Please don't do that! Use Events instead - that's how the rest of .NET does this kind of thing, after all. Exactly how depends on the "relationship" between the two forms.
Have a look at these, one of them will fit your circumstances.
The form that creates an instance of another:
MyForm mf = new MyForm();
mf.Show();
Is the "parent", the other form is the "child".
(This doesn't imply any formal MDI relationship)
Transferring information between two forms, Part 1: Parent to Child[
^]
Transferring information between two forms, Part 2: Child to Parent[
^]
Transferring information between two forms, Part 3: Child to Child[
^]
But ... do remember that ShowDialog creates a Modal form - which means that the thread calling ShowDialog does not return from the call until the new form has been closed - so absolutely nothing you try top do to it will occur until that happens. That's by design, and intended for things like Open and Save dialogs - use the Show method for everything else.