties[PropertyName];
32.         for (int i = 0; i < rvpc.Count; i++)
33.         {
34.             valstring += o.ToString() + "\r\n";
35.         }
36.         dr["Value"] = valstring;
37.         dt.Rows.Add(dr);
38.       }
39.  
40.   }
41.   catch (Exception ex)
42.   {
43.       throw new Exception("Error: " + ex.Message);
44.   }
45. }
46.  

Then I looked at it in a DataGridView, there was nothing about "memberof" or "givenName"

So did the MemberOf thing work for you?
I happen to know I'm in the DomainAdmins group but I saw no group memberships listed?

It did in fact work without me changing anything except inserting my own domain.
Out came all the groups I was registered in. The code knocked it down to only the first group, but stepping through showed me all of them.

Interesting, but now I've forgotten what you used this code for?