LINQ 学习路程 -- 查询操作 OrderBy & OrderByDescending

Sorting Operator	Description
OrderBy	通过给定的字段进行升序 降序 排序
OrderByDescending	通过给定字段进行降序排序，仅在方法查询中使用
ThenBy	第二级升序排序，仅在方法查询中使用
ThenByDescending	第二级降序排序，仅在方法查询中使用
Reverse	反转集合，仅在方法查询中使用

IList<Student> studentList = new List<Student>() { 
    new Student() { StudentID = 1, StudentName = "John", Age = 18 } ,
    new Student() { StudentID = 2, StudentName = "Steve",  Age = 15 } ,
    new Student() { StudentID = 3, StudentName = "Bill",  Age = 25 } ,
    new Student() { StudentID = 4, StudentName = "Ram" , Age = 20 } ,
    new Student() { StudentID = 5, StudentName = "Ron" , Age = 19 } 
};

var orderByResult = from s in studentList
                   orderby s.StudentName 
                   select s;

var orderByDescendingResult = from s in studentList
                   orderby s.StudentName descending
                   select s;

OrderBy扩展方法有两个重载方法，第一个方法接受一个类型参数，你可以指定通过哪个字段进行排序

第二个方法接受一个实现IComparer的类型，用户可以自定义排序

public static IOrderedEnumerable<TSource> OrderBy<TSource, TKey>(this IEnumerable<TSource> source, 
                                                                 Func<TSource, TKey> keySelector);

public static IOrderedEnumerable<TSource> OrderBy<TSource, TKey>(this IEnumerable<TSource> source, 
                                                                 Func<TSource, TKey> keySelector, 
                                                                 IComparer<TKey> comparer);

IList<Student> studentList = new List<Student>() { 
    new Student() { StudentID = 1, StudentName = "John", Age = 18 } ,
    new Student() { StudentID = 2, StudentName = "Steve",  Age = 15 } ,
    new Student() { StudentID = 3, StudentName = "Bill",  Age = 25 } ,
    new Student() { StudentID = 4, StudentName = "Ram" , Age = 20 } ,
    new Student() { StudentID = 5, StudentName = "Ron" , Age = 19 } 
};

var studentsInAscOrder = studentList.OrderBy(s => s.StudentName);

OrderByDescending

IList<Student> studentList = new List<Student>() { 
    new Student() { StudentID = 1, StudentName = "John", Age = 18 } ,
    new Student() { StudentID = 2, StudentName = "Steve",  Age = 15 } ,
    new Student() { StudentID = 3, StudentName = "Bill",  Age = 25 } ,
    new Student() { StudentID = 4, StudentName = "Ram" , Age = 20 } ,
    new Student() { StudentID = 5, StudentName = "Ron" , Age = 19 } 
};

var studentsInDescOrder = studentList.OrderByDescending(s => s.StudentName);

多个排序

可以使用多个字段以逗号隔开进行排序，集合首先以第一个字段进行排序，如果第一个字段有相同的值，则根据第二个字段进行排序

注意：

LINQ包含五种排序操作:OrderBy、OrderByDescending、ThenBy、ThenByDescending、Reverse

查询语言不支持OrderByDescending、ThenBy、ThenByDescending、Reverse，它仅支持Order By从句后面跟ascending、descending

查询语法支持多字段排序，