private void AddExpressionToList() { String addedEexpression = ExpressionTextBox.Text; if (String.IsNullOrEmpty(addedEexpression) || String.IsNullOrWhiteSpace(addedEexpression)) { MessageBox.Show("Expression cannot be null or empty"); return; } try { _cl.AddExpression(addedEexpression); ExpressionList.Items.Add(addedEexpression); ExpressionTextBox.Clear(); } catch (Exception) { MessageBox.Show("Some exception thrown during parce entered expression. Expression can't added to list. Please check it"); } }