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> { ,  };

bool hasElements = numbers.Any();

bool hasElementsEquals2 = numbers.Any(num=>num==);

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

Average 求平均值

List<int> grades = new List<int> { , , , ,  };

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();

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 > );

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

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

            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().First();

            Console.WriteLine("The value of the firstMonth2 variable is {0}", firstMonth2);

Last、LastOrDefault

int[] numbers = { , , , , , , , ,

        , , , , ,  };

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

int last1 = numbers.Last();

Console.WriteLine(last);

Console.WriteLine(last1);

int[] numbers1 = { };

Console.WriteLine(numbers1.LastOrDefault());

Console.WriteLine(numbers1.DefaultIfEmpty().Last());

LongCount

namespace ConsoleApp4

{

    class Program

    {

        static void Main(string[] args)

        {

            Pet[] pets = { new Pet { Name="Barley", Age= },

                   new Pet { Name="Boots", Age= },

                   new Pet { Name="Whiskers", Age= } };

            const int Age = ;

            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; }

    }

}

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 =  };

Pet pet2 = new Pet { Name = "Peanut", Age =  };

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 >);

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().Single();

Console.WriteLine("The value of the pageNumber2 variable is {0}", pageNumber2);

float?[] points = { null, , 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

巴特西

LINQ标准查询运算符的执行方式-即时

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