如何使用LINQ to SQL验证用户？

I'm trying to work out how to validate a user in my application but i don't know how. I wish create a form with Username and Password to enter in the application(WPF) using Linq toSql to store and validate Username and password in the database but i don't know so much about Linq to Sql so i keep in stuck now. I have a database with this fields"UserID,UserTypeID,Name,Surname,Username,Password,Email" and here there is the code that i use :I'm trying to work out how to validate a user i