var qGroup = from dispatch in _dispatchRepository.GetAll()
                         group dispatch by dispatch.DeviceId into g
                         select g.Key + "_" + g.Max(d => d.BeginTime);


var query = from dispatch in _dispatchRepository.GetAll()
                         where qGroup.Contains(dispatch.DeviceId + "
 
_" + dispatch.BeginTime) 
                        select dispatch;

EF 分組取最後一條的實例