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);
原文：http://www.cnblogs.com/lanpingwang/p/6603101.html