ways of writing connections in Asp.net

• The very elementary idea is to write the connection    string in a class file. and then use it's object  on our page where ever we required.
• The second is we can write the connection string  in web.config file.
• And the third  idea  is to  write the connection in  class file and make the DLL of that.This one is a standard way of writing  the connection in Asp.Net .