Union部分只能放在最后，这和C的不太一样，要放在前面先用Type定义一个单独的Union类型就可以了。

unit Unit1;interfaceuses  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,  Dialogs;type  TForm1 = class(TForm)    procedure FormCreate(Sender: TObject);  private    { Private declarations }  public    { Public declarations }  end;type  TMyUnion = packed record    ID: Integer;//    case Boolean of//      true: (cmd: DWORD);//      false: (hiCmd:Word;loCmd:Word);//    case Byte of//    0: (cmd: DWORD);//    1: (hiCmd:Word;loCmd:Word);//    case Integer of//    0: (cmd: DWORD);//    1: (hiCmd:Word;loCmd:Word);    case aCmd:Integer of    0: (cmd: DWORD);    1: (hiCmd:Word;loCmd:Word);  end;var  Form1: TForm1;implementation{$R *.dfm}procedure TForm1.FormCreate(Sender: TObject);var  tmp: TMyUnion;begin  tmp.ID := 10;  tmp.cmd := $0000FFFF;  tmp.aCmd := 100;  ShowMessageFmt('%d',[SizeOf(TMyUnion)]);  ShowMessageFmt('%d',[tmp.hiCmd]);  ShowMessageFmt('%d',[tmp.aCmd]);  Application.Terminate;end;end.