string message = "You haven't satisfied all " +
                  "of the prerequisites for this course.";
        MessageBox.Show(message, "Request Denied",
               MessageBoxButtons.OK, MessageBoxIcon.Warning);
      }
      else {
        if (status == Section.PREVIOUSLY_ENROLLED) {
          string message = "You are enrolled in or have successfully " +
                    "completed a section of this course.";
          MessageBox.Show(message, "Request Denied",
               MessageBoxButtons.OK, MessageBoxIcon.Warning);
        }
        else {  // success!
          string message = "Seat confirmed in " +
             selected.RepresentedCourse.CourseNo + ".";
          MessageBox.Show(message, "Request Successful",
                 MessageBoxButtons.OK, MessageBoxIcon.Information);

          // Update the list of sections 
          // that this student is registered for.
          registeredListBox.Items.Clear();
          IEnumerator ie = currentUser.GetEnrolledSections();
          while ( ie.MoveNext() ) {
            registeredListBox.Items.Add((Section)ie.Current);
          }

          // Update the field representing student's course total.
          totalTextBox.Text = "" + currentUser.GetCourseTotal();

          // Clear the selection in the schedule of classes list.
          scheduleListBox.SelectedItem = null;
        }
      }
    }

    // Check states of the various buttons.
    ResetButtons();
  }

  // event handling method for "Log Off" Button
  public void LogOffButtonClicked(object source, EventArgs e) {
    ClearFields();
    ssnTextBox.Text = "";
    currentUser = null;

    // Clear the selection in the
    // schedule of classes list.
    scheduleListBox.SelectedItem = null;

    // Check states of the various buttons.
    ResetButtons();

  }

  // event handling method for the "Schedule of Classes" ListBox
  public void ScheduleSelectionChanged(object source, EventArgs e) {
    // When an item is selected in this list,
    // we clear the selection in the other list.
    if (scheduleListBox.SelectedItem != null)  {
      registeredListBox.SelectedItem = null;
    }

    // reset the enabled state of the buttons
    ResetButtons();
  }

  // event handling method for the "Registered For:" ListBox
  public void RegisteredSelectionChanged(object source, EventArgs e) {
    // When an item is selected in this list,
    // we clear the selection in the other list.
    if (registeredListBox.SelectedItem != null)  {
      scheduleListBox.SelectedItem = null;
    }

    // reset the enabled state of the buttons
    ResetButtons();
  }

  // event handling method for the ssn TextBox
  public void SsnTextBoxKeyUp(object source, KeyEventArgs e) {
    // We only want to act if the Enter key is pressed
    if ( e.KeyCode == Keys.Enter ) {
  
      // First, clear the fields reflecting the
      // previous student's information.
      ClearFields();

      // We'll try to construct a Student based on
      // the ssn we read, and if a file containing
      // Student's information cannot be found,
      // we have a problem.

      currentUser = new Student(ssnTextBox.Text);

      // Test to see if the Student fields were properly
      // initialized. If not, reset currentUser to null
      // and display a message box

      if (!currentUser.StudentSuccessfullyInitialized()) {
        // Drat!  The ID was invalid.
        currentUser = null;

        // Let the user know that login failed,
        string message = "Invalid student ID; please try again.";
        MessageBox.Show(message, "Invalid Student ID",
                 MessageBoxButtons.OK, MessageBoxIcon.Warning);
      }
      else {
        // Hooray!  We found one!  Now, we need
        // to request and validate the password.
        passwordDialog = new PasswordForm();
        passwordDialog.ShowDialog(this);

        string password = passwordDialog.Password;
        passwordDialog.Dispose();

        if (currentUser.ValidatePassword(password)) {
          // Let the user know that the
          // login succeeded.
          string message = 
               "Log in succeeded for " + currentUser.Name + ".";
          MessageBox.Show(message, "Log In Succeeded",
                 MessageBoxButtons.OK, MessageBoxIcon.Information);

          // Load the data for the current user into the TextBox and
          // ListBox components.
          SetFields(currentUser);
        }
        else {
          // The ssn was okay, but the password validation failed;
          // notify the user of this.
          string message = "Invalid password; please try again.";
          MessageBox.Show(message, "Invalid Password",
                 MessageBoxButtons.OK, MessageBoxIcon.Warning);
        }
      }
      // Check states of the various buttons.
      ResetButtons();
    }
  }

  // These are private housekeeping methods

  // Because there are so many different situations in which one or
  // more buttons need to be (de)activated, and because the logic is
  // so complex, we centralize it here and then just call this method
  // whenever we need to check the state of one or more of the buttons.  
  // It is a tradeoff of code elegance for execution efficiency:  
  // we are doing a bit more work each time (because we don't need to 
  // reset all four buttons every time), but since the execution time
  // is minimal, this seems like a reasonable tradeoff.
  private void ResetButtons() {
    // There are four conditions which collectively govern the
    // state of each button:
    //	
    // o  Whether a user is logged on or not.
    bool isLoggedOn;
    if (currentUser != null) {
      isLoggedOn = true;
    }
    else {
      isLoggedOn = false;
    }
		
    // o   Whether the user is registered for at least one course.
    bool atLeastOne;
    if (currentUser != null && currentUser.GetCourseTotal() > 0) {
      atLeastOne = true;
    }
    else {
      atLeastOne = false;
    }


    // o   Whether a registered course has been selected.
    bool courseSelected;
    if (registeredListBox.SelectedItem == null) {
      courseSelected = false;
    }
    else {
      courseSelected = true;
    }
		
    // o   Whether an item is selected in the Schedule of Classes.
    bool catalogSelected;
    if (scheduleListBox.SelectedItem == null)  {
      catalogSelected = false;
    }
    else {
      catalogSelected = true;
    }

    // Now, verify the conditions on a button-by-button basis.

    // Drop button:
    if (isLoggedOn && atLeastOne && courseSelected) {
      dropButton.Enabled = true;
    }
    else {
      dropButton.Enabled = false;
    }

    // Add button:
    if (isLoggedOn && catalogSelected) {
      addButton.Enabled = true;
    }
    else {
      addButton.Enabled = false;
    }

    // Save My Schedule button:
    if (isLoggedOn) {
      saveButton.Enabled = true;
    }
    else {
      saveButton.Enabled = false;
    }

    // Log Off button:
    if (isLoggedOn) {
      logOffButton.Enabled = true;  
    }
    else {
      logOffButton.Enabled = false;  
    }
  }

  // Called whenever a user is logged off.
  private void ClearFields() {
    nameTextBox.Text = "";
    totalTextBox.Text = "";
    registeredListBox.Items.Clear();
  }

  // Set the various fields, lists, etc. to reflect the information
  // associated with a particular student.  (Used when logging in.)
  private void SetFields(Student theStudent) {
    nameTextBox.Text = theStudent.Name;
    int total = theStudent.GetCourseTotal();
    totalTextBox.Text = ""+total;

    // If the student is registered for any courses, list these, too.
    if (total > 0) {
      // Use the GetEnrolledSections() method to obtain a list
      // of the sections that the student is registered for and
      // add the sections to the registered ListBox

      IEnumerator e = theStudent.GetEnrolledSections();
      while ( e.MoveNext() ) {
        registeredListBox.Items.Add((Section)e.Current);
      }
    }
  }
}