Visual Basic Instinct : Using Files: Section VI

Using Files: Section VI

This section will cover file error handling, and demonstrate how to use error trapping to check the status of files.

Before You Start

This section will not explain error trapping in general. You should be familiar with the On Error statement and how to use error codes and error messages.

Error Messages

These are the error messages in Visual Basic related to files:

52 Bad file name or number
53 File not found
54 Bad file mode
55 File already open
57 Device I/O error
58 File already exists
59 Bad record length
61 Disk full
62 Input past end of file
63 Bad record number
67 Too many files
68 Device unavailable
70 Permission denied
71 Disk not ready
74 Can't rename with different drive
75 Path/File access error
76 Path not found

When you try to open a file, any one of these errors can occur, especially the ones concerning file names (Bad file name or number) and file locations (File not found, Path not found). It is important for an application to either check the filenames before it tries to open them, or trap the errors that occure when it opens them.

The error trapping procedures are similar to every other error trapping procedure:

On Error Goto FileError Open "C:\Windows\Non-existing Filename.txt" For Input As #1 Close #1 Exit Sub FileError: Select Case Err.Number Case 53 ' Error handling code would go here Case 55 ' Error handling code would go here End Select

This is a very crude example, but it demonstrates the principle. Now, when you open a file, you could do one out of two things: Either you decide to trap every error that could occure, or you figure out which errors are more likely to occure and trap them.

With the second approach, there's always the possibility that an unexpected error could crash the application. This is a calculated risk -- creating error handling code for file accesses in 200 different procedures is a large job, especially due to Visual Basic's lack of a global error trapping system.

Checking File Status

The error trapping can be useful if you want to check if a file name is valid or not, and also if you have implemented file locking in your applications.

Here's how to test if a file is locked or not:

Public Function FileLocked(ByVal Filename As String) As Boolean

Dim fHandle As Integer On Error Goto FileError fHandle = FreeFile Open Filename For Input As #fHandle Close #fHandle FileLocked = False Goto ExitFunction FileError: Select Case Err.Number Case 70 FileLocked = True Case Else ' An unexpected error. We do not deal with this in this example End Select ExitFunction: End Function