NPOI.XSSF.UserModel.XSSFWorkbook.this C# (CSharp) Метод

        public ISheet this[int index]
        {
            get
            {
                return this.GetSheetAt(index);
            }
            set
            {
                if (this.sheets[index] != null)
                {
                    this.sheets[index] = (XSSFSheet)value;
                }
                else
                {
                    this.sheets.Insert(index, (XSSFSheet)value);
                }
            }
        }