Nested select on Entity Framework

This is what I used in my project and it worked perfectly, it searches for the necessary variables, it changes the variables:

you can try one of these can be very helpful. these check you if they exist only change the variable: one number:

var usuarioexist = (from u in conn.USUARIOS where u.LOGIN == txtLogin.Text select u.LOGIN).ToList();
    if (usuarioexist.Count == 0)

two number:

var auten = (from u in conn0.USUARIOS where u.LOGIN == Login1.UserName && u.PASSWORD == Login1.Password select u.LOGIN).ToList();
    if (auten.Count > 0)

luck.