C# code to set a Password for an Existing Excel(not for the new excel file), without any 3rd party dll's

Question

I have a Microsoft Excel file on my desktop. How to set a password to that excel using C#? I need this because I will attach this excel and mail it to certain people.

I don't want to use any external dll's for this

Answer 1

Finally I was able to crack this, its a simple bit of code which resolved My issue.

I added "Microsoft Excel 16.0 Object Library" in References. Below is the code

//Namespace

using Excel = Microsoft.Office.Interop.Excel;

//Inside class declare 

private static Excel.Workbook MyBook = null;

private static Excel.Application MyApp = null;

//I have created one button in the design and on that button click event I have written the below code

        private void btn_Pwd_Click(object sender, EventArgs e)
        {
            try
            {
                string DB_PATH = "Your path to select that existing Excel file";
                MyApp = new Excel.Application();
                MyApp.Visible = false;
                MyBook = MyApp.Workbooks.Open(DB_PATH);
                MyBook.Password = "abc";
                MyBook.SaveAs("Your Path to save the copy of that excel which is password protected");
                MyBook.Close();
            }
            catch(Exception ex)
            {
                MessageBox.Show(ex.Message, "Message", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }

            finally
            {

            }


        }

This will create a copy of that Excel file and save it in a path which you specify and this copy will ask for the password to open that excel, in my above code the password is "abc"