>How can test all .dbf files in a folder and check if any of them is in use
>
>If at least any of them is in use, to display an error message
>
>Moisés

Very simple with a TRY/CATCH in VFP9.

TRY
  use myTable exclusive in 0
catch
  =messagebox("Sorry")
endtry

You can create it as a separate procedure, say, OpenTable, and then just use its result.

Bellow is my old OpenTble function pre VFP8:

********************************************************************
*  Description.......: OpenTble && Opens the passed table
*  Calling Samples...: OpenTble('TranMstr','WorkFile')
*  Parameter List....: tcTable, tcAlias, tcMode, tcTagOrder, tnBufferMode
*  Created by........:
*  Modified by.......: Nadya Nosonovsky 05/01/2001 11:34:13 AM
********************************************************************
function OpenTble && Opens, but doesn't select a table
lparameters tcTable, tcAlias, tcMode, tcTagOrder, tnBufferMode
* tcTable - required: name of table to open (with or without full path)
* tcAlias - optional: alias name, if different from tcTable
* tcMode - optional:  mode in which table should be opened, like shared noupdate
* tcTagOrder - optional: Tag name
* tnBufferMode - optional: Set buffer mode
local lcTableName, lnPos, llDBCPrefix, lcDBCName, lcPath
* Check parameters first
if vartype(m.tcTable)<>'C' or empty(m.tcTable)
* First parameter is required
	return .f.
endif
lnPos=rat('!',m.tcTable)
if m.lnPos>0 && DataBase Name was specified && example: Support!PostOff
	lcTableName=juststem(substr(m.tcTable,m.lnPos+1)) && PostOff
	lcDBCName=upper(left(m.tcTable,m.lnPos-1)) && DBC Name
	llDBCPrefix=.t.
else
	lcTableName=m.tcTable
endif
lcPath = justpath(m.tcTable)
if vartype(m.tcAlias)<>'C' or empty(m.tcAlias)
	tcAlias=juststem(m.lcTableName)
endif

if vartype(m.tcMode)<>'C' or empty(m.tcMode)
	tcMode='shared'
endif
local lcCheckFile
if empty(justext(m.lcTableName))
	lcCheckFile=forceext(m.lcTableName,"dbf")
else
	lcCheckFile=m.lcTableName
endif
if !file(m.lcCheckFile)  && File doesn't exist
	=messagebox(m.tcTable +' does not exist!. Can not proceed...',48,'Error')
	return .f.
endif
local lnOldSelect
lnOldSelect=select()
if !used(m.tcAlias) or ; && Alias is not already in use or
	(m.llDBCPrefix and not juststem(cursorgetprop("DATABASE",m.tcAlias))==m.lcDBCName) or ; && it's not the same database
	(!empty(m.lcPath) and m.lcPath<>justpath(dbf(m.tcAlias)))
	local lcOldError, lnErr, lcOldSetExclusive
	use in select(m.tcAlias) && Close table && Nadya Nosonovsky 11/20/2001 02:58:33 PM allways
	lnErr=0
	lcOldError=on('error') && Save current Error Handler
	lcOldSetExclusive=set('exclusive') && Save current exclusive status
	set exclusive off && This should be always set this way
	on error lnErr=error()
	use (m.tcTable) again &tcMode in 0 alias (m.tcAlias)
	on error &lcOldError && Restore previous Error Handler
	if m.lcOldSetExclusive='ON'
		set exclusive on && Restore it back to original settings
	endif
	do case
	case m.lnErr=0 && Everything is OK
	case m.lnErr=1705  && File Access is denied
		=messagebox(m.tcTable +' is opened by another user. Can not proceed...',48,'Error '+transform(m.lnErr))
		return .f. && Table could not be opened (already used exclusively)
	otherwise && Another error
		=messagebox(m.tcTable+' could not be opened!',48,'Error '+transform(m.lnErr))
		return .f.
	endcase
endif
select (m.tcAlias) && If it's here, everything is all right
if not empty(m.tcTagOrder) and vartype(m.tcTagOrder)='C' and tagno(m.tcTagOrder)>0
	set order to tag (m.tcTagOrder) in (m.tcAlias)
endif
if vartype(m.tnBufferMode)='N' and between(m.tnBufferMode,1,5)
	set multilocks on
	=cursorsetprop('Buffering',m.tnBufferMode) && Note, we can not set buffering mode less than 3 on a view - no check for this
endif
select (m.lnOldSelect) && Return back to the original area
return .t.

So, in the loop you can do

for lnI = 1 to lnFiles
   if OpenTble(laFiles[m.lnI,1])
      * do processing
   else
     * write exception log
   endif
next