LINQ标准查询运算符的执行方式-即时

即时，声明查询的位置立即执行。查询返回如果是不可以枚举的的结果，都会立即执行。

执行方式为“”即时”的查询运算符有下面这些。

• Aggregate 应用累计器函数和结果选择器，返回传入泛型类型TSource

//找出字符串最长值
string[] fruits = { "apple", "mango", "orange", "passionfruit", "grape" };
string longestName =
    fruits.Aggregate("banana",
                    (longest, next) =>
                        next.Length > longest.Length ? next : longest,
                    fruit => fruit.ToUpper());
Console.WriteLine(
    "The fruit with the longest name is {0}.",
    longestName);

• All 判断所有函数是否满足条件，返回bool

string[] names = { "ab","abc","abcd"};

bool allStartWithab = names.All(p => p.StartsWith("ab"));
bool allStartWithabc = names.All(p => p.StartsWith("abc"));

Console.WriteLine(
    "{0} pet names start with 'ab'. {1} pet names start with 'abc'",
    allStartWithab ? "All" : "Not all", allStartWithabc ? "all":"not all");

• Any 判断序列是否包含元素，返回bool

List<int> numbers = new List<int> { 1, 2 };
bool hasElements = numbers.Any();
bool hasElementsEquals2 = numbers.Any(num=>num==2);

Console.WriteLine($"{hasElements} {hasElementsEquals2}");

• Average 求平均值

List<int> grades = new List<int> { 78, 92, 100, 37, 81 };
var average = grades.Average();
Console.WriteLine("The average grade is {0}.", average);

• Contains 是否包含特定的元素

string[] fruits = { "apple", "banana", "mango", "orange", "passionfruit", "grape" };
string fruit = "mango";
bool hasMango = fruits.Contains(fruit);

• Count 计算序列的数目，返回类型Int

string[] fruits = { "apple", "banana", "mango", "orange", "passionfruit", "grape" };
fruits.Count();           

• ElementAt 返回源序列指定位置的元素

string[] names =
{ "Hartono, Tommy", "Adams, Terry", "Andersen, Henriette Thaulow",
    "Hedlund, Magnus", "Ito, Shu" };
Random random = new Random(DateTime.Now.Millisecond);
var name = names.ElementAt(5);
Console.WriteLine("The name chosen at random is '{0}'.", name);

ElementAtDefault

First

Empty

FirstDefault 返回序列的第一个元素 ，如果为空则返回类型的默认值

string[] names = { "Hartono, Tommy", "Adams, Terry",
                     "Andersen, Henriette Thaulow",
                     "Hedlund, Magnus", "Ito, Shu" };

            string firstLongName = names.FirstOrDefault(name => name.Length > 20);

            Console.WriteLine("The first long name is '{0}'.", firstLongName);

            string firstVeryLongName = names.FirstOrDefault(name => name.Length > 30);

            Console.WriteLine(
                "There is {0} name longer than 30 characters.",
                string.IsNullOrEmpty(firstVeryLongName) ? "not a" : "a");

            //如果为空，希望设另一个值，可以使用DefaultIfEmpty 后面必须使用First方法.
            List<int> months = new List<int> { };            
            int firstMonth2 = months.DefaultIfEmpty(1).First();
            Console.WriteLine("The value of the firstMonth2 variable is {0}", firstMonth2);        

•  Last、LastOrDefault

int[] numbers = { 9, 34, 65, 92, 87, 435, 3, 54,
        83, 23, 87, 67, 12, 19 };

int last = numbers.Last(num => num > 80);
int last1 = numbers.Last();

Console.WriteLine(last);           
Console.WriteLine(last1);

int[] numbers1 = { };
Console.WriteLine(numbers1.LastOrDefault());
Console.WriteLine(numbers1.DefaultIfEmpty(9999).Last());

• LongCount

namespace ConsoleApp4
{
    class Program
    {
        static void Main(string[] args)
        {
            Pet[] pets = { new Pet { Name="Barley", Age=8 },
                   new Pet { Name="Boots", Age=4 },
                   new Pet { Name="Whiskers", Age=1 } };

            const int Age = 3;

            int count0 = pets.Count(pet => pet.Name.StartsWith("B"));
            long count = pets.LongCount(pet => pet.Age > Age);
            long count1 = pets.LongCount();
            long count2 = pets.LongLength;

