private void RequiringExemptionExamSelectionChanged(object sender, SelectionChangedEventArgs e) { if (RequiringExemptionExams.SelectedValue != null) { Exam ex = (Exam)RequiringExemptionExams.SelectedValue; Student st = (Student)RequiringExemption.SelectedValue; PreviousAttempts.ItemsSource = ex.AttemptsBy((Student)RequiringExemption.SelectedValue); RequiringGiveExemptionButton.IsEnabled = st.CanGetExemption(ex); } else { PreviousAttempts.ItemsSource = null; RequiringGiveExemptionButton.IsEnabled = false; } }