how to declare a class( or a method ) that is only accessible through package (namespace)?

i.e.

Assume that
X is a class in package A
Y is a class in package A
Z is a class in package B

Y can access X
Z can not access X

how to declare such X class?

This is easy in Java. juct declare it as
"class X {}" instead of "public class X {}".
How do we do it in C#?