Question: I want to schedule the email with asp.net and C# code.

what is basic idea or code for schedule email?

Answer: 
Use a scheduler perhaps? [Quartz.NET](http://quartznet.sourceforge.net/) is a pretty decent one.

I assume you already know how to send a mail, so just schedule a new job, and roll with it.
The most simple way to differ the sending of email is to schedule a task in the windows scheduled tasks tool.
This task is a simple call to a vbs file.
This vbs file open an url from your web application.
Behind this url, put a webpage that do your scheduled work inside the app, in this case, the sending of emails.
It doesn't need windows service, just a simple vbs.
The called page is in your app, so no need to do some extra work to interface data or treatment outside of the web app.

Hope this will help,

Regards,

Pierre.