巧用表达式树动态构建查询条件

项目中有个高级查询的功能，根据不同查询条件的各种组合来查询mongo，这种情况，可使用Expression来动态拼接查询条件来简化操作

BLL中相关代码如下

public DataResult GetRateStaticList(object param, bool paginating = false)
        {
            object result = null;
            long count = 0;
            var dicParam = FromParam(param);

            ParameterExpression p_rate = Expression.Parameter(typeof(Entity.RateStaticMessage), "rate");

            MemberExpression m_chain = Expression.PropertyOrField(p_rate, "Chain");
            ConstantExpression c_chain = Expression.Constant(dicParam["Chain"]);
            var q_chain = Expression.Equal(m_chain, c_chain);
            if (dicParam.ContainsKey("Hotel"))
            {
                MemberExpression m_hotel = Expression.PropertyOrField(p_rate, "Hotel");
                ConstantExpression c_hotel = Expression.Constant(dicParam["Hotel"]);
                var q_hotel = Expression.Equal(m_hotel, c_hotel);
                q_chain = Expression.AndAlso(q_chain, q_hotel);
            }
            if (dicParam.ContainsKey("Channel"))
            {
                MemberExpression m_hotel = Expression.PropertyOrField(p_rate, "Channel");
                ConstantExpression c_hotel = Expression.Constant(dicParam["Channel"]);
                var q_hotel = Expression.Equal(m_hotel, c_hotel);
                q_chain = Expression.AndAlso(q_chain, q_hotel);
            }
            if (dicParam.ContainsKey("RatePlan"))
            {
                MemberExpression m_hotel = Expression.PropertyOrField(p_rate, "RatePlan");
                ConstantExpression c_hotel = Expression.Constant(dicParam["RatePlan"]);
                var q_hotel = Expression.Equal(m_hotel, c_hotel);
                q_chain = Expression.AndAlso(q_chain, q_hotel);
            }

            MemberExpression m_start = Expression.PropertyOrField(p_rate, "Start");
            ConstantExpression c_start = Expression.Constant(dicParam["Start"].ToString());
            var q_start = Expression.LessThanOrEqual(Expression.Call(m_start, typeof(string).GetMethod("CompareTo", new Type[] { typeof(string) }), c_start), Expression.Constant(0));
            q_chain = Expression.AndAlso(q_chain, q_start);

            MemberExpression m_end = Expression.PropertyOrField(p_rate, "End");
            ConstantExpression c_end = Expression.Constant(dicParam["End"].ToString());
            var q_end = Expression.GreaterThanOrEqual(Expression.Call(m_end, typeof(string).GetMethod("CompareTo", new Type[] { typeof(string) }), c_end), Expression.Constant(0));
            q_chain = Expression.AndAlso(q_chain, q_end);

            var exp = Expression.Lambda<Func<Entity.RateStaticMessage, Boolean>>(q_chain, p_rate);
            if (paginating)
                result = _dal.List(exp, true, Convert.ToInt32(dicParam["Offset"]), Convert.ToInt32(dicParam["Limit"]));
            else
                result = _dal.List(exp);
            count = _dal.Count(exp);

            return FromObject(result, count);
        }

 记录如上，以后遇到相同情况可参考。