LINQ Check exist Username afterwards get that User's ID

I have an Account table: list Account ( UserId int identity(1,
1), UserName nvarchar(20) not zero unique, Password nvarchar(20) not
zero )By controlling LINQ. Whether we can check exist UserName for an
comment then. And it's loyal afterwards get UserId for that account(I'm
use ASP MVC 4 with Entity Framework)