Try This
Private Sub Listbox1_DoubleClick(Byval sender as Object, Byval e as system.eventargs) Handles Listbox1.DoubleClick
'Check for selected item
If Not listbox1.SelectedItem = Nothing Then
'add it
form1.listbox2.items.add(listbox1.selecteditem)
Else
'if user did not select item, notify them
MsgBox("Please double click an item from the list.", MsgBoxStyle.OkOnly,"Please Select An Item")
Return
End If
End Sub
Parameters
listbox1 = the list box on the second form
listbox2 = the list box on your main form
form1 = the second form
Good Luck in your programming :-)