VB On Error 使用详解

打不开MSDN，转载网友一篇，原帖地址VB On Error 使用详解

1. 微软提供的On Error Goto 的使用

To prevent error-handling code from running when no error has occurred, place an Exit Sub, Exit Function, or Exit Property statement immediately before the error-handling routine, as in the following fragment:

Public Sub InitializeMatrix(ByVal Var1 As Object, ByVal Var2 As Object)
   On Error GoTo ErrorHandler
   ' Insert code that might generate an error here
   Exit Sub
ErrorHandler:
   ' Insert code to handle the error here
   Resume Next
End Sub

Here, the error-handling code follows the Exit Sub statement and precedes the End Sub statement to separate it from the procedure flow.

2 运行时错误要用On Error Resume Next，然后If Err.Number=...

The On Error Resume Next construct may be preferable to On Error GoTowhen handling errors generated during access to other objects. Checking Err after each interaction with an object removes ambiguity about which object was accessed by the code. You can be sure which object placed the error code in Err.Number, as well as which object originally generated the error (the object specified in Err.Source).

3.Example

This example first uses the On Error GoTo statement to specify the location of an error-handling routine within a procedure. In the example, an attempt to divide by zero generates error number 6. The error is handled in the error-handling routine, and control is then returned to the statement that caused the error. The On Error GoTo 0 statement turns off error trapping. Then the On Error Resume Next statement is used to defer error trapping so that the context for the error generated by the next statement can be known for certain. Note that Err.Clear is used to clear the Errobject's properties after the error is handled.

Public Sub OnErrorDemo()
   On Error GoTo ErrorHandler   ' Enable error-handling routine.
   Dim x As Integer = 32
   Dim y As Integer = 0
   Dim z As Integer
   z = x / y   ' Creates a divide by zero error
   On Error GoTo 0   ' Turn off error trapping.
   On Error Resume Next   ' Defer error trapping.
   z = x / y   ' Creates a divide by zero error again
   If Err.Number = 6 Then
      ' Tell user what happened. Then clear the Err object.
      Dim Msg As String
      Msg = "There was an error attempting to divide by zero!"
      MsgBox(Msg, , "Divide by zero error")
      Err.Clear() ' Clear Err object fields.
   End If
Exit Sub      ' Exit to avoid handler.
ErrorHandler:  ' Error-handling routine.
   Select Case Err.Number   ' Evaluate error number.
      Case 6   ' Divide by zero error
         MsgBox("You attempted to divide by zero!")
         ' Insert code to handle this error
      Case Else
         ' Insert code to handle other situations here...
   End Select
   Resume Next  ' Resume execution at same line
                ' that caused the error.
End Sub

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