LINQ 学习路程 -- 查询操作 Aggregate

聚合操作执行数学的运算，如平均数、合计、总数、最大值、最小值

Method	Description
Aggregate	在集合上执行自定义聚集操作
Average	求平均数
Count	求集合的总数
LongCount	求集合的总数
Max	最大值
Min	最小值
Sum	总数

public static TSource Aggregate<TSource>(this IEnumerable<TSource> source, 
                                         Func<TSource, TSource, TSource> func);

public static TAccumulate Aggregate<TSource, TAccumulate>(this IEnumerable<TSource> source, 
                                         TAccumulate seed, 
                                         Func<TAccumulate, TSource, TAccumulate> func);

public static TResult Aggregate<TSource, TAccumulate, TResult>(this IEnumerable<TSource> source, 
                                         TAccumulate seed, 
                                         Func<TAccumulate, TSource, TAccumulate> func, 
                                         Func<TAccumulate, TResult> resultSelector);

Aggregate接受2个参数，一般第一个参数是称为累积数（默认情况下等于第一个值），而第二个代表了下一个值。第一次计算之后，计算的结果会替换掉第一个参数，继续参与下一次计算。

public static TAccumulate Aggregate<TSource, TAccumulate>(this IEnumerable<TSource> source, 
                                         TAccumulate seed, 
                                         Func<TAccumulate, TSource, TAccumulate> func);

seed作为种子值进行累加 

// Student collection
IList<Student> studentList = new List<Student>>() { 
        new Student() { StudentID = 1, StudentName = "John", Age = 13} ,
        new Student() { StudentID = 2, StudentName = "Moin",  Age = 21 } ,
        new Student() { StudentID = 3, StudentName = "Bill",  Age = 18 } ,
        new Student() { StudentID = 4, StudentName = "Ram" , Age = 20} ,
        new Student() { StudentID = 5, StudentName = "Ron" , Age = 15 } 
    };

string commaSeparatedStudentNames = studentList.Aggregate<Student, string>(
                                        "Student Names: ",  // seed value
                                        (str, s) => str += s.StudentName + "," ); 

Console.WriteLine(commaSeparatedStudentNames); 

 第三个参数对结果进行构造返回

public static TResult Aggregate<TSource, TAccumulate, TResult>(this IEnumerable<TSource> source, 
                                         TAccumulate seed, 
                                         Func<TAccumulate, TSource, TAccumulate> func, 
                                         Func<TAccumulate, TResult> resultSelector);

IList<Student> studentList = new List<Student>>() { 
        new Student() { StudentID = 1, StudentName = "John", Age = 13} ,
        new Student() { StudentID = 2, StudentName = "Moin",  Age = 21 } ,
        new Student() { StudentID = 3, StudentName = "Bill",  Age = 18 } ,
        new Student() { StudentID = 4, StudentName = "Ram" , Age = 20} ,
        new Student() { StudentID = 5, StudentName = "Ron" , Age = 15 } 
    };

string commaSeparatedStudentNames = studentList.Aggregate<Student, string,string>(
                                            String.Empty, // seed value
                                            (str, s) => str += s.StudentName + ",", // returns result using seed value, String.Empty goes to lambda expression as str
                                            str => str.Substring(0,str.Length - 1 )); // result selector that removes last comma

Console.WriteLine(commaSeparatedStudentNames);