private void button1_Click(object sender, EventArgs e)
        {
            string strUserADsPath = "LDAP://" + txtServer.Text + "/cn=" + textBox1.Text + ",ou=Accounts,dc=Gotham,dc=local";
            DirectoryEntry oUser;
            oUser = new DirectoryEntry(strUserADsPath,txtUserName.Text,txtPassword.Text);
                listBox1.Items.Add("Groups to which " + oUser.Name + " belongs:");
                // Invoke IADsUser::Groups method.
                object groups = oUser.Invoke("Groups");
                foreach (object group in (System.Collections.IEnumerable)groups)
                {
                    // Get the Directory Entry.
                    DirectoryEntry groupEntry = new DirectoryEntry(group);
                    listBox1.Items.Add(groupEntry.Name);
                }
        }

DirectoryEntry oUser = new DirectoryEntry(strUserADsPath,txtUserName.Text,txtPassword.Text);
if(oUser!=null) {
...
}

DirectoryEntry oUser = null;
try {
   oUser = new DirectoryEntry(strUserADsPath,txtUserName.Text,txtPassword.Text);
} catch {
}
if(oUser!=null) {
...
}

listBox1.Items.Add("Groups to which " + oUser.Name + " belongs:");

private void button1_Click(object sender, EventArgs e)
        {
            string strUserADsPath = "LDAP://" + txtServer.Text + "/cn=" + textBox1.Text + ",ou=Accounts,dc=Gotham,dc=local";
            DirectoryEntry oUser = new DirectoryEntry(strUserADsPath, txtUserName.Text, txtPassword.Text);
            try
            {
                listBox1.Items.Add("Groups to which " + oUser.Name + " belongs:");
                // Invoke IADsUser::Groups method.
                object groups = oUser.Invoke("Groups");
                foreach (object group in (System.Collections.IEnumerable)groups)
                {
                    // Get the Directory Entry.
                    DirectoryEntry groupEntry = new DirectoryEntry(group);
                    listBox1.Items.Add(groupEntry.Name);
                }
            }
            catch
            {
                listBox1.Items.Add("User NOT found!");
            }
        }

public class UserInfo {
    public string UserName;
    public string DisplayName;
    public List<string> Groups = new List<string>();
    public bool IsValid = false;
    public UserInfo(string userName) { this.UserName = userName; }
}

public class UserInfoFactory {
    protected DirectoryEntry rootEntry;

    public UserInfoFactory(string domain, string rootPath, string username, string password) {
        this.rootEntry = new DirectoryEntry("LDAP://" + domain + "/" + rootPath,
            domain + @"\" + username, password
            );
    }

    protected string GetFirstValue(DirectoryEntry entry, string propertyName) {
        ICollection props = entry.Properties[propertyName];
        if (props == null || props.Count == 0) { return null; }
        IEnumerator e = props.GetEnumerator();
        e.MoveNext();
        return (e.Current == null) ? null : e.Current.ToString();
    }


    protected void AddGroups(UserInfo info, DirectoryEntry entry) {
        // Invoke IADsUser::Groups method.
        foreach (object group in (System.Collections.IEnumerable)entry.Invoke("Groups")) {
            string groupName = new DirectoryEntry(group).Name;
            int equalsIndex = groupName.IndexOf("=", 1);
            if (equalsIndex != -1) {
                groupName = groupName.Substring((equalsIndex + 1));
            }
            info.Groups.Add(groupName);
        }
    }

    public UserInfo GetUser(string sAMAccountName) {
        UserInfo info = new UserInfo(sAMAccountName);
        try {
            DirectorySearcher search = new DirectorySearcher(this.rootEntry);
            search.Filter = "(SAMAccountName=" + sAMAccountName + ")";
            search.PropertiesToLoad.Add("displayName");
            SearchResult result = search.FindOne();
            if (result==null) { return info; }
            DirectoryEntry entry = result.GetDirectoryEntry();
            info.DisplayName = GetFirstValue(entry, "displayName");
            if (info.DisplayName == null) { info.DisplayName = info.UserName; }
            AddGroups(info, entry);
            info.IsValid = true;
        } catch (Exception ex) {
            Debug.WriteLine(ex.Message);
        }
        return info;
    }

private void button1_Click(object sender, EventArgs e) {
    UserInfoFactory factory = new UserInfoFactory(txtServer.Text, "dc=Gotham,dc=local", txtUserName.Text, txtPassword.Text);
    UserInfo oUser = factory.GetUser(textBox1.Text);
    if (!oUser.IsValid) {
        listBox1.Items.Add("User NOT found!");
    } else {
        listBox1.Items.Add("Groups to which " + oUser.DisplayName + " belongs:");
        foreach (string groupName in oUser.Groups) {
            listBox1.Items.Add(groupName);
        }
        // since it's a List, this might also work, didn't try it
        //listBox1.Items.AddRange(oUser.Groups);
    }
}

        public void DumpEntryProps(DirectoryEntry entry) {
            foreach (PropertyValueCollection pvc in entry.Properties) {
                Debug.WriteLine(pvc.PropertyName + "(" + pvc.Count + ")");
                for (int i = 0; i < pvc.Count; i++) {
                    object obj = pvc[i];
                    if (obj != null) {
                        Debug.WriteLine("\t" + obj.ToString());
                    }
                }
            }
        }

        protected string[] GetGroups(ICollection pvc) {
            List<string> list = new List<string>();
            if ( (pvc!=null) && (pvc.Count>0) ) {
                foreach(string groupName in pvc) {
                    int equalsIndex = groupName.IndexOf("=", 1);
                    int commaIndex = groupName.IndexOf(",", 1);
                    if (equalsIndex!=-1) {
                        string roleName = groupName.Substring((equalsIndex + 1), (commaIndex - equalsIndex) - 1);
                        list.Add(roleName);
                    }
                }
            }
            return list.ToArray();
        }

    public interface IUserInfo : IPrincipal, IIdentity {
        string UserName { get; set; }
        string DisplayName { get; set; }
        string RoleAttributeName { get; }
        string[] GetAttributes();
        string[] GetAttributeValues(string attrName);
        void AddAttribute(string attrName, string attrValue);
        void AddAttribute(string attrName, string[] attrValues);
        void DropAttribute(string attrName);
    }