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
internal
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:
http://en.wikipedia.org/wiki/Accidental_complexity[
^],
http://en.wikipedia.org/wiki/Loose_coupling[
^].
Also, better don't use MDI. Who needs MDI, ever? Why torturing yourself and scaring off your users?
Do yourself a great favor: do not use MDI at all. You can do much easier to implement design without it, with much better quality. MDI is highly discouraged even by Microsoft, in fact, Microsoft dropped it out of WPF and will hardly support it. More importantly, you will scare off all your users if you use MDI. Just don't. I can explain what to do instead. Please see:
http://en.wikipedia.org/wiki/Multiple_document_interface#Disadvantages[
^],
How to Create MDI Parent Window in WPF?[
^].
Please also see my past answers:
Question on using MDI windows in WPF[
^],
MDIContainer giving error[
^],
How to set child forms maximized, last childform minimized[
^].
—SA