Pre & Post image plugin issue

            if (context.InputParameters.Contains("Target") && context.InputParameters["Target"] is Entity)
            {
                Entity entity = (Entity)context.InputParameters["Target"];

                if (entity.LogicalName == "contact")
                {
                    Entity contact = (Entity)context.InputParameters["Target"];
                    Entity preImageContact = (Entity)context.PreEntityImages["ContactPreImage"];
                    Entity postImageContact = (Entity)context.PostEntityImages["ContactPreImage"];
                    string preImagefullname = preImageContact.Attributes["fullname"].ToString();
                    string preImagetradename = preImageContact.Attributes["tradename"].ToString();

                    string postImagefullname = postImageContact.Attributes["fullname"].ToString(); ;
                    string postmagetradename = postImageContact.Attributes["tradename"].ToString();
                    tracingService.Trace("Pre-image fullname: {0}, Pre-image tradename: {1}", preImagefullname, preImagetradename);
                    tracingService.Trace("Post-image fullname: {0}, Post-image tradename: {1}", postImagefullname, postmagetradename);
                    Entity mycontact = new Entity("contact");
                    mycontact.Id = contact.Id;
                    mycontact["previousname"] = preImagefullname;
                    mycontact["previoustradename"] = preImagetradename;
                    mycontact["changedate"] = DateTime.Now;
                    service.Update(mycontact);
                    tracingService.Trace("updated");
                }
            }