            Console.WriteLine("There are {0} animals over age {1}.{2},{3}", count, Age,count1,count2);
        }
    }
    class Pet
    {
        public string Name { get; set; }
        public int Age { get; set; }
    }
}

•  Max

List<long> longs = new List<long> { 4294967296L, 466855135L, 81125L };
long max = longs.Max();
Console.WriteLine("The largest number is {0}.", max);            
Pet[] pets = { new Pet { Name="Barley", Age=8 },
       new Pet { Name="Boots", Age=4 },
       new Pet { Name="Whiskers", Age=1 } };

int min = pets.Min(pet => pet.Age+pet.Name.Length);
Console.WriteLine("The youngest animal is age {0}.", min)

• SequenceEquals 比较两个序列是否相等，使用默认比较器，也可以传一个新的比较器。返回bool值

Pet pet1 = new Pet { Name = "Turbo", Age = 2 };
Pet pet2 = new Pet { Name = "Peanut", Age = 8 };

List<Pet> pets1 = new List<Pet> { pet1, pet2 };
List<Pet> pets2 = new List<Pet> { pet1, pet2 };

bool equal = pets1.SequenceEqual(pets2);

Console.WriteLine(
    "The lists {0} equal.",
    equal ? "are" : "are not");

List<Pet> pets3 = new List<Pet> { pet2, pet1 };
equal = pets1.SequenceEqual(pets3);
Console.WriteLine(
    "The lists {0} equal.",
    equal ? "are" : "are not");

•  Single 返回序列中特定的元素，元素不唯一和找不到元素都抛异常。

string[] fruits = { "apple", "banana", "mango",
          "orange", "passionfruit", "grape" };

string fruit1 = fruits.Single(fruit => fruit.Length >10);
Console.WriteLine(fruit1);

//抛异常 
//string fruit2 = fruits.Single(fruit => fruit.Length >0);

//抛异常 
//string fruit3 = fruits.Single();

• SingleDefault 返回序列中特定的元素，元素不唯一则抛异常

int[] pageNumbers = { };

int pageNumber1 = pageNumbers.SingleOrDefault();           
Console.WriteLine("The value of the pageNumber1 variable is {0}", pageNumber1);

int pageNumber2 = pageNumbers.DefaultIfEmpty(1).Single();
Console.WriteLine("The value of the pageNumber2 variable is {0}", pageNumber2);

•  Sum

float?[] points = { null, 0, 92.83F, null, 100.0F, 37.46F, 81.1F };

float? sum = points.Sum();

Console.WriteLine("Total points earned: {0}", sum);           

• ToArray 

List<Package> packages =
    new List<Package>
 { new Package { Company = "Coho Vineyard", Weight = 25.2 },
  new Package { Company = "Lucerne Publishing", Weight = 18.7 },
  new Package { Company = "Wingtip Toys", Weight = 6.0 },
  new Package { Company = "Adventure Works", Weight = 33.8 } };

string[] companies = packages.Select(pkg => pkg.Company).ToArray();

• ToDictionary

List<Package> packages = new List<Package>
 new Package { Company = "Coho Vineyard", Weight = 25.2, TrackingNumber = 89453312L },
  new Package { Company = "Lucerne Publishing", Weight = 18.7, TrackingNumber = 89112755L },
  new Package { Company = "Wingtip Toys", Weight = 6.0, TrackingNumber = 299456122L },
  new Package { Company = "Adventure Works", Weight = 33.8, TrackingNumber = 4665518773L } };

Dictionary<long, Package> dictionary =
    packages.ToDictionary(p=>p.TrackingNumber);

foreach (var kvp in dictionary)
{
    Console.WriteLine(
        "Key {0}: {1}, {2} pounds",
        kvp.Key,
        kvp.Value.Company,
        kvp.Value.Weight);
}

• ToList

string[] fruits = { "apple", "passionfruit", "banana", "mango",
          "orange", "blueberry", "grape", "strawberry" };

List<int> lengths = fruits.Select(fruit => fruit.Length).ToList();

foreach (int length in lengths)
{
    Console.WriteLine(length);
}

• ToLookUp