How to use Lock and Unlock statement in VBA?

In VBA (Visual Basic for Applications), the Lock statement is used in conjunction with file operations, specifically when working with files opened in Binary, Random, or Input/Output (I/O) modes. The Lock and Unlock statements control access to a file, or a portion of it, to prevent conflicts when reading from or writing to the file. This is particularly useful in multi-user environments where several processes might access the same file simultaneously.

Basic Syntax

Lock [#]fileNumber [, recordRange]
Unlock [#]fileNumber [, recordRange]
  • fileNumber: The file number used in the Open statement.
  • recordRange: (Optional) Specifies the range of records to lock or unlock. If omitted, the entire file is locked or unlocked.

Steps to Use Lock and Unlock

  1. Open a File: Use the Open statement to open a file for Binary, Random, or Input/Output access.
  2. Lock the File or Part of It: Use Lock to lock the entire file or a specific range of records.
  3. Read from or Write to the File: Perform the file operations while the file or records are locked.
  4. Unlock the File or Part of It: After the operations are complete, use Unlock to release the lock.


Here’s an example of using Lock and Unlock in a Random file access scenario:

Sub LockFileExample()
    Dim fileNum As Integer
    fileNum = FreeFile()

    ' Open file for Random access
    Open "C:\path\to\your\file.dat" For Random As #fileNum Len = Len(YourRecordStructure)

    ' Lock the entire file
    Lock #fileNum

    ' Perform file operations
    ' ...

    ' Unlock the file
    Unlock #fileNum

    ' Close the file
    Close #fileNum
End Sub

In this example, YourRecordStructure would be a user-defined type (UDT) representing the structure of the records in the file. The file is locked before the operations and unlocked afterward.

Important Points

  • Use in Multi-user Environments: The main purpose of Lock and Unlock is to prevent conflicts in multi-user or multi-process environments.
  • File Access Modes: Lock and Unlock are relevant only for files opened in Binary, Random, or Input/Output modes.
  • Avoid Deadlocks: Ensure that locks are not held longer than necessary to avoid deadlocks or performance issues.
  • Error Handling: Always include error handling in your file operations, especially when dealing with locks, to handle cases where a file might already be locked by another process.

Using Lock and Unlock correctly can help maintain data integrity in scenarios where files are accessed concurrently by multiple users or processes.

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