Security :: Write Custom MemberShip Provider Only To Read Connection String From Other File

In asp.net mvc project I am using MYSQlMemberShipProvider. Now I want that instead of reading the connection string from web.config file, it will read the connection string from external file every time. So that I am implementing the cutsom mebership provider class, this class inherits the MemberShipProvider class. But the problem is that if I inherits the MemberShipProvider class then I have to impelment all of its method in my cutsom membership provider class, But I want to use all other inbuilt methods of MemeberShip. What can i do. I only want to add the code like below:

public class CustomSqlMembershipProvider :MembershipProvider
{
public override void Initialize(string name, NameValueCollection configs)
{
base.Initialize(name, configs);
Connectionstring objProducts = // redaing the connection string.
}
}

But on compiltaion it is giving me the error does not implement inherit abstract member.