Linq排序方式与Lambda排序方式比较以及OrderByThenBy的使用

Posted 2020-10-02 Jichan Jong

沿用之前某一篇文章的实体类与EF操作类代码。数据库中增加几条数据

 

Linq 的排序方式，下面例子是根据RoleId 升序，Name降序

 

EFContext<Member> efMemberContext = new EFContext<Member>();
var memberSet = efMemberContext.Set<Member>().Include("Role");
var memberList = from m in memberSet
                 orderby m.RoleId ascending, m.Name descending
                 select m;

foreach (Member item in memberList)
{
    Console.WriteLine("{0},Role:{1}",item.Name,item.Role.Name);
}

输出结果如下：

SQL Profiler如下：

 

 同样的方式如果用Lambda排序，先猜想如下，

EFContext<Member> efMemberContext = new EFContext<Member>();
var memberSet = efMemberContext.Set<Member>().Include("Role");
var memberList = memberSet.OrderBy(m => m.RoleId).OrderByDescending(m => m.Name);
foreach (Member item in memberList)
{
    Console.WriteLine("{0},Role:{1}",item.Name,item.Role.Name);
}

运行程序发现与linq方式的不一致，

SQL Profile如下：

发现连续调用OrderBy或者OrderByDescending之后，以最后一个为准排序，这时ThenBy就派上了用场。

ThenBy是使用OrderBy或者OrderByDescending之后，再次对结果进行二次排序

EFContext<Member> efMemberContext = new EFContext<Member>();
var memberSet = efMemberContext.Set<Member>().Include("Role");
var memberList = memberSet.OrderBy(m => m.RoleId).ThenByDescending(m => m.Name);
foreach (Member item in memberList)
{
    Console.WriteLine("{0},Role:{1}",item.Name,item.Role.Name);
}

如果想数据库中多个列排序一致，可以先OrderBy(或者OrderByDescending)后ThenBy(或者ThenByDescending)，也可以在OrderBy或者OrderByDescending直接指定多个属性，这样生成的SQL就是多个列的排序(排序一致asc/desc)

EFContext<Member> efMemberContext = new EFContext<Member>();
var memberSet = efMemberContext.Set<Member>().Include("Role");
var memberList = memberSet.OrderBy(m => new { m.RoleId, m.Name });
foreach (Member item in memberList)
{
    Console.WriteLine("{0},Role:{1}",item.Name,item.Role.Name);
}