This is the popular question about form collaboration. The most robust solution is implementation of an appropriate interface in form class and passing the interface reference instead of reference to a "whole instance" of a Form. Please see my past solution for more detail: How to copy all the items between listboxes in two forms
Please also see other solutions in this discussion. If the application is simple enough, the solution could be as simple as declaring of some
property in one form and passing a reference to the instance of one form to the instance of another form. For more complex projects, such violation of strictly encapsulated style and loose coupling
could add up the the accidental complexity
of the code and invite mistakes, so the well-encapsulated solution would be preferable.
Please see also: