Microsoft KB Archive/192909

= XL98: How to Use "On Error" to Handle Errors in a Macro =

Article ID: 192909

Article Last Modified on 6/17/2005

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APPLIES TO


 * Microsoft Excel 98 for Macintosh

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This article was previously published under Q192909





SUMMARY
In Microsoft Excel 98 Macintosh Edition, 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 INFORMATION
Microsoft provides programming examples for illustration only, without warranty either expressed or implied. This includes, but is not limited to, the implied warranties of merchantability or fitness for a particular purpose. This article assumes that you are familiar with the programming language that is being demonstrated and with the tools that are used to create and to 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 requirements. The 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  . ..

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 Statement
The 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

Enables the error-handling routine that starts at, 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 Statement
Your 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

Resumes macro execution at the line number or line label specified

by.

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 Function
You 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 HANDLING
In these examples, the error handler may be called if there is not a disk in your floppy drive named "DataDisk", or if there is no Workbook1 file in the DataDisk:XLFiles folder.

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 "DataDisk" ChDir "DataDisk:" ChDir "DataDisk:XLFiles" Workbooks.Open "Workbook1"

' 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 Workbook1 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 Message
This 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 "DataDisk" ChDir "DataDisk:" ChDir "DataDisk:XLFiles" Workbooks.Open "Workbook1"

' 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 your floppy drive."

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:


 * If the error is either 68 or 75, the message "There is an error reading drive your floppy drive" will be displayed, and the macro will end.
 * If the error is 76, the message "The specified path is not found" will be displayed, and the macro will end.
 * If the error is an error other than 68, 75, or 76, the message "Error : " will be displayed and the macro will end.

If the Workbook1 workbook is successfully opened, then a message will be displayed showing the destination workbook and the macro will end because there is an Exit Sub statement before the error handler label "ErrHandler."

Example 3--Macro that Uses the Resume Statement
This 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 "DataDisk" ChDir "DataDisk:" ChDir "DataDisk:XLFiles" Workbooks.Open "Workbook1"

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 your floppy drive." & _               "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 your floppy drive 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 " & _ "DataDisk:XLFiles:Workbook1. 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 Workbook1 is successfully opened, a message will be displayed showing the destination workbook as Workbook1 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:

 If the error is either 68 or 75, then the following message is displayed:

There is an error reading drive your floopy drive. Please insert a disk and then press OK to continue or press Cancel to end this operation.

If the user clicks OK in this dialog box, the macro resumes at the line where the error occurred. If the user clicks Cancel, the macro will end. If the error is 76, then the following message will be displayed:

The disk in your floppy drive does not have an XLFiles directory. Please insert the correct disk.

If the user clicks OK in this dialog box, the macro resumes at the line where the error occurred. If the user clicks Cancel, the macro will end. If the error is an error other than 68, 75 or 76, then the following error message is displayed:

An error has occurred opening DataDisk:XLFiles:Workbook1. Use the active workbook as the destination?

If the user clicks Yes in the dialog box, the macro resumes at the line labeled "NewWorkbook." The currently active workbook is displayed as the destination workbook. If the user clicks No, the macro will end.

Example 4--Centralizing Error Handling
You 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 "DataDisk" ChDir "DataDisk:" ChDir "DataDisk:XLFiles" Workbooks.Open "Workbook1"

' 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

