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type
  PWindows = ^TWindows;
  TWindows = record
    WindowHandle: HWND;
    WindowText: string;
  end;


type
  PMyEnumParam = ^TMyEnumParam;
  TMyEnumParam = record
    Nodes: TTreeNodes;
    Current: TTreeNode;
  end;

function EnumWindowsProc(Wnd: HWND; Param: PMyEnumParam): BOOL; stdcall;
const
  MyMaxName = 64;
  MyMaxText = 64;
var
  ParamChild: TMyEnumParam;
  ClassName: string;
  WindowText: string;
begin
  Result := True;
  SetLength(ClassName, MyMaxName);
  SetLength(ClassName, GetClassName(Wnd, PChar(ClassName), MyMaxName));
  SetLength(WindowText, MyMaxText);
  SetLength(WindowText, SendMessage(Wnd, WM_GETTEXT, MyMaxText, lParam(PChar(WindowText))));
  ParamChild.Nodes   := Param.Nodes;
  ParamChild.Current := Param.Nodes.AddChildObject(Param.Current,
    '[' + ClassName + '] "' + WindowText + '"' + ' Handle: ' + IntToStr(Wnd), Pointer(Wnd));
  EnumChildWindows(Wnd, @EnumWindowsProc, lParam(@ParamChild));
end;


procedure TForm1.Button1Click(Sender: TObject);
var
  Param: TMyEnumParam;
begin
  Param.Nodes := TreeView1.Items;
  Param.Current := TreeView1.TopItem;
  TreeView1.Items.BeginUpdate;
  EnumWindows(@EnumWindowsProc, lParam(@Param));
  TreeView1.Items.EndUpdate;
end;