VB.NET Language in a Nutshell

2017-07-07 02:10:07

Unlock Procedure

Class

Microsoft.VisualBasic.FileSystem

Syntax

Unlock( filenumber [, record ) Unlock( filenumber [, fromrecord [, torecord ]])

filenumber (required; Integer): Any valid file number
record (required; Long): The record or byte number at which to commence the lock
fromrecord (required; Long): The first record or byte number to lock
torecord (required; Long): The last record or byte number to lock

Description

Use the Unlock procedure in situations where more than one part of your program may need read and write access to the same data file. The Unlock procedure removes a lock that the Lock procedure placed on a section of the file or the whole file.

Rules at a Glance

Use the Unlock procedure only with the filenumber parameter to unlock the whole file.

The Unlock procedure unlocks an entire file opened in Input or Output (sequential) mode, regardless of the record , fromrecord , or torecord arguments.

Records and bytes in a file are always numbered sequentially from 1 up.

To unlock a particular record, specify its record number as record , and only that record will be unlocked.

To unlock a range of bytes (in a binary file) or of records (in a random file), indicate the starting position as fromrecord and the ending position as torecord .

Programming Tips and Gotchas

You must take care to remove all file locks using the Unlock procedure before either closing a file or ending the application; otherwise , you can leave the file in an unstable state. This means that, where appropriate, your error-handling routines must be made aware of any locks you currently have in place so that they may be removed if necessary.

You use the Lock and Unlock procedures in pairs, and the argument lists of both statements must match exactly.

VB.NET/VB 6 Differences

In VB 6, it is possible to omit the fromrecord argument and provide only the torecord argument, in which case all records (in random mode) or bytes (in binary mode) from the beginning of the file to torecord would be unlocked. In VB.NET, this syntax is not allowed.

VB 6 allows you to precede the filenumber argument with the # symbol. In VB.NET, this syntax is not permitted.

When specifying starting and ending records in VB 6, you use the To keyword to separate them. In VB.NET, this syntax is not permitted; instead, you must use a comma to separate the two arguments.