Since Form B is a dialog, control won't return to Form A until the user closes Form B. When a user closes a DialogForm, the form is not disposed. So, your calling form can access public properties on the DialogForm. So, all you have to do is make the property on Form B public, and then access it from Form A once control returns there.

frmSelect.ShowDialog()  
int i = frmSelect.TreeItemSelected

~~Bonnie