两个步骤：先设Cancel为True，然后调用该控件的Undo方法。

Private Sub ProductName_BeforeUpdate(Cancel As Integer) If(Not IsNull(DLookup("[ProductName]",_ "Products","[ProductName] ='" _ & Me!ProductName & "'"))) Then Msg<a href="/tag/Box/" target="_blank" class="keywords">Box</a> "Product has already been entered in the database." Cancel = True Me!ProductName.Undo End If End Sub

示例来自微软VB帮助