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' This example illustrates the use of the LOCK and UNLOCK statements. A
' sample data record is created in a random-access file containing a customer
' record. The program allows you to update the file, locking the file while
' you use it to prevent access from another terminal.
 
' Note: To use this program, you must have MS-DOS version 3.1 or later, and
' you must run the MS-DOS command SHARE (SHARE.EXE) to enable locking
' operations before you start Visual Basic.
 
TYPE AccountRec                        ' Define the record
   Payer AS STRING * 20
   Address AS STRING * 20
   Place AS STRING * 20
   Owe AS SINGLE
 END TYPE
 DIM CustRec AS AccountRec
 ON ERROR GOTO ErrHandler              ' This section creates a
                                       ' sample record to use
 OPEN "MONITOR" FOR RANDOM SHARED AS #1 LEN = LEN(CustRec)
 CustRec.Payer = "George Washington"
 CustRec.Address = "1 Cherry Tree Lane"
 CustRec.Place = "Mt. Vernon, VA"
 CustRec.Owe = 12!
 PUT #1, 1, CustRec                    ' Put one record in the file
 CLOSE #1
 
' This sections opens the sample record for updating.
 OPEN "MONITOR" FOR RANDOM SHARED AS #1 LEN = LEN(CustRec)
 DO
     Number% = 0                       ' Reset to zero
     DO UNTIL Number% = 1              ' Force user to input 1
          CLS : LOCATE 10, 10
         INPUT "Enter Customer Number? (enter 1)"; Number%
     LOOP
                          ' Lock the current record so another process
     LOCK #1, Number%     ' doesn't change it while you're using it
     GET #1, Number%, CustRec
     LOCATE 11, 10: PRINT "Customer: "; CustRec.Payer
     LOCATE 12, 10: PRINT "Address:  "; CustRec.Address
     LOCATE 13, 10: PRINT "Currently owes: $"; CustRec.Owe
     LOCATE 15, 10: INPUT "Change (+ or - amount)", Change!
     CustRec.Owe = CustRec.Owe + Change!
     PUT #1, Number%, CustRec
     UNLOCK #1, Number%   ' Unlock the record so others can use it
     LOCATE 17, 10: INPUT "Update another? ", Continue$
     Update$ = UCASE$(LEFT$(Continue$, 1))
 LOOP WHILE Update$ = "Y"
 CLOSE #1
 KILL "MONITOR"                        ' Remove file from disk
 END
 
ErrHandler:
 IF ERR = 70 THEN                      ' Permission denied error
     CLS
     PRINT "You must run SHARE.EXE before running this example."
     PRINT "Exit the programming environment, run SHARE.EXE, and"
     PRINT "reenter the programming environment to run this"
     PRINT "example. Do not shell to MS-DOS to run SHARE.EXE or you"
     PRINT "may not be able to run other programs until you reboot."
 END IF
 END