When working with collections, use an error handler to easily determine if a given key exists in the collection. If you try to access an item from a collection where the key doesn’t exist, you’ll get an error. Likewise, if you try to add an item that exists, you’ll also get an error. This example shows an error handler for adding an item to a collection. To trap for errors where an item exists, trap error code 457:

 Private Function BuildCustCol(CustList As ListBox) As _	Collection	On Error GoTo ProcError	Dim colCust As Collection	Dim lngCustCnt As Long	Dim J As Long	Set colCust = New Collection	For J = 0 To CustList.ListCount - 1		lngCustCnt = colCust(CStr(CustList.List(J))) + 1		colCust.Remove (CStr(CustList.List(J)))		colCust.Add Item:=lngCustCnt, _			Key:=CStr(CustList.List(J))	Next J	Set BuildCustCol = colCust	Set colCust = Nothing	Exit FunctionProcError:	Select Case Err		Case 5 'collection item doesn't exist, so add it			colCust.Add Item:=0, _				Key:=CStr(CustList.List(J))			Resume		Case Else		'untrapped error	End SelectEnd Function