Dynamically Change Connection String for Database-First Entity Framework

Entity Framework get connection string directly from web.config, thus we don't need to offer connection string to establish the Entity Framework as it will automatically setup, open, and close the database. In most case, it is very convenient, however, in real project, we usually need to dynamically change connection string for different working environment.

Reference:
https://msdn.microsoft.com/en-us/library/orm-9780596520281-01-16.aspx