On Error Statement (Visual Basic)
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. The On Error statement is used in unstructured error handling and can be used instead of structured exception handling. Structured exception handling is built into .NET, is generally more efficient, and so is recommended when handling runtime errors in your application.
Without error handling or exception handling, any run-time error that occurs is fatal: an error message is displayed, and execution stops.
Note
The Error keyword is also used in the Error Statement, which is supported for backward compatibility.
Syntax
On Error { GoTo [ line | 0 | -1 ] | Resume Next }
Parts
| Term | Definition |
|---|---|
GoTo line |
Enables the error-handling routine that starts at the 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 the specified line, making the error handler active. The specified line must be in the same procedure as the On Error statement or a compile-time error will occur. |
GoTo 0 |
Disables enabled error handler in the current procedure and resets it to Nothing. |
GoTo -1 |
Disables enabled exception in the current procedure and resets it to Nothing. |
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 from that point. Use this form rather than On Error GoTo when accessing objects. |
Remarks
Note
We recommend that you use structured exception handling in your code whenever possible, rather than using unstructured exception handling and the On Error statement. For more information, see Try...Catch...Finally Statement.
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 cannot 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 such 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. Once 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.
Note
An error-handling routine is not a Sub procedure or a Function procedure. It is a section of code marked by a line label or a line number.
Number Property
Error-handling routines rely on the value in the Number property of the Err object to determine the cause of the error. The 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.
Throw Statement
An error that is raised with the Err.Raise method sets the Exception property to a newly created instance of the Exception class. In order to support the raising of exceptions of derived exception types, a Throw statement is supported in the language. This takes a single parameter that is the exception instance to be thrown. The following example shows how these features can be used with the existing exception handling support:
On Error GoTo Handler
Throw New DivideByZeroException()
Handler:
If (TypeOf Err.GetException() Is DivideByZeroException) Then
' Code for handling the error is entered here.
End If
Notice that the On Error GoTo statement traps all errors, regardless of the exception class.
On Error Resume Next
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.
Note
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
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.
On Error GoTo -1
On Error GoTo -1 disables the exception in the current procedure. It does not specify line -1 as the start of the error-handling code, even if the procedure contains a line numbered -1. Without an On Error GoTo -1 statement, an exception 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:
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. You can place error-handling code anywhere in a procedure.
Untrapped Errors
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 returned to the controlling application only 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 any unhandled errors they pass back. If you cannot, map the error codes 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
Caution
System errors during calls to Windows dynamic-link libraries (DLLs) 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.
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 Err object'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 the statement immediately
' following the statement where the error occurred.
End Sub
Requirements
Namespace: Microsoft.VisualBasic
Assembly: Visual Basic Runtime Library (in Microsoft.VisualBasic.dll)
