How can I make a cell mandatory?

Paste the following code into the code section of ThisWorkbook.


Private Sub Workbook_BeforeSave(ByVal SaveAsUI As Boolean, Cancel As
Boolean)

On Error GoTo ErrorHandler

If Sheets(1).Range("A1") <> "" And Sheets(1).Range("B1") = "" Then
Err.Raise vbObjectError + 1, , _
"Value not in B1 when there is a Value in A1"
End If
Exit Sub

ErrorHandler:

Response = MsgBox("Enter a Value into Cell B1", vbOKOnly, "Your
Program Name")

End Sub