How do I wait to delete a file until after the program I started has finished using it?

Question

I have been looking for a way to open a file saved to my computer via a Delphi app with its appropriate application. The file is stored in a Varbinary field in a SQL databas

Answer 1

Hers a snip from a unit I use for a similar purpose. I found these functions online somewhere over the the years so I take no credit and make no promises.

I personally use the WaitExec() function to launch a pdf (retrieved from a database) in Acrobat for editing and then re-save it to our database when done.

I have used the two other functions at other times as well so I know they all work to one degree or another but I think WaitExec() worked best in an interactive mode, while Launch() worked better from a thread or non-interactive mode.

The IsFileInUse function can tell you if the file you created is in use by any other processes and may be a viable option as well.

uses SysUtils, Windows, ShellAPI, Forms, Registry, Classes, Messages, Printers,
    PSAPI, TlHelp32, SHFolder;



function IsFileInUse(fName: string): boolean;
var
    HFileRes: HFILE;
begin
    Result := False;
    if not FileExists(fName) then
        Exit;
    HFileRes := CreateFile(pchar(fName), GENERIC_READ or GENERIC_WRITE,
        0 {this is the trick!}, nil, OPEN_EXISTING,
        FILE_ATTRIBUTE_NORMAL, 0);
    Result := (HFileRes = INVALID_HANDLE_VALUE);
    if not Result then
        CloseHandle(HFileRes);
end;

function Launch(sCommandLine: string; bWait: Boolean; AppHandle: HWND): Boolean;
var
    SEI: TShellExecuteInfo;
    Mask: Longint;
begin
    Mask := SEE_MASK_NOCLOSEPROCESS;
    FillChar(SEI, Sizeof(SEI), #0);
    SEI.cbsize := Sizeof(SEI);
    SEI.wnd := AppHandle;
    SEI.fmask := Mask;

    //if FExeStyleString<>'' then SEI.LPVERB:=pchar(FExeStyleString);
    SEI.LPFile := pchar(sCommandline);

    //SEI.LPParameters := pchar(FExeParameters);
    //SEI.LPDirectory  := pchar(FExepath);
    SEI.nshow := SW_SHOWNORMAL; // SW_SHOWMINIMIZED, SW_SHOWMAXIMIZED
    ShellexecuteEx(@SEI);

    if bWait then
        WaitforSingleObject(SEI.hProcess, INFINITE);
    Result := True;
end;

function WaitExec(const CmdLine:AnsiString;const DisplayMode:Integer):Integer;
  {Execute an app, wait for it to terminate then return exit code.  Returns -1
   if execution fails. DisplayMode is usually either sw_ShowNormal or sw_Hide.}
 var
   S:TStartupInfo;
   P:TProcessInformation;
   M:TMsg;
   R:DWord;
 begin
   FillChar(P,SizeOf(P),#0);
   FillChar(S,Sizeof(S),#0);
   S.cb := Sizeof(S);
   S.dwFlags := STARTF_USESHOWWINDOW;
   S.wShowWindow := DisplayMode;
   if not CreateProcess(nil,
     PChar(CmdLine),                { pointer to command line string }
     nil,                           { pointer to process security attributes }
     nil,                           { pointer to thread security attributes }
     False,                         { handle inheritance flag }
     CREATE_NEW_CONSOLE or          { creation flags }
     NORMAL_PRIORITY_CLASS,
     nil,                           { pointer to new environment block }
     nil,                           { pointer to current directory name }
     S,                             { pointer to STARTUPINFO }
     P)                             { pointer to PROCESS_INF }
   then begin
    ShowMessage('Create Process failed. Save this message for IT: ' + CmdLine);
    Result:=-1
   end
   else begin
//     WaitforSingleObject(P.hProcess,INFINITE);
//   The following replacement better satisfies DDE requirements
     repeat
       R := MsgWaitForMultipleObjects(1, // One event to wait for
       P.hProcess, // The array of events
       FALSE, // Wait for 1 event
       INFINITE, // Timeout value
       QS_ALLINPUT); // Any message wakes up
       if R>WAIT_OBJECT_0 then begin
         M.Message := 0;
         while PeekMessage(M,0,0,0,PM_REMOVE) do begin
           TranslateMessage(M);
           DispatchMessage(M);
         end
       end;

     until R=WAIT_OBJECT_0;

     // put value into Result.... non zero = success
     GetExitCodeProcess(P.hProcess,DWord(Result));

     CloseHandle(P.hProcess);
     CloseHandle(P.hThread);
     P.hProcess:=0;
     P.hThread:=0;
   end;
end;