REST WCF 服务不返回响应

我创建了一个 rest wcf 服务。服务本身工作正常。从本地主机浏览中，我可以调用所有函数。下面是合约和函数。

问题是当我尝试使用 db.savechanges（） 保存时，该服务不返回响应。如果我注释掉该行，则会返回 JSON 响应。

我的代码做错了吗？

 [DataContract]
     public class RouteObject{
     public List<DetailRouteInfo>routeDetail {get; set;} /// this is Entity Model Class; 
    }
[OperationContract]
        [WebGet(UriTemplate="GRDI?route={route}&cloc={cloc}", ResponseFormat=WebMessageFormat.Json)]
        RouteObject GRDI(string route,string cLoc);
public RouteObject GRDI(string route, string cLoc) { 
        MapApiClient mClient = new MapApiClient();
        Response mData= new Response();
        var coord=route.Split('|');
        RouteObject rObject = new RouteObject();
        rObject.routeDetail = new List<DriverRouteInfo>(); ;
        //this variable needs to updated later
        //List<string> dummyCoord= new List<string>();
        List<Task> wtask = new List<Task>();
        mClient.getRouteData(coord.ToList(), (x) =>
            {
                mData = x;
            },
            (y) => 
            {
                wtask.Add(y);
            }
            );
        if (wtask != null)
        {
            Task.WaitAll(wtask.ToArray());
            wtask.Clear();
        }
        DriverRouteInfo droute=null;
// DO SOME MANIPULATION WITH THE DROUTE //
// THEN ADD LIST OF DROUTE INTO ROUTOBJECT.DETAILROUTE //
        try 
        {
            using (var db = new F2PDatabaseEntities())
            {
                UserInfo userinfo = db.UserInfoes.SingleOrDefault(u => u.UserId == 1);
                //userinfo.DriverRouteInfoes = listRoute;
                foreach(var dr in rObject.routeDetail){
                    userinfo.DriverRouteInfoes.Add(dr);
                }
               db.SaveChanges();
                return rObject;
            }
            var check = droute.RouteId;

        }
        catch(SqlException ex) {
            throw new FaultException(ex.Message); ;
        }
    }