protected void BtnOk_Click(object sender, EventArgs e)
        {
            // the user clicked the OK button...

            // ensure the user is an administrator, then execute the remaining code within a
            // LearningStorePrivilegedScope (which grants full access to database views)
            if (!SPFarm.Local.CurrentUserIsAdministrator())
            {
                throw new UnauthorizedAccessException(
                          "Access is denied. Only adminstrators can access this page.");
            }
            using (new LearningStorePrivilegedScope())
            {
                // if the user didn't select an "original instructor", do nothing
                if (OriginalInstructor.SelectedValue.Length == 0)
                {
                    return; // real code would display an error message here
                }
                // if the user didn't enter any "new instructors", do nothing
                if (NewInstructors.Accounts.Count == 0)
                {
                    return; // the <NewInstructors> control already displays a validation error
                }
                // set <originalInstructorId> to the SLK identifier of the selected "original
                // instructor"
                UserItemIdentifier originalInstructorId = new UserItemIdentifier(
                    long.Parse(OriginalInstructor.SelectedValue, CultureInfo.InvariantCulture));

                // execute the following code within the context of the current SharePoint Web site;
                // in fact, the operations below are actually done across the entire site *collection*,
                // but SlkStore.GetStore needs to be passed a Web site, so we use the current site
                using (SPWeb spWeb = SPControl.GetContextWeb(HttpContext.Current))
                {
                    // set <assignmentIds> to a list containing the IDs of the assignments for which
                    // <originalInstructorId> is an instructor
                    List <AssignmentItemIdentifier> assignmentIds = new List <AssignmentItemIdentifier>();
                    SlkStore           slkStore = SlkStore.GetStore(spWeb);
                    LearningStoreJob   job      = slkStore.LearningStore.CreateJob();
                    LearningStoreQuery query    = slkStore.LearningStore.CreateQuery("AllAssignmentIds");
                    query.AddCondition("SPSiteGuid", LearningStoreConditionOperator.Equal,
                                       spWeb.Site.ID);
                    query.AddCondition("InstructorId", LearningStoreConditionOperator.Equal,
                                       originalInstructorId);
                    query.AddColumn("AssignmentId");
                    job.PerformQuery(query);
                    DataRowCollection rows = job.Execute <DataTable>().Rows;
                    OriginalInstructor.Items.Add(String.Empty);
                    foreach (DataRow row in rows)
                    {
                        assignmentIds.Add(new AssignmentItemIdentifier(
                                              (LearningStoreItemIdentifier)row["AssignmentId"]));
                    }

                    // set <newInstructorIds> to a list of SLK numeric user IDs corresponding to the
                    // users in the <NewInstructors> control
                    List <UserItemIdentifier> newInstructorIds = new List <UserItemIdentifier>();
                    foreach (string loginName in NewInstructors.Accounts)
                    {
                        // set <spUser> to the SharePoint SPUser corresponding to <loginName> (which
                        // was retrieved from the <NewInstructors> control>; quit with an error
                        // message if that user isn't in "All People" for this site collection
                        SPUser spUser;
                        try
                        {
                            spUser = spWeb.AllUsers[loginName];
                        }
                        catch (SPException)
                        {
                            NewInstructors.ErrorMessage = "User isn't in \"All People\": " +
                                                          loginName;
                            return;
                        }

                        // set <userKey> to the SLK "user key", which is a string used to identify a
                        // user in the SLK database; SLK uses a user's security identifier (SID) if
                        // they have one, or their login name if they don't (e.g. in the case of forms
                        // authentication)
                        string userKey = String.IsNullOrEmpty(spUser.Sid)
                                                ? spUser.LoginName : spUser.Sid;

                        // set <userId> to the SLK UserItemIdentifier of the user <spUser> by
                        // searching the SLK UserItem table; if the user isn't found in UserItem,
                        // add them
                        UserItemIdentifier userId;
                        job   = slkStore.LearningStore.CreateJob();
                        query = slkStore.LearningStore.CreateQuery("UserItem");
                        query.AddCondition("Key", LearningStoreConditionOperator.Equal, userKey);
                        query.AddColumn("Id");
                        job.PerformQuery(query);
                        rows = job.Execute <DataTable>().Rows;
                        if (rows.Count != 0)
                        {
                            // found user in the SLK UserItem table
                            userId = new UserItemIdentifier(
                                (LearningStoreItemIdentifier)rows[0]["Id"]);
                        }
                        else
                        {
                            // user not found in SLK UserItem table -- add them; we use
                            // LearningStoreJob.AddOrUpdateItem rather than LearningStoreJob.AddItem
                            // to account for the rare case where the user may be added simultaneously
                            // by another process
                            job = slkStore.LearningStore.CreateJob();
                            Dictionary <string, object> findProperties = new Dictionary <string, object>();
                            findProperties["Key"] = userKey;
                            Dictionary <string, object> setProperties = new Dictionary <string, object>();
                            setProperties["Name"] = spUser.Name;
                            job.AddOrUpdateItem("UserItem", findProperties, setProperties, null, true);
                            userId = new UserItemIdentifier(
                                job.Execute <LearningStoreItemIdentifier>());
                        }

                        // update <newInstructorIds>
                        newInstructorIds.Add(userId);
                    }

                    // add each user in <newInstructorIds> as an instructor to each assignment in
                    // <assignmentIds>; set <updatedAssignmentCount> to the number of assignments that
                    // were updated (note that we don't update assignments for which the new
                    // instructors are already instructors)
                    Dictionary <UserItemIdentifier, bool> oldInstructors =
                        new Dictionary <UserItemIdentifier, bool>();
                    int updatedAssignmentCount = 0;
                    foreach (AssignmentItemIdentifier assignmentId in assignmentIds)
                    {
                        AssignmentProperties assignmentProperties =
                            slkStore.GetAssignmentProperties(assignmentId, SlkRole.Instructor);
                        oldInstructors.Clear();
                        foreach (SlkUser slkUser in assignmentProperties.Instructors)
                        {
                            oldInstructors[slkUser.UserId] = true;
                        }
                        int oldInstructorCount = oldInstructors.Count;
                        foreach (UserItemIdentifier userId in newInstructorIds)
                        {
                            if (!oldInstructors.ContainsKey(userId))
                            {
                                assignmentProperties.Instructors.Add(new SlkUser(userId));
                            }
                        }
                        if (assignmentProperties.Instructors.Count != oldInstructorCount)
                        {
                            slkStore.SetAssignmentProperties(assignmentId, assignmentProperties);
                            updatedAssignmentCount++;
                        }
                    }

                    // provide user feedback
                    SuccessPanel.Visible = true;
                    SuccessLabel.Text    =
                        String.Format("Found {0} assignment(s); updated {1} assignment(s).",
                                      assignmentIds.Count, updatedAssignmentCount);
                    OriginalInstructorSection.Visible = false;
                    NewInstructorsSection.Visible     = false;
                    ButtonSection.Visible             = false;
                }
            }
        }