XL: How to Use "On Error" to Handle Errors in a MacroLast reviewed: February 3, 1998Article ID: Q141571 |
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SUMMARYIn Microsoft Excel, you can use the On Error statement in a Microsoft Visual Basic for Applications macro (Sub procedure) to trap errors and direct procedure flow to the error-handling statements in a procedure. By using error handling, you make your macros and your application easier to use by intercepting run-time errors before the user sees them. This article describes how to use error-handling in a macro and provides several examples of error trapping macro code.
MORE INFORMATIONMicrosoft provides programming examples for illustration only, without warranty either expressed or implied, including, but not limited to, the implied warranties of merchantability and/or fitness for a particular purpose. This article assumes that you are familiar with the programming language being demonstrated and the tools used to create and debug procedures. Microsoft support engineers can help explain the functionality of a particular procedure, but they will not modify these examples to provide added functionality or construct procedures to meet your specific needs. If you have limited programming experience, you may want to contact the Microsoft fee-based consulting line at (800) 936-5200. For more information about the support options available from Microsoft, please see the following page on the World Wide Web:
http://www.microsoft.com/support/supportnet/refguide/default.aspThe following is an example of the basic structure of a macro that uses error-handling:
Sub MyMacro()
On Error GoTo ErrorHandler
. . .
Exit Sub
ErrorHandler:
. . .
Resume <or Exit Sub>
. . .
End Sub
The example contains the following elements: an On Error statement (On
Error GoTo ErrorHandler) and a Resume statement. The error handler might
contain an Error Statement and/or an Error function. Each of these elements
is discussed in greater detail in later sections of this article.
Note that in this example an Exit Sub statement precedes the error handler label. By placing Exit Sub or Exit Function before the error handling routine, you prevent the error-handling code from being run when the macro runs without error.
On Error StatementThe On Error statement enables your application to handle errors that your macro encounters. If you do not use an On Error statement in your procedures, any run-time error that occurs is fatal: that is, Microsoft Excel will generate a run-time error message and the macro will stop running. The following table outlines the On Error statement syntax and describes each type of statement.
This On Error statement Does the following -------------------------------------------------------------------- On Error GoTo <line> Enables the error-handling routine that starts at <line>, which is any line label or line number. The specified line must be in the same procedure as the On Error statement. On Error Resume Next Specifies that when a run-time error occurs, control goes to the statement immediately following the statement where the error occurred. In other words, execution continues. On Error GoTo 0 Disables any enabled error handler in the current procedure. Resume StatementYour error-handling routine will need to determine where macro control should go when an error has occurred. To end the macro when an error has occurred, place the error handling routine either immediately before the End Sub (or End Function) statement or use the Exit Sub (or Exit Function) statement. To return control to another location within the macro, use the Resume statement.
This Resume statement Does the following -------------------------------------------------------------------- Resume [0] Resumes macro execution with the line that caused the error. [0] is an optional argument. Resume Next Resumes macro execution with the line following the line that caused the error. Resume <line> Resumes macro execution at the line number or line label specified by <line>.NOTE: A very common mistake in writing error handling routines is to use a GoTo statement in the routine to specify where macro control should go. Generally, it is not recommended that you use GoTo in an error handling routine; you should use the Resume statement to resume macro execution outside of the error handler.
Error Statement and Error FunctionYou can make your error-handling routine more effective by determining what error has occurred. When a run-time error occurs, you can use the Err function to determine the error number; the Error statement is used to get the text that describes the error. For example, you can use the following statement to display the description of the error that was encountered:
MsgBox Err & ": " & Error(Err)If the error handling routine encountered the error 13 (a type mismatch error), the following text would appear in a dialog box:
13: Type Mismatch EXAMPLES OF MACROS THAT USE ERROR HANDLINGIn these examples, the error handler may be called if there is no disk in drive B, if the B:\XLFiles path cannot be located, or if there is no Book1.xls file in the B:\XLFiles folder (directory).
