How to compare case sensitive string in EF?
public override bool ValidateUser(string username, string password)
{
    // Validate User Credentials
    var r = db.ST_Users.FirstOrDefault(
                           x => x.Username.Equals(username) &&
                                x.Password.Equals(password));
    return r != null ? true : false;
}

var r = db.ST_Users.SingleOrDefault(x => x.Username == username);

return r != null && r.Password == password;

x => x.Username == username && x.Password == password