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.
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