Example 1--Basic Error Handling Macro
Sub MyMacro()
Dim MyWorkbook As Workbook
' Run the Error handler "ErrHandler" when an error occurs.
On Error GoTo Errhandler
ChDrive "B:"
ChDir "B:\"
ChDir "B:\XLFiles"
Workbooks.Open "Book1.xls"
' Disable the error handler.
On Error GoTo 0
Set MyWorkbook = ActiveWorkbook
MsgBox "The destination workbook is " & MyWorkbook.Name
' Exit the macro so that the error handler is not executed.
Exit Sub
Errhandler:
' If an error occurs, display a message and end the macro.
MsgBox "An error has occurred. The macro will end."
End Sub
This example uses the On Error statement to display a message and end a
macro when an error occurs. If an error occurs in the macro, the error
handler displays the following error message and the macro execution is
halted:
An error has occurred. The macro will end.If the workbook Book1.xls is successfully opened, a message is displayed, showing the destination workbook, and the macro ends because there is an Exit Sub statement before the error handler label "ErrHandler."
Example 2--Error Handling Macro That Displays a Specific Error MessageThis next example is similar to the macro in Example 1; however, this macro implements the Error statement and the Err function to show a more descriptive error message when an error is encountered.
Sub MyMacro()
Dim MyWorkbook As Workbook
' Run the Error handler "ErrHandler" when an error occurs.
On Error GoTo Errhandler
ChDrive "B:"
ChDir "B:\"
ChDir "B:\XLFiles"
Workbooks.Open "Book1.xls"
' Disable the error handler.
On Error GoTo 0
Set MyWorkbook = ActiveWorkbook
MsgBox "The destination workbook is " & MyWorkbook.Name
' Exit the macro so that the error handler is not executed.
Exit Sub
Errhandler:
Select Case Err
Case 68, 75: ' Error 68: "Device not available"
' Error 75: "Path/File Access Error"
MsgBox "There is an error reading drive B."
Case 76: ' Error 76: "Path not found"
MsgBox "The specified path is not found."
Case Else: ' An error other than 68, 75 or 76 has occurred.
' Display the error number and the error text.
MsgBox "Error # " & Err & " : " & Error(Err)
End Select
' End the macro.
End Sub
If an error occurs in the macro one of the following will occur:
Example 3--Macro that Uses the Resume StatementThis next example uses the Resume statement to resume macro execution based on choices that the user makes when an error occurs.
Sub MyMacro()
Dim Result as Integer
Dim ErrMsg as String
Dim MyWorkbook as Workbook
' Run the Error handler "ErrHandler" when an error occurs.
On Error GoTo Errhandler
ChDrive "B:"
ChDir "B:\"
ChDir "B:\XLfiles"
Workbooks.Open "Book1.xls"
NewWorkbook:
' Disable the error handler.
On Error GoTo 0
Set MyWorkbook = ActiveWorkbook
MsgBox "The destination workbook is " & MyWorkbook.Name
' Exit the macro so that the error handler is not executed.
Exit Sub
Errhandler:
Select Case Err
Case 68, 75: ' Error 68: "Device not available"
' Error 75: "Path/File access error
ErrMsg = "There is an error reading drive B. Please " & _
"insert a disk and then press OK to continue or " & _
"press Cancel to end this operation."
Result = MsgBox(ErrMsg, vbOKCancel)
' Resume at the line where the error occurred if the user
' clicks OK; otherwise end the macro.
If Result = vbOK Then Resume
Case 76: ' Error 76: Path not found
ErrMsg = "The disk in drive B does not have an XLFiles " & _
"directory. Please insert the correct disk."
Result = MsgBox(ErrMsg, vbOKCancel)
' Resume at the line where the error occurred if the user
' clicks OK; otherwise end the macro.
If Result = vbOK Then Resume
Case Else: ' A different error occurred.
ErrMsg = "An error has occurred opening " & _
"B:\XLFiles\Book1.xls. Use the active workbook as " & _
"the destination?"
