This example demonstrates pessimistic locking by setting the LockEdits property to True, and then demonstrates optimistic locking by setting the LockEdits property to False. It also demonstrates what kind of error handling is required in a multiuser database environment in order to modify a field. The PessimisticLock and OptimisticLock functions are required for this procedure to run.
Sub LockEditsX()
Dim dbsNorthwind As Database
Dim rstCustomers As Recordset
Dim strOldName As String
Set dbsNorthwind = OpenDatabase("Northwind.mdb")
Set rstCustomers = _
dbsNorthwind.OpenRecordset("Customers", _
dbOpenDynaset)
With rstCustomers
' Store original data.
strOldName = !CompanyName
If MsgBox("Pessimistic locking demonstration...", _
vbOKCancel) = vbOK Then
' Attempt to modify data with pessimistic locking
' in effect.
If PessimisticLock(rstCustomers, !CompanyName, _
"Acme Foods") Then
MsgBox "Record successfully edited."
' Restore original data...
.Edit
!CompanyName = strOldName
.Update
End If
End If
If MsgBox("Optimistic locking demonstration...", _
vbOKCancel) = vbOK Then
' Attempt to modify data with optimistic locking
' in effect.
If OptimisticLock(rstCustomers, !CompanyName, _
"Acme Foods") Then
MsgBox "Record successfully edited."
' Restore original data...
.Edit
!CompanyName = strOldName
.Update
End If
End If
.Close
End With
dbsNorthwind.Close
End Sub
Function PessimisticLock(rstTemp As Recordset, _
fldTemp As Field, strNew As String) As Boolean
dim ErrLoop as Error
PessimisticLock = True
With rstTemp
.LockEdits = True
' When you set LockEdits to True, you trap for errors
' when you call the Edit method.
On Error GoTo Err_Lock
.Edit
On Error GoTo 0
' If the Edit is still in progress, then no errors
' were triggered; you may modify the data.
If .EditMode = dbEditInProgress Then
fldTemp = strNew
.Update
.Bookmark = .LastModified
Else
' Retrieve current record to see changes made by
' other user.
.Move 0
End If
End With
Exit Function
Err_Lock:
If DBEngine.Errors.Count > 0 Then
' Enumerate the Errors collection.
For Each errLoop In DBEngine.Errors
MsgBox "Error number: " & errLoop.Number & _
vbCr & errLoop.Description
Next errLoop
PessimisticLock = False
End If
Resume Next
End Function
Function OptimisticLock(rstTemp As Recordset, _
fldTemp As Field, strNew As String) As Boolean
dim ErrLoop as Error
OptimisticLock = True
With rstTemp
.LockEdits = False
.Edit
fldTemp = strNew
' When you set LockEdits to False, you trap for errors
' when you call the Update method.
On Error GoTo Err_Lock
.Update
On Error GoTo 0
' If there is no Edit in progress, then no errors were
' triggered; you may modify the data.
If .EditMode = dbEditNone Then
' Move current record pointer to the most recently
' modified record.
.Bookmark = .LastModified
Else
.CancelUpdate
' Retrieve current record to see changes made by
' other user.
.Move 0
End If
End With
Exit Function
Err_Lock:
If DBEngine.Errors.Count > 0 Then
' Enumerate the Errors collection.
For Each errLoop In DBEngine.Errors
MsgBox "Error number: " & errLoop.Number & _
vbCr & errLoop.Description
Next errLoop
OptimisticLock = False
End If
Resume Next
End Function