Question

i’m writing a delphi app that communicates with excel. one thing i noticed is that if i call the Save method on the Excel workbook object, it can appear to hang because excel has a dialog box open for the user. i’m using the late binding.

i’d like for my app to be able to notice when Save takes several seconds and then take some kind of action like show a dialog box telling this is what’s happening.

i figured this’d be fairly easy. all i’d need to do is create a thread that calls Save and have that thread call Excel’s Save routine. if it takes too long, i can take some action.

procedure TOfficeConnect.Save;
var
  Thread:TOfficeHangThread;
begin
  // spin off as thread so we can control timeout
  Thread:=TOfficeSaveThread.Create(m_vExcelWorkbook);

  if WaitForSingleObject(Thread.Handle, 5 {s} * 1000 {ms/s})=WAIT_TIMEOUT then
    begin
      Thread.FreeOnTerminate:=true;
      raise Exception.Create(_('The Office spreadsheet program seems to be busy.'));
    end;

  Thread.Free;
end;

  TOfficeSaveThread = class(TThread)
  private
    { Private declarations }
    m_vExcelWorkbook:variant;
  protected
    procedure Execute; override;
    procedure DoSave;
  public
    constructor Create(vExcelWorkbook:variant);
  end;

{ TOfficeSaveThread }

constructor TOfficeSaveThread.Create(vExcelWorkbook:variant);
begin
  inherited Create(true);

  m_vExcelWorkbook:=vExcelWorkbook;

  Resume;
end;

procedure TOfficeSaveThread.Execute;
begin
  m_vExcelWorkbook.Save;
end;

i understand this problem happens because the OLE object was created from another thread (absolutely).

how can i get around this problem? most likely i’ll need to “re-marshall” for this call somehow…

any ideas?

Answer 1

Rather than accessing the COM object from two threads, just show the message dialog in the secondary thread. The VCL isn’t thread-safe, but Windows is.

type
  TOfficeHungThread = class(TThread)
  private
    FTerminateEvent: TEvent;
  protected
    procedure Execute; override;
  public
   constructor Create;
   destructor Destroy; override;
   procedure Terminate; override;
  end;

...

constructor TOfficeHungThread.Create;
begin
  inherited Create(True);
  FTerminateEvent := TSimpleEvent.Create;
  Resume;
end;

destructor TOfficeHungThread.Destroy;
begin
  FTerminateEvent.Free;
  inherited;
end;

procedure TOfficeHungThread.Execute;
begin
  if FTerminateEvent.WaitFor(5000) = wrTimeout then
    MessageBox(Application.MainForm.Handle, 'The Office spreadsheet program seems to be busy.', nil, MB_OK);
end;

procedure TOfficeHungThread.Terminate;
begin
  FTerminateEvent.SetEvent;
end;

...

procedure TMainForm.Save;
var
  Thread: TOfficeHungThread;
begin
  Thread := TOfficeHungThread.Create;
  try
    m_vExcelWorkbook.Save;
    Thread.Terminate;
    Thread.WaitFor;
  finally
    Thread.Free;
  end;
end;