Expression<Func<your class, bool>> whereExp = f => true;//类似1=1，初始化条件

            if (!string.IsNullOrWhiteSpace(userID))

            {

                whereExp = whereExp.And(f => f.UserID.Equals(userID));

            }

            if (!string.IsNullOrWhiteSpace(userName))

            {

                whereExp = whereExp.And(f => f.userName.Equals(userName));

            }

YourService.QueryListByLinq(whereExp: whereExp).FirstOrDefault();