I am working on C# project and i am using Linq to Sql as Database i have a window form for holding a sale Invoice Detail like ac

I am working on C# project and i am using Linq to Sql as Database i have a window form for holding a sale Invoice Detail like account detail ,invoice value etc and for calculation i have created a class which containing some function to done some spacial task on the invoice and some Database function is already defined in another class which is generated by OR designer in LINQ My question is "is this a right way to using three classes to do one work" can i make my class as a child class of that class which is generated by OR designer in LINQ, or can i make my class as a child class of windows form class ? please gide me the right way