C# - Set ListBox.Items on Parent Form to Child Form using an eventListener maybe?

I don't think your forms are really child and parent. Without setting Forms.TopLevel to false (don't do it!), you would get an exception at attempt to set any form's Parent.

It looks like a 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[^].

—SA

As the question turned out to be very popular, and my previous answers often were not well understood, probably were not clear enough, I decided to write a Tips/Trick article complete with detailed code samples and explanations: Many Questions Answered at Once — Collaboration between Windows Forms or WPF Windows.

—SA