Getting error while creating a object

Try adding a set of brackets to specify the operator priority:

Service obj = new Service();
obj = (RACS.Completion.Web.Private.Activity.Service)(Session["obj"]);

Alternatively, check that the Session["obj"] return value is a Service, as the error message implies it is a Session[] instead.

BTW: It is a bad idea to always create a new object to fill a variable when you construct it - particularly when you are going to replace it's content immediately. It wastes memory, and possibly other resources in the object constructor. Either leave it null, or create it when you do the initial assignment (from Session in your example)

Seems like you have

List<service></service>

in

Session["obj"]

.

It's better to try something like that:

   if(Session["obj"]!=null)
   {
       var objList=Session["obj"] as List<racs.completion.web.private.activity.service>;
       var objRes=objRes!=null ? objList.FirstOrDefault() : new racs.completion.web.private.activity.service();
       if(objRest!=null) ..
   }
</racs.completion.web.private.activity.service>

After that you should think - why you have List<racs.completion.web.private.activity.service> in Session["obj"] and not RACS.Completion.Web.Private.Activity.Service, or you'll have new problems later.

import namespace RACS.Completion.Web.Private.Activity.Service
and the use

Service obj=new Service();
problem fixed!