var personData =
    db.Person
    .Where(p => p.PersonId == currentUserId)
    .Select(p => new
    {
        UserId = p.PersonId,
        Name = p.PersonName,
        Surname = p.PersonSurname,
        Patronymic = p.PersonPatronymic
    })
    .Join(
        db.Membership.
            Where(m => m.UserId == currentUserId)
            .Select(m => new
            {
                UserId = m.UserId,
                UserName = m.Users.UserName,
                Email = m.Email
            }),
        person => person.UserId,
        membershipUser => membershipUser.UserId,
        (person, membershipUser) => new
            {
                UserId = person.UserId,
                UserName = membershipUser.UserName,
                Name = person.Name,
                Surname = person.Surname,
                Patronymic = person.Patronymic,
                Email = membershipUser.Email
            })
    .First();

var personData =
    db.Person
    .Where(p => p.PersonId == currentUserId)
    .Join(
        db.Membership.
            Where(m => m.UserId == currentUserId)
        person => person.UserId,
        membershipUser => membershipUser.UserId,
        (person, membershipUser) => new
            {
                UserId = person.UserId,
                UserName = membershipUser.UserName,
                Name = person.Name,
                Surname = person.Surname,
                Patronymic = person.Patronymic,
                Email = membershipUser.Email
            })
    .First();