On Error statement
Enables an error-handling routine and specifies the location of the routine within a procedure; can also be used to disable an error-handling routine.
On Error GoTo line
On Error Resume Next
On Error GoTo 0
The On Error statement syntax can have any of the following forms:
|On Error GoTo line||Enables the error-handling routine that starts at line specified in the required line argument.
The line argument is any line label or line number.
If a run-time error occurs, control branches to line, making the error handler active.
The specified line must be in the same procedure as the On Error statement; otherwise, a compile-time error occurs.
|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 and execution continues. Use this form rather than On Error GoTo when accessing objects.|
|On Error GoTo 0||Disables any enabled error handler in the current procedure.|
If you don't use an On Error statement, any run-time error that occurs is fatal; that is, an error message is displayed and execution stops.
An "enabled" error handler is one that is turned on by an On Error statement; an "active" error handler is an enabled handler that is in the process of handling an error. If an error occurs while an error handler is active (between the occurrence of the error and a Resume, Exit Sub, Exit Function, or Exit Property statement), the current procedure's error handler can't handle the error. Control returns to the calling procedure.
If the calling procedure has an enabled error handler, it is activated to handle the error. If the calling procedure's error handler is also active, control passes back through previous calling procedures until an enabled, but inactive, error handler is found. If no inactive, enabled error handler is found, the error is fatal at the point at which it actually occurred.
Each time the error handler passes control back to a calling procedure, that procedure becomes the current procedure. After an error is handled by an error handler in any procedure, execution resumes in the current procedure at the point designated by the Resume statement.
Error-handling routines rely on the value in the Number property of the Err object to determine the cause of the error. The error-handling routine should test or save relevant property values in the Err object before any other error can occur or before a procedure that might cause an error is called. The property values in the Err object reflect only the most recent error. The error message associated with Err.Number is contained in Err.Description.
On Error Resume Next causes execution to continue with the statement immediately following the statement that caused the run-time error, or with the statement immediately following the most recent call out of the procedure containing the On Error Resume Next statement. This statement allows execution to continue despite a run-time error. You can place the error-handling routine where the error would occur, rather than transferring control to another location within the procedure. An On Error Resume Next statement becomes inactive when another procedure is called, so you should execute an On Error Resume Next statement in each called routine if you want inline error handling within that routine.
The On Error Resume Next construct may be preferable to On Error GoTo when 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).
On Error GoTo 0 disables error handling in the current procedure. It doesn't specify line 0 as the start of the error-handling code, even if the procedure contains a line numbered 0. Without an On Error GoTo 0 statement, an error handler is automatically disabled when a procedure is exited.
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:
Sub InitializeMatrix(Var1, Var2, Var3, Var4) On Error GoTo ErrorHandler . . . Exit Sub ErrorHandler: . . . 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. Error-handling code can be placed anywhere in a procedure.
Untrapped errors in objects are returned to the controlling application when the object is running as an executable file. Within the development environment, untrapped errors are only returned to the controlling application if the proper options are set. See your host application's documentation for a description of which options should be set during debugging, how to set them, and whether the host can create classes.
If you create an object that accesses other objects, you should try to handle errors passed back from them unhandled. If you cannot handle such errors, map the error code in Err.Number to one of your own errors, and then pass them back to the caller of your object. You should specify your error by adding your error code to the vbObjectError constant. For example, if your error code is 1052, assign it as follows:
Err.Number = vbObjectError + 1052
System errors during calls to Windows dynamic-link libraries (DLL) or Macintosh code resources do not raise exceptions and cannot be trapped with Visual Basic error trapping. When calling DLL functions, you should check each return value for success or failure (according to the API specifications), and in the event of a failure, check the value in the Err object's LastDLLError property. LastDLLError always returns zero on the Macintosh.
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 delete an open file generates error number 55. 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.
The On Error Resume Next statement is then 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 Err object's properties after the error is handled.
Sub OnErrorStatementDemo() On Error GoTo ErrorHandler ' Enable error-handling routine. Open "TESTFILE" For Output As #1 ' Open file for output. Kill "TESTFILE" ' Attempt to delete open ' file. On Error Goto 0 ' Turn off error trapping. On Error Resume Next ' Defer error trapping. ObjectRef = GetObject("MyWord.Basic") ' Try to start nonexistent ' object, then test for 'Check for likely Automation errors. If Err.Number = 440 Or Err.Number = 432 Then ' Tell user what happened. Then clear the Err object. Msg = "There was an error attempting to open the Automation object!" MsgBox Msg, , "Deferred Error Test" 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 55 ' "File already open" error. Close #1 ' Close open file. Case Else ' Handle other situations here... End Select Resume ' Resume execution at same line ' that caused the error. End Sub
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