You are not initializing your collection. And extension methods can be called in a better way.
ICollection<UserInApplication> userInAppRole=new Collection<UserInApplication>(); //Initialize this
IEnumerable<UserInApplication> result=null;
result = _userService.UserInApplicationRoles(iAppRoleId,collection["displayName"])
.AsEnumerable();
userInAppRole = result.AddTo(userInAppRole);