Result = MsgBox(ErrMsg, vbYesNo)
' Resume at the label "NewWorkbook" if the user clicks Yes;
' otherwise end the macro.
If Result = vbYes Then Resume NewWorkbook
End Select
' End the macro.
End Sub
If the workbook Book1.xls is successfully opened, a message will be
displayed showing the destination workbook as Book1.xls and the macro will
end because there is an Exit Sub statement before the error handler label
"ErrHandler." If an error occurs in the macro, the error handler will do
one of the following:
Example 4--Centralizing Error HandlingYou can reduce the length of overall code in your application by centralizing the error handling. You can centralize error-handling by creating one or more procedures that handle common errors. The following is a procedure called ErrorHandling that will display a message corresponding to the error number (ErrorValue) that was passed to it and, where possible, allow the user to choose a button to specify which action should be taken following the error. Based on the choice that the user makes, the ErrorHandling procedure will return a value (ReturnValue) for the course of action to the calling procedure. The ReturnValue can be Err_Exit (exit the macro where the error occurred), Err_Resume (resume at the line in the macro where the error occurred), or Err_Resume_Next (resume at the line following the line in the macro where the error occurred).
Public Const Err_Exit = 0
Public Const Err_Resume = 1
Public Const Err_Resume_Next = 2
Sub ErrorHandling(ErrorValue As Integer, ReturnValue As Integer)
Dim Result as Integer
Dim ErrMsg as String
Dim Choices as Integer
Select Case ErrorValue
Case 68: ' Device not available.
ErrMsg = "The device you are trying to access is either " & _
"not online or does not exist. Retry?"
Choices = vbOKCancel
Case 75: ' Path/File access error.
ErrMsg = "There is an error accessing the path and/or " & _
"file specified. Retry?"
Choices = vbOKCancel
Case 76: ' Path not found.
ErrMsg = "The path and/or file specified was not found. Retry?"
Choices = vbOKCancel
Case Else: 'An error other than 68, 75 or 76 has occurred
ErrMsg = "An unrecognized error has occurred ( " & _
Error(Err) & " ). The macro will end."
MsgBox ErrMsg, vbOKOnly
ReturnValue = Err_Exit
Exit Sub
End Select
' Display the error message.
Result = MsgBox(ErrMsg, Choices)
' Determine the ReturnValue based on the user's choice from MsgBox.
If Result = vbOK Then
ReturnValue = Err_Resume
Else
ReturnValue = Err_Exit
End If
End Sub
This next macro demonstrates how you could use the ErrorHandling
procedure when an error is encountered:
Sub MyMacro()
Dim Action As Integer
' Run the Error handler "ErrHandler" when an error occurs.
On Error GoTo Errhandler
ChDrive "B:"
ChDir "B:\"
ChDir "B:\XLFiles"
Workbooks.Open "Book1.xls"
' Exit the macro so that the error handler is not executed.
Exit Sub
Errhandler:
' Run the ErrorHandling macro to display the error and to
' return a value for Action which will determine the appropriate
' action to take (Resume the macro or end the macro)
ErrorHandling Err, Action
If Action = Err_Exit Then
Exit Sub
ElseIf Action = Err_Resume Then
Resume
Else
Resume Next
End If
End Sub
REFERENCESIn Microsoft Excel 97, for more information about handling errors in a macro, click the Index tab in the Microsoft Visual Basic for Applications Help, type the following text:
On Errorand then double-click the selected text to go to the "On Error Statement" topic. In Microsoft Excel 7.0, for more information about handling errors in a macro, click the Index tab in Microsoft Excel Help, type the following text
Error Trappingand then double-click the selected text to go to the "Error Trapping" topic. In Microsoft Excel 5.0, for more information about error trapping see "Visual Basic User's Guide," "Handling Errors and Error Values" For additional information about getting help with Visual Basic for Applications, please see the following article in the Microsoft Knowledge Base:
ARTICLE-ID: Q163435 TITLE : VBA: Programming Resources for Visual Basic for Applications |
Additional query words: 5.00 5.00a 5.00c 7.00 8.00
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