//Instantiate the Application object.
            Excel.Application excelApp = Application;
            //Excel.Application excelApp = Application;

            //Specify the template excel file path.
            string myPath = "Protect and unProtect Worksheets.xlsx";

            //Open the excel file.

            excelApp.Workbooks.Open(myPath, Missing.Value, Missing.Value,
            Missing.Value, Missing.Value,
            Missing.Value, Missing.Value,
            Missing.Value, Missing.Value,
            Missing.Value, Missing.Value,
            Missing.Value, Missing.Value,
            Missing.Value, Missing.Value);

            //Protect the worksheet specifying a password with Structure and Windows attributes.
            ((Excel.Worksheet)excelApp.ActiveSheet).Protect("thispassword",
                missing, missing, missing, missing, missing, missing, missing, missing,
                missing, missing, missing, missing, true, missing, missing);

            //Unprotect the worksheet specifying its password.
            ((Excel.Worksheet)excelApp.ActiveSheet).Unprotect("thispassword");

            //Save the file.
            excelApp.ActiveWorkbook.Save();

            //Quit the Application.
            excelApp.Quit();

//Specify the template excel file path.
            string myPath = "Protect and unProtect Worksheets.xlsx";

            //Instantiate a new Workbook.
            //Open the excel file.
            Workbook workbook = new Workbook(myPath);

            //Protect the worksheet specifying a password with Structure and Windows attributes.
            workbook.Worksheets[workbook.Worksheets.ActiveSheetIndex].Protect(ProtectionType.All, "thispassword", "");

            //Unprotect the worksheet specifying its password.
            workbook.Worksheets[workbook.Worksheets.ActiveSheetIndex].Unprotect("thispassword");

            //Save As the excel file.
            workbook.Save(myPath);