OpenDialog does not show up in Delphi MultiThreaded application

Question

i tried to use the openDialog in new thread but it made so strange behavior ..

if i put the if opendialog.execute then in the create constructor like this :

constructor TChatMemberThread.Create(Name: string);
begin
  inherited Create(True);
  FName := Name;
  FreeOnTerminate := True;
  Opendialog := TOpenDialog.create(nil);
  if opendialog.execute then
    for 0 to opendialog.filescount do
      somecodeishere
    end;
end;

the opendialog open normally but when i put it in the execute producer of the thread it didn't open at all !!

i'm so beginner in threads so can any one explain for me what happened ?

Thanks in advance .

[Edit]

unit Unit1;

interface

uses
  Classes,Dialogs,ComCtrls,SysUtils,DCPcrypt2, DCPmd5;

type
  TOpenThread = class(TThread)
  private
    { Private declarations }
    OpenDlG : TOpenDialog;
    LI : TListItem;
    Procedure Openit;
    Function MD5it(Const filename : string ):String;
  protected
    procedure Execute; override;
  Public
    Constructor Create;
    Destructor Destroy;Override;
  end;

implementation
uses Main;

{ TOpenThread }

Constructor TOpenThread.Create;
begin
 inherited Create(True);
 opendlg := Topendialog.Create(nil);
 opendlg.Filter := 'All Files | *.*';
 openDlg.Options := [OfAllowMultiSelect];
 openDlg.InitialDir := GetCurrentDir;
end;

Destructor TOpenThread.Destroy;
begin
  OpenDlg.Free;
  inherited;
end;

Function TOpenThread.MD5it(Const filename : string ):String;
var
hash : TDCP_MD5 ;
Digest: array[0..15] of byte;
Source: TFileStream;
i: integer;
s: string;
begin
  Source:= nil;
    try
      Source:= TFileStream.Create(filename,fmOpenRead);  // open the file specified by Edit1
    except
      MessageDlg('Unable to open file',mtError,[mbOK],0);
    end;
    if Source <> nil then
    begin
      Hash:= TDCP_MD5.Create(nil);         // create the hash
      Hash.Init;                                   // initialize it
      Hash.UpdateStream(Source,Source.Size);       // hash the stream contents
      Hash.Final(Digest);                          // produce the digest
      Source.Free;
      s:= '';
      for i:= 0 to 15 do
        s:= s + IntToHex(Digest[i],2);
    Result := s;
    end;
    Hash.Free;
end;

Procedure TOpenThread.Openit;
var
I: Integer;
begin
 if opendlg.Execute then
 begin
   for I := 0 to openDlg.Files.Count - 1 do begin
      LI := Form1.LV1.Items.Add;
      LI.Caption := ExtractFileName(openDlg.Files[i]);
      LI.SubItems.Add(MD5it(openDlg.Files[i]));
      LI.SubItems.add(openDlg.Files[i]);
   end;
  //SB.Panels[0].Text := ' '+IntToStr(LV1.Items.Count)+' File(s)';
  OpenDlg.Free;
end;end;

procedure TOpenThread.Execute;
begin
  { Place thread code here }
  Synchronize(OpenIt);
end;

end.

Answer 1

It works when you call it in the constructor because the constructor runs in the context of the calling thread (ie the main thread), whereas Execute() runs in the context of the worker thread. The VCL is not thread-safe, and UI components in particular rarely if ever work correctly outside of the main thread. If you want to display an open dialog in a thread, then have your TThread.Execute() method either:

1) call TThread.Synchronize() to access the TOpenDialog within the context of the main thread.

2) call the Win32 API GetOpenFileName() function directly instead. API dialogs can be safely used in threads when used properly.

Answer 2

I just hit a similar case in Delphi XE2 but I suppose it can happen in 2009 too.

Delphi was uplifted to use new Windows Vista open/save dialogs, which are COM-based components instead of old flat C-style API. https://msdn.microsoft.com/library/windows/desktop/bb776913.aspx

I was adding a debug logging function, and it used to call PromptForFileName if the dump file name was not set yet. The function never did a thing.

I traced into Delphi RTL/VCL internals and reached function TCustomFileSaveDialog.CreateFileDialog in Dialogs.pas.

The said function was calling into Microsoft COM API, but then - oops! - just suppressed all the errors that could be returned. I used CPU Window in the Delphi Debugger and saw EAX register having $800401f0 error, which stands for 'COM was not initialized yet' situation. https://msdn.microsoft.com/en-us/library/cc704587.aspx

I knew for sure that the said function worked perfectly in other places of the program, so I supposed it was - unexpectedly for me - executing in a separate thread. That was the case. In your case you DO KNOW already you have multithreading issues, and I think you may try the direct solution, instead of the workaround with Synchronize

uses ActiveX, Windows;
constructor TChatMemberThread.Create(Name: string);
var COM_Init_Here: Boolean;
begin
  inherited Create(True);
  FName := Name;
  FreeOnTerminate := True;
  COM_Init_Here := S_OK = CoInitialize(nil); // ***
  try                                        // ***        
    Opendialog := TOpenDialog.create(nil);
    if opendialog.execute then
      for 0 to opendialog.filescount do
        somecodeishere
      end;
  finally                                    // ***
    if COM_Init_Here then CoUnInitialize();  // ***
  end;                                       // ***
end;