/// <summary>
        /// Unselect answer sheet
        /// </summary>
        /// <returns></returns>
        public AnswerSheetItemData UnselectAnswerSheet()
        {
            AnswerSheetItemData selected = this.selectedSheet;

            this.selectedSheet = null;

            foreach (AnswerGroupItemData g in this.answerGroupDataList)
            {
                foreach (AnswerSheetItemData sheet in g.AnswerSheetData)
                {
                    sheet.SelectCanvasVisibility = System.Windows.Visibility.Collapsed;
                }
            }

            return selected;
        }
        /// <summary>
        /// Select answer sheet
        /// </summary>
        /// <param name="sheet"></param>
        public void SelectAnswerSheet(AnswerSheetItemData sheet)
        {
            if (this.selectedSheet != null)
            {
                UnselectAnswerSheet();
            }

            this.selectedSheet = sheet;
        }