Routing Execution When an Error Occurs
An error handler specifies what happens within a procedure when an error occurs. For example, you may want the procedure to end if a certain error occurs, or you may want to correct the condition that caused the error and resume execution. The On Error and Resume statements determine how execution proceeds in the event of an error.
The On Error Statement
The On Error statement enables or disables an error-handling routine. If an error-handling routine is enabled, execution passes to the error-handling routine when an error occurs.
There are three forms of the On Error statement: On Error GoTo label, On Error GoTo 0, and On Error Resume Next. The On Error GoTo label statement enables an error-handling routine, beginning with the line on which the statement is found. You should enable the error-handling routine before the first line at which an error could occur. When the error handler is active and an error occurs, execution passes to the line specified by the label argument.
The line specified by the label argument should be the beginning of the error-handling routine. For example, the following procedure specifies that if an error occurs, execution passes to the line labeled Error_MayCauseAnError:
Function MayCauseAnError()
' Enable error handler.
On Error GoTo Error_MayCauseAnError
. ' Include code here that may generate error.
.
.
Error_MayCauseAnError:
. ' Include code here to handle error.
.
.
End Function
The On Error GoTo 0 statement disables error handling within a procedure. It doesn't specify line 0 as the start of the error-handling code, even if the procedure contains a line numbered 0. If there's no On Error GoTo 0 statement in your code, the error handler is automatically disabled when the procedure has run completely. The On Error GoTo 0 statement resets the properties of the Err object, having the same effect as the Clear method of the Err object.
The On Error Resume Next statement ignores the line that causes an error and routes execution to the line following the line that caused the error. Execution isn't interrupted. You can use the On Error Resume Next statement if you want to check the properties of the Err object immediately after a line at which you anticipate an error will occur, and handle the error within the procedure rather than in an error handler.
The Resume Statement
The Resume statement directs execution back to the body of the procedure from within an error-handling routine. You can include a Resume statement within an error-handling routine if you want execution to continue at a particular point in a procedure. However, a Resume statement isn't necessary; you can also end the procedure after the error-handling routine.
There are three forms of the Resume statement. The Resume or Resume 0 statement returns execution to the line at which the error occurred. The Resume Next statement returns execution to the line immediately following the line at which the error occurred. The Resume label statement returns execution to the line specified by the label argument. The label argument must indicate either a line label or a line number.
You typically use the Resume or Resume 0 statement when the user must make a correction. For example, if you prompt the user for the name of a table to open, and the user enters the name of a table that doesn't exist, you can prompt the user again and resume execution with the statement that caused the error.
You use the Resume Next statement when your code corrects for the error within an error handler, and you want to continue execution without rerunning the line that caused the error. You use the Resume label statement when you want to continue execution at another point in the procedure, specified by the label argument. For example, you might want to resume execution at an exit routine, as described in the following section.
Exiting a Procedure
When you include an error-handling routine in a procedure, you should also include an exit routine, so that the error-handling routine will run only if an error occurs. You can specify an exit routine with a line label in the same way that you specify an error-handling routine.
For example, you can add an exit routine to the example in the previous section. If an error doesn't occur, the exit routine runs after the body of the procedure. If an error occurs, then execution passes to the exit routine after the code in the error-handling routine has run. The exit routine contains an Exit statement.
Function MayCauseAnError()
' Enable error handler.
On Error GoTo Error_MayCauseAnError
. ' Include code here that may generate error.
.
.
Exit_MayCauseAnError:
Exit Function
Error_MayCauseAnError:
. ' Include code to handle error.
.
.
' Resume execution with exit routine to exit function.
Resume Exit_MayCauseAnError
End Function
