How to password protect an excel file in C#

If we need to protect an excel document with a password by some coding how will we do that? We may require to protect a report by some password after exporting it to MS Excel format. I'm going to demonstrate this by a Console Application with Visual C# code.

First create a console application and add a reference Microsoft.Office.Interop.Excel to it and just write the following piece of code. I've a standard excel file called "Test.xls".

Code:

using System;
using System.Reflection;
using System.Runtime.InteropServices;
using Microsoft.Office.Interop.Excel;

namespace MyConsoleApplication
{
   class Program
   {
       static void Main(string[] args)
       {
           string filePath = @"c:\Test.xls";
           Application application = new Application();
           try
           {
               Workbook myWorkBook = application.Workbooks.Open(filePath,
                                                                Missing.Value,
                                                                false,
                                                                5,
                                                                Missing.Value,
                                                                Missing.Value,
                                                                Missing.Value,
                                                                XlPlatform.xlWindows,
                                                                Missing.Value,
                                                                false,
                                                                false,
                                                                Missing.Value,
                                                                Missing.Value,
                                                                Missing.Value,
                                                                Missing.Value);
              
               myWorkBook.Password = "koushik";
               myWorkBook.Close(true, @"c:\Test2.xls", null);
               Marshal.ReleaseComObject(myWorkBook);
           }
           catch (Exception ex)
           {
               application.Quit();
               Console.WriteLine(ex.Message);
               Console.ReadLine();
           }
       }
   }
}