IF + Message box

Hi,

i need a piece of code that asks the question whether another WB is open, if the answer is Yes, i need the msgbox+then exit sub, if the answer is No, i want to carry on through the sub

I have attempted to start it below but cannot seem to make a messagebox and exit sub work!

Thanks
Seamus

If MsgBox("Have you another Workbook Open?", vbYesNo) = vbYes Then MsgBox "Close that File, you can only have this file open"