VB.NET Language in a Nutshell, Second Edition

Name

Lock Procedure

Class

Microsoft.VisualBasic.FileSystem

Syntax

Lock(filenumber[, record])

or:

Lock(filenumber[, fromrecord,torecord]

filenumber (required; Integer)

Any valid file number

record (optional; Long)

The record or byte number at which to commence the lock

fromrecord (optional; Long)

The first record or byte number to lock

torecord (optional; Long)

The last record or byte number to lock

Description

The Lock procedure prevents another process from accessing a record, section, or whole file until it is unlocked by the Unlock function.

Use the Lock procedure in situations where multiple programs or more than one instance of your program may need read and write access to the same data file.

Rules at a Glance

• Use the Lock procedure with only the filenumber argument to lock the whole file.

• record is interpreted as a record number in the case of random files and a byte number in the case of binary files. Records and bytes in a file are always numbered sequentially from 1 onward.

• To lock a particular record, specify its record number as record, and only that record will be locked.

• The Lock procedure locks an entire file opened in Input or Output (sequential) mode, regardless of the record argument.

• If you omit the start argument, Lock will lock all records from the start of the file to record or byte number end.

• Attempting to access a locked file or portion of a file returns runtime error 70, “Permission denied.”

Programming Tips and Gotchas

• You must take care to remove all file locks ...