C#中利用Linq.Dynamic实现简单的动态表达式构建查询

背景介绍

在ADO.NET中我们可以根据用户输入的查询条件拼接出指定的SQL语句进行查询或者筛选出所需的数据，但是在ORM框架如EF中，我们一般用LINQ操作数据查询，LINQ是否可以像SQL一样拼接查询条件呢？答案是可以的。这一技术叫Linq.Dynamic(动态Linq)，Linq.Dynamic的出现解决了本人心中的疑惑，不然总感觉少了什么。不管怎么样，我们首先要做的是去网上找各种大神的资料，本文参考如下几篇资料：

Linq.Dynamic基本介绍1：https://www.cnblogs.com/myzony/p/9143692.html

Linq.Dynamic基本介绍2：https://blog.csdn.net/flyingdream123/article/details/79962525

另外Linq.Dynamic是由外国大神所创，源码在其个人主页上也有：源码地址

首先，需要在NuGet程序包中引入Linq.Dynamic.Core并引用其命名空间才能使用。

其次，使用动态查询必须先调用AsQueryable()方法。

        static void Main(string[] args){List<Phone> PhoneLists = new List<Phone>(){new Phone { Country = "中国", City = "北京", Name = "小米" },new Phone { Country = "中国",City = "北京",Name = "华为"},new Phone { Country = "中国",City = "北京",Name = "联想"},new Phone { Country = "中国",City = "台北",Name = "魅族"},new Phone { Country = "日本",City = "东京",Name = "索尼"},new Phone { Country = "日本",City = "大阪",Name = "夏普"},new Phone { Country = "日本",City = "东京",Name = "松下"},new Phone { Country = "美国",City = "加州",Name = "苹果"},new Phone { Country = "美国",City = "华盛顿",Name = "三星"},new Phone { Country = "美国",City = "华盛顿",Name = "HTC"}};var Lists = PhoneLists.AsQueryable().Where("Country = @0 And City = @1", "中国", "北京").OrderBy("Name,City").Select("new (Country as  Country,City as City,Name as Name)");var dLists = Lists.ToDynamicList();foreach (var list in dLists){Console.WriteLine(list.Country + "-" + list.City + "-" + list.Name);}Console.Read();}

运行效果如下图所示：

当然也支持GroupBy()查询，代码如下：

        static void Main(string[] args){List<Phone> PhoneLists = new List<Phone>(){new Phone { Country = "中国", City = "北京", Name = "小米" },new Phone { Country = "中国",City = "北京",Name = "华为"},new Phone { Country = "中国",City = "北京",Name = "联想"},new Phone { Country = "中国",City = "台北",Name = "魅族"},new Phone { Country = "日本",City = "东京",Name = "索尼"},new Phone { Country = "日本",City = "大阪",Name = "夏普"},new Phone { Country = "日本",City = "东京",Name = "松下"},new Phone { Country = "美国",City = "加州",Name = "苹果"},new Phone { Country = "美国",City = "华盛顿",Name = "三星"},new Phone { Country = "美国",City = "华盛顿",Name = "HTC"}};var Lists = PhoneLists.AsQueryable().GroupBy("new (Country as Country,City as City)");var dLists = Lists.ToDynamicList();foreach (var list in dLists){Console.WriteLine(list + ":");foreach (var lis in list){Console.WriteLine(lis.Country + "-" + lis.City + "-" + lis.Name);Console.WriteLine($"{lis.Country} - {lis.City} - {lis.Name}");}}Console.Read();}

执行效果如下图所示：