如何用C#为桌面已有Excel文件设置密码（不使用第三方DLL）

Got it, let's break this down into two straightforward steps—no third-party DLLs required, just using .NET's built-in libraries for both tasks.

We'll use Microsoft.Office.Interop.Excel for this, which is part of the .NET Framework's Office interop assemblies. Note: This requires Microsoft Excel to be installed on the machine running the code.

First, add a reference to Microsoft.Office.Interop.Excel in your project (you can find it under the COM tab in Visual Studio's Add Reference dialog).

Here's the code to set a password and save the file:

using Microsoft.Office.Interop.Excel;
using System;

class ExcelPasswordProtector
{
    public static void ProtectExcelFile(string filePath, string password)
    {
        Application excelApp = null;
        Workbook workbook = null;

        try
        {
            // Initialize Excel application (run in background)
            excelApp = new Application();
            excelApp.Visible = false;

            // Open the target workbook
            workbook = excelApp.Workbooks.Open(filePath);

            // Set password required to open the file
            workbook.Password = password;

            // Save changes and close the workbook
            workbook.Save();
            workbook.Close();
        }
        catch (Exception ex)
        {
            Console.WriteLine($"Error protecting Excel file: {ex.Message}");
        }
        finally
        {
            // Clean up COM objects to avoid lingering Excel processes
            if (workbook != null)
                System.Runtime.InteropServices.Marshal.ReleaseComObject(workbook);
            if (excelApp != null)
            {
                excelApp.Quit();
                System.Runtime.InteropServices.Marshal.ReleaseComObject(excelApp);
            }
        }
    }
}

You can call this method like so (it targets your desktop file):

string desktopPath = Environment.GetFolderPath(Environment.SpecialFolder.Desktop);
string excelFilePath = $@"{desktopPath}\YourExcelFile.xlsx";
ProtectExcelFile(excelFilePath, "YourSecurePassword123!");

For this, we'll use System.Net.Mail—a fully native .NET library for email operations.

Here's the code to send the email with your protected Excel file:

using System.Net;
using System.Net.Mail;

class EmailSender
{
    public static void SendEmailWithAttachment(string recipientEmail, string excelFilePath)
    {
        try
        {
            // Build the email message
            MailMessage mail = new MailMessage();
            mail.From = new MailAddress("your-email@example.com");
            mail.To.Add(recipientEmail);
            mail.Subject = "Password-Protected Excel Document";
            mail.Body = "Please find the password-protected Excel file attached. The password is: YourSecurePassword123!";

            // Attach the protected Excel file
            Attachment attachment = new Attachment(excelFilePath);
            mail.Attachments.Add(attachment);

            // Configure your SMTP client (update with your provider's settings)
            SmtpClient smtpClient = new SmtpClient("smtp.example.com", 587);
            smtpClient.Credentials = new NetworkCredential("your-email@example.com", "your-account-password");
            smtpClient.EnableSsl = true; // Most modern providers require this

            // Send the email
            smtpClient.Send(mail);

            // Clean up resources
            attachment.Dispose();
            mail.Dispose();
            smtpClient.Dispose();

            Console.WriteLine("Email sent successfully!");
        }
        catch (Exception ex)
        {
            Console.WriteLine($"Error sending email: {ex.Message}");
        }
    }
}

Call this method right after protecting your Excel file:

SendEmailWithAttachment("recipient@example.com", excelFilePath);

Key Notes #

• SMTP Settings: Replace smtp.example.com, port number, and credentials with your email provider's details. For example:
  • Outlook/Office 365: smtp.office365.com on port 587
  • Gmail: smtp.gmail.com on port 587 (use an app password if 2FA is enabled)
• Excel Interop Limitation: This approach requires Excel to be installed on the machine running the code. Without Excel, there's no native .NET way to manipulate Excel files—third-party libraries would be needed, but you specified avoiding those.
• COM Cleanup: Always release COM objects in the finally block to prevent Excel processes from running in the background unnoticed.

内容的提问来源于stack exchange，提问作者Mayur