vb.net - Diagnosing cause of SQLite locking -

on vb.net application, have case users actions in 1 portion of form creating sqlite lock causes problems later in application (in case, closing down).

here sub-routine called when users add data list of items printed:

private sub btnadd_click(byval sender system.object, byval e system.eventargs) handles btnadd.click     ' add item printqueue => regenerate listprint.items     dim queueitem new dictionary(of string, string)     queueitem("quantity") = inputquantity.value.tostring     queueitem("description") = textdesc.text     queueitem("sizeuk") = inputsize.value.tostring.replace(".5", "½").replace(".0", "")     queueitem("sku") = liststyles.selecteditem.tostring     queueitem("colour") = textcolour.text     queueitem("date") = textdatecode.text     queueitem("name") = textname.text      try         queueitem("sizeeu") = sizeeu(inputsize.value).tostring.replace(".5", "½")     catch ex exception         queueitem("sizeeu") = "??"     end try      ' size: m = uk + 1; f = uk + 1.5     queueitem("sizeus") = (inputsize.value + 1.5 - (chksex.checkstate * 0.5)).tostring.replace(".5", "½")      ' add image data string     dblocalquery = new sqlitecommand("select * tblimages id ='" & liststyles.selecteditem.tostring & "'", dblocal)      if dblocal.state = connectionstate.closed         dblocal.open()     end if      dim r sqlitedatareader = dblocalquery.executereader()      try         r.read()         queueitem("image") = byte2string((r("image")))     catch ex exception         queueitem("image") = settings("imgnotfound")             if dblocal.state = connectionstate.open             dblocal.close()         end if     end try      printqueue.add(printqueue.count + 1, queueitem)     msgframe.items.add(printqueue(printqueue.count)("sku") & " x" & printqueue(printqueue.count)("quantity"))     msgframe.selectedindex = msgframe.items.count - 1     ' update print queue list     populatelist() end sub 

the table in database touched query tblimages. then, connection closed once finished.

when closing form down, function called captures data written dictionary called settings on load , reinserts database.

public function dblocalsave() boolean     'recreates tblsettings current values stored in settings      try         if dblocal.state = connectionstate.closed             dblocal.open()         end if          if dblocal.state = connectionstate.open             msgframe.items.add("flushing local settings table")             msgframe.selectedindex = msgframe.items.count - 1              each pair in settings                 debug.print("+ tblsettings: " & pair.key & " = " & pair.value)                  dblocalquery = new sqlitecommand("delete tblsettings name = '" & pair.key & "';", dblocal)                 dblocalquery.executenonquery()                  dblocalquery = new sqlitecommand("insert tblsettings (`name`,`value`) values ('" & pair.key & "','" & pair.value & "');", dblocal)                 dblocalquery.executenonquery()             next             return true         else             return false         end if     catch sqlex sqliteexception         messagebox.show(sqlex.message, "sql error", messageboxbuttons.ok, messageboxicon.error)     catch ex exception         messagebox.show(ex.message, "error", messageboxbuttons.ok, messageboxicon.error)             if dblocal.state = connectionstate.open             dblocal.close()         end if     end try  end function 

this function fails @ dblocalquery.executenonquery() stage db lock error, cannot understand why. not handling db closing correctly? under impression sqlite created locks on writes, never happens in explicit case described.

if not 'add' item print queue (fundamental functionality of program), program closes gracefully.

dispose each command after executing should solve problem

dblocalquery.executenonquery() dblocalquery.dispose() 

the problem because have delete , insert command concurrently executing causing error

            dblocalquery = new sqlitecommand("delete tblsettings name  = '" & pair.key & "';", dblocal)             dblocalquery.executenonquery()             'insert here '             dblocalquery.dispose()             dblocalquery = new sqlitecommand("insert tblsettings (`name`,`value`) values ('" & pair.key & "','" & pair.value & "');", dblocal)             dblocalquery.executenonquery()             'here '             dblocalquery.dispose() 

ref : http://www.sqlite.org/cvstrac/wiki?p=databaseislocked