Linq :Aggregate;SequenceEqual;join

1. Aggregate(使用 Aggregate 创建数组的连乘，计算所有元素的总乘积。):

　　double[] doubles = { 1.7, 2.3, 1.9, 4.1, 2.9 };
            
      double product = doubles.Aggregate((runningProduct, nextFactor) => runningProduct * nextFactor);

2. Aggregate重载（使用 Aggregate 创建一个流水账余额， 从最初余额 100 减去每次取出的金额，直到余额减少到 0 以下为止。）：

　　　　double startBalance = 100.0;
           
            int[] attemptedWithdrawals = { 20, 10, 40, 50, 10, 70, 30 };
           
            double endBalance =
                attemptedWithdrawals.Aggregate(startBalance,
                    (balance, nextWithdrawal) =>
                        ( (nextWithdrawal <= balance) ? (balance - nextWithdrawal) : balance ) );

3.  SequenceEqual（使用 SequenceEquals 查看两个序列中所有元素是否以相同顺序匹配。）：

　　　　var wordsA = new string[] { "cherry", "apple", "blueberry" };
            var wordsB = new string[] { "cherry", "apple", "blueberry" };
           
            bool match = wordsA.SequenceEqual(wordsB);

4. SequenceEqual重载（自定义比较方法）：

　　List<Student> list = new List<Student>();

　　List<Student> list2 = new List<Student>();

　　Student a = new Student
  　　{
         UserId = 1,
         StudentName = "Eric"

  　　};
  　　Student b = new Student
  　　{
         UserId = 1,
         StudentName = "Eric"

 　　 };

 　　 Student c = new Student
  　　{
         UserId = 2,
         StudentName = "laoyi"

  　　};
 　　 list.Add(a);
  　　list.Add(b);

  　　list.Add(c);

　　  list2.Add(c);
  　　list2.Add(b);

  　　list2.Add(a);

  　　var tt = list.SequenceEqual(list, new StudentComparer());
 

　　public class Student
　　{
  　　public int UserId { get; set; }
  　　public string StudentName { get; set; }
　　}
 

　　自定义的比较类:
　　public class StudentComparer : IEqualityComparer<Student>
　　{
  　　public bool Equals(Student x, Student y)
  　　{
         　　return x.UserId.Equals(y.UserId);
  　　}

  　　public int GetHashCode(Student obj)
  　　{
         　　return obj.UserId.GetHashCode();
  　　}
　　}

 5. join in (左外部联接和复合键,使用匿名类型封装多个键值):

　　　　 List<Customer> customers = GetCustomerList();
            List<Supplier> suppliers = GetSupplierList();

            var supplierCusts =
                from sup in suppliers
                join cust in customers on new { sup.City, sup.Country } equals new { cust.City, cust.Country } into cs
                from c in cs.DefaultIfEmpty() //移除 DefaultIfEmpty 方法调用可以使之成为内部联接
                orderby sup.SupplierName
                select new { Country = sup.Country,
                             City = sup.City,
                             SupplierName = sup.SupplierName,
                             CompanyName = c == null ? "(No customers)" : c.CompanyName
                           };