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procedure TDownload.Execute;
var
  TempTabSheet:TTabSheet;
begin
  CoInitialize(nil);
  try
    age_cs.Enter;
    //kritischer Prozess Begin
    TempTabSheet:= CreateTabBrowser('about:blank', 'TAB');
  finally
    age_cs.Leave;
  end;

  try
    age_cs.Enter;
    //kritischer Prozess Begin
    TempTabSheet.Free;
  finally
    age_cs.Leave;
  end;

  CoUnInitialize;
end;					

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initialization
  OleInitialize(nil);
  CoInitialize(nil);
  age_cs := TCriticalSection.Create;
finalization
  OleUninitialize;
  CoUninitialize;
  age_cs.Free;
end.					

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function CreateTabBrowser(sURL, sCaption: string): TTabSheet;
var
  ts: TTabSheet;
  WB: TWebbrowser;
begin
  // Create a new TabSheet
  ts := TTabSheet.Create(PageControl);
  Result := ts;
  try
    // Assign TTabSheet Properties
    ts.PageControl := PageControl;
    ts.Parent := PageControl;
    ts.Caption := sCaption;
    //ts.PageIndex := PageControl.ActivePageIndex + 1;

    // Create a TWebbrowser instance
    WB := TWebbrowser.Create(ts);

    // put TWebbrowser on TTabSheet
    TControl(WB).Parent := ts;
    // or:   Ts.InsertControl(WB);

    // Assign Webbrowser Properties
    WB.Align := alClient;
    WB.Silent := True;
    WB.Visible := True;

    PageControl.ActivePage := ts;

    // Assign Webbrowser Events
    //WB.OnProgressChange := WebBrowser1ProgressChange;
    //WB.OnStatusTextChange := WebBrowser1StatusTextChange;
    //WB.OnTitleChange := WebBrowser1TitleChange;
    WB.OnNewWindow2 := Form1.WebBrowser_Tab_NewWindow2;
    //WB.OnCommandStateChange := WebBrowser1CommandStateChange;
    //WB.OnDownloadComplete := WebBrowser1DownloadComplete;
    //WB.OnDocumentComplete := WebBrowser1DocumentComplete;
    //WB.OnDownloadBegin := WebBrowser1DownloadBegin;
    //WB.FNavForward := False;
    //WB.FNavBack := False;

    // Navigate to a URL
    if Trim(sURL) <> '' then
    begin
      WB.Navigate(sURL);
      WB_Warten_spec(WB,0);
    end;
  except
    ts.Free;
    Log('Fehler beim Tab createn',3);
  end;
end;					

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procedure TForm1.Button15Click(Sender: TObject);
var
  j:Integer;
  DownloadThreads: array [0..9] of TDownload;
begin
// die ersten 10 threads starten
  for j := 0 to 9 do
  begin
    DownloadThreads[j] := TDownload.Create(True);
    DownloadThreads[j].FreeOnTerminate := True;
    DownloadThreads[j].Index := j;
    DownloadThreads[j].Resume;
  end;
end;					

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Procedure TMyThread.Execute;
Begin
  ...
  MainForm.Control.Caption := 'Foobar'; // <<--- Zugriff auf VCL, hier wird der Thread hängen bleiben
  ...
End;					

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Procedure TMyThread.DoUpdateControl;
Begin
  MainForm.Control.Caption := 'Foobar'; // <<--- Zugriff auf VCL
End;

Procedure TMyThread.Execute;
Begin
  ...
  Synchronize (DoUpdateControl);        // <<--- So klappts
  ...
End;