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

what is basic idea or code for schedule email?

Answer: 
You could have a database table EmailSchedule(ID, SendTo, Subject, MessageBody, SendDateTime)
When you want to schedule an email to be sent, write to the table.

Then have a process that runs every x minutes and sends all emails where SendDateTime <= Now
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.