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procedure TForm1.Button1Click(Sender: TObject);

var
   f: file of Byte;
   size: Longint;
  begin
  if OpenDialog1.Execute then
  begin
    AssignFile(f, OpenDialog1.FileName);
    Reset(f);
    try
      size := FileSize(f);
      Edit1.Text:=IntToStr(size);
    finally
      CloseFile(f);
    end;
  end;
end;					

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function GetFileSizeEx(const sFileName: String): Int64;
type
  TInt64 = packed record
    Low : Cardinal;
    High: Cardinal;
  end;
var lFindData: TWin32FindData;
    lHandle  : Cardinal;
    lGiveback: TInt64;
begin
  lHandle := FindFirstFile(PChar(sFileName), lFindData);
  if (lHandle <> INVALID_HANDLE_VALUE)
  then begin
    lGiveback.Low := lFindData.nFileSizeLow;
    lGiveback.High := lFindData.nFileSizeHigh;
    Windows.FindClose(lHandle);
    Result := Int64(lGiveback);
  end else
    Result := -1;
end;					

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  Reset(f,1);