LINQ之延迟执行标准查询操作符(中)

操作符：Concat描述：用于连接2个序列原型：1种

public static IEnumerable<TSource> Concat<TSource>(
this IEnumerable<TSource> first,
IEnumerable<TSource> second
)

string [] dogs = {"kelly","belly","shelly"};
string[] cats = {"kiti", "lili"};

var pets = dogs.Concat(cats);

foreach (var pet in pets)
{
Console.WriteLine(pet);
}

返回的结果是：kellybellyshellykitilili操作符：OrderBy描述：允许输入序列根据一个keySelector方法进行升序排序，并返回一个排序后的结果：IOrderedEnumerable<TSource>原型：2种原型一：

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

在这种原型中，TKey必须是实现了IComparable<TKey>

            string [] dogs = {"kelly","belly","shelly"};string[] cats = {"kiti", "lili"};var pets = dogs.Concat(cats).OrderBy(item=>item);foreach (var pet in pets){Console.WriteLine(pet);}

需要注意的是，OrderBy后返回的是一个已经排序的结果：IOrderedEnumerable<TSource>，此时我们不应该在对这个序列调用OrderBy方法，因为这样会破坏原有的OrderBy的排序，如果我们需要再次针对其他的key进行排序，可以调用ThenBy，稍后会提到。原型二：

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

原型二和原型一很类似，只是多提供了一个IComparer的参数，允许我们传入自己的排序算法。这样子的话，TKey就不一定要是想IComparer接口了。我们还是来看个例子：我们有一个Student类：

public class Student{public string Name { get; set; }public List<int > Scores { get; set; }}

我们希望根据Student的Scores的和来做一个降序排序，因此，我们另外建一个类来实现IComparer：

public class SortStudentScore:IComparer<Student>{public int Compare(Student x, Student y){decimal xScores = x.Scores.Sum(item => item);decimal yScores = y.Scores.Sum(item => item);return yScores.CompareTo(xScores);}}

这样，我们的查询可以写成：

//定义数据源List<Student> students = new List<Student>{new Student{Name = "Terry",Scores = new List<int>{98,88,93,75,82}},new Student{Name = "Tina",Scores = new List<int>{85,99,87,93,97}},new Student{Name = "Linda",Scores = new List<int>{57,100,83,89,92}},new Student{Name = "Leon",Scores = new List<int>{100,98,72,77,84}},new Student{Name = "Echo",Scores = new List<int>{79,80,97,55,88}}};SortStudentScore sortStudentScore = new SortStudentScore();//定义查询表达式，找出不及格的学生及其分数var scoreQuery = students.OrderBy((s => s),sortStudentScore).Select(item=>new {item.Name,TotalScore= item.Scores.Sum(score=>score)});//输出结果)foreach (var query in scoreQuery){Console.WriteLine("Student:{0} TotalScore:{1}", query.Name, query.TotalScore);}

我们的输出结果就是：Student:Tina TotalScore:461Student:Terry TotalScore:436Student:Leon TotalScore:431Student:Linda TotalScore:421Student:Echo TotalScore:399操作符：OrderByDescending描述：与OrderBy操作符类似，只是排序方式是按照降序来排序的。原型：与OrderBy一样，支持2种原型具体例子可以参照OrderBy的，不再赘述。操作符：ThenBy描述：将一个已经排序后的序列，根据新的key进行再排序。主要，与大多数LINQ操作不一样的地方是，ThenBy(以及后面要介绍的ThenByDescending)的输入序列都必须是实现了IOrderedEnumerable<T>。由于ThenBy是稳定排序，因此对于OrderBy/OrderByDescending后，ThenBy的key相同的2个元素，不会再打乱其排序。原型：2种，跟OrderBy一样为了说明这点，我们举个例子：

//定义数据源List<Student> students = new List<Student>{new Student{Id = 1,Name = "Leon",Scores = new List<int>{98,88,93,75,82}},new Student{Id = 2,Name = "Tina",Scores = new List<int>{85,99,87,93,97}},new Student{Id = 3,Name = "Linda",Scores = new List<int>{57,100,83,89,92}},new Student{Id = 4,Name = "Leon",Scores = new List<int>{100,98,77,77,84}},new Student{Id = 5,Name = "Echo",Scores = new List<int>{79,80,97,55,88}}};SortStudentScore sortStudentScore = new SortStudentScore();var orderByScoreQuery = students.OrderBy((s => s), sortStudentScore).Select(item => new {item.Id, item.Name, TotalScore = item.Scores.Sum(score => score) });Console.WriteLine("Using OrderBy only");//输出结果));foreach (var query in orderByScoreQuery){Console.WriteLine("{0}.{1} TotalScore:{2}",query.Id, query.Name, query.TotalScore);}Console.WriteLine();Console.WriteLine("After add ThenBy");//定义查询表达式，找出不及格的学生及其分数var scoreQuery = students.OrderBy((s => s), sortStudentScore).ThenBy(item=>item.Name).Select(item => new {item.Id, item.Name, TotalScore = item.Scores.Sum(score => score) });//输出结果)foreach (var query in scoreQuery){Console.WriteLine("{0}.{1} TotalScore:{2}", query.Id, query.Name, query.TotalScore);}

为了说明这个例子，我们让两个学生同名，并且总分也一样，用Id来区别他们，以下是输出结果：Using OrderBy only2.Tina TotalScore:4611.Leon TotalScore:4364.Leon TotalScore:4363.Linda TotalScore:4215.Echo TotalScore:399After add ThenBy2.Tina TotalScore:4611.Leon TotalScore:4364.Leon TotalScore:4363.Linda TotalScore:4215.Echo TotalScore:399我们发现Id 1和Id 4经过ThenBy后，并没有调整顺序。对于像List.Sort<T>这种不稳定排序，相同的值排序后可能打乱掉原有的顺序的。操作符：Reverse描述：Reverse很简单，就是将序列逆序一下原型：

public static IEnumerable<TSource> Reverse<TSource>(this IEnumerable<TSource> source)

看个简单的例子吧：

int[] nums = new int[] { 0,1,2,3,4,5 };var evenNums = nums.Reverse();foreach (var evenNum in evenNums){Console.WriteLine(evenNum);}

输出的结果为：543210To Be Continue…参考：《Pro.LINQ.Language.Integrated.Query.in.Csharp.2010》