想读取I:\test.xlsx中的第一个单元格的字符串数据，用下面的代码运行时出错，提示问题出在第4行，请高手帮忙修改，或者给出更简单的方法。         
Excel.Application excelApp = new Excel.ApplicationClass();
Excel.Workbook workbook = excelApp.Workbooks.Open(@"I:\test.xlsx");
Excel.Worksheet worksheet = workbook.Sheets[1] as Excel.Worksheet;
Excel.Range range = worksheet.Range[worksheet.Cells[1, 1],Type.Missing];
MessageBox.Show(range.Value2.ToString());