LINQ标准查询运算符

Posted 2021-03-07 bibi-feiniaoyuan

标准查询运算符：

标准查询运算符是一组方法，提供包括筛选where、投影select、聚合（例如max)、排序order by等在内的查询功能。

　　　　string sentence = "the quick brown fox jumps over the lazy dog";
            string[] words = sentence.Split(‘ ‘);

            var query = from word in words
                        group word.ToUpper() by word.Length into gr
                        orderby gr.Key
                        select new { Length = gr.Key, Words = gr };

            //与上面表达同一种意思的方法调用写法
            var query1 = words.GroupBy(x => x.Length, w => w.ToUpper())
                .OrderBy(o => o.Key)
                .Select(p => new { Length = p.Key, Words = p });

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各个标准查询运算符在执行时间上有所不同， 返回单一值的方法例如Max、Sum会立即执行，返回值序列的延迟执行。 

运算符对应的查询表达式：

Cast方法 public static System.Collections.Generic.IEnumerable<TResult> Cast<TResult> (this System.Collections.IEnumerable source);

例如 from string fruit in fruits

System.Collections.ArrayList fruits = new System.Collections.ArrayList();
fruits.Add("mango");
fruits.Add("apple");
fruits.Add("lemon");

//查询表达式
var query = from string fruit in fruits
            orderby fruit
            select fruit;
//方法调用
IEnumerable<string> query1 =
    fruits.Cast<string>().OrderBy(fruit => fruit).Select(fruit => fruit);

GroupBy方法 group ... by ... 或 group ... by ... into ...

GroupJoin分组联接方法  join ... in ... on ... equals ... into ...

namespace ConsoleApp4
{
    class Program
    {
        static void Main(string[] args)
        {
            Person magnus = new Person { Name = "Hedlund, Magnus" };
            Person terry = new Person { Name = "Adams, Terry" };
            Person charlotte = new Person { Name = "Weiss, Charlotte" };

            Pet barley = new Pet { Name = "Barley", Owner = terry };
            Pet boots = new Pet { Name = "Boots", Owner = terry };
            Pet whiskers = new Pet { Name = "Whiskers", Owner = charlotte };
            Pet daisy = new Pet { Name = "Daisy", Owner = magnus };

            List<Person> people = new List<Person> { magnus, terry, charlotte };
            List<Pet> pets = new List<Pet> { barley, boots, whiskers, daisy };

            //join … in … on … equals … into …   查询表达式
            var query = from person in people
                        join pet in pets on person equals pet.Owner into petGroup
                        select new { OwnerName = person.Name, Pets = petGroup.Select(p=>p.Name) };
　　 　　　　 //方法调用
            var query1 = people.GroupJoin(pets,
                person => person,
                 pet => pet.Owner,
                 (person, petCollection) =>
                 new
                 {
                     OwnerName = person.Name,
                     Pets = petCollection.Select(pet => pet.Name)
                 }
                );

            Console.WriteLine(" 查询表达式 :");
            foreach (var obj in query)
            {
                // Output the owner‘s name.
                Console.WriteLine("{0}:", obj.OwnerName);
                // Output each of the owner‘s pet‘s names.
                foreach (string pet in obj.Pets)
                {
                    Console.WriteLine("  {0}", pet);
                }
            }

            Console.WriteLine(" 方法调用 :");
            foreach (var obj in query1)
            {
                // Output the owner‘s name.
                Console.WriteLine("{0}:", obj.OwnerName);
                // Output each of the owner‘s pet‘s names.
                foreach (string pet in obj.Pets)
                {
                    Console.WriteLine("  {0}", pet);
                }
            }
        }
    }

    class Person
    {
        public string Name { get; set; }
    }

    class Pet
    {
        public string Name { get; set; }
        public Person Owner { get; set; }
    }
}

Join方法 join...in...on...equals...

SelectMany 方法将序列的每个元素投影到 IEnumerable<T> 并将结果序列合并为一个序列。

SelectMany方法对应复合from查询表达式

namespace ConsoleApp4
{
    class Program
    {
        static void Main(string[] args)
        {
            PetOwner[] petOwners =
        { new PetOwner { Name="Higa",
              Pets = new List<string>{ "Scruffy", "Sam" } },
          new PetOwner { Name="Ashkenazi",
              Pets = new List<string>{ "Walker", "Sugar" } },
          new PetOwner { Name="Price",
              Pets = new List<string>{ "Scratches", "Diesel" } },
          new PetOwner { Name="Hines",
              Pets = new List<string>{ "Dusty" } } };

            var query1 = from owner in petOwners
                         from pet in owner.Pets
                         select new { petOwner = owner.Name, petName = pet };

            foreach (var obj in query1)
            {
                Console.WriteLine(obj);
            }

            Console.WriteLine("方法调用结果：");

            var query =
                petOwners
                .SelectMany(petOwner => petOwner.Pets, (petOwner, petName) => new { petOwner.Name, petName })
               ;

            // Print the results.
            foreach (var obj in query)
            {
                Console.WriteLine(obj);
            }
        }
    }

    class PetOwner
    {
        public string Name { get; set; }
        public List<string> Pets { get; set; }
    }
}

 OrderBy 、OrderByDescending、ThenByDescending 、ThenBy等排序方法对应表达式orderby...descending/ascending(默认)。

Where方法对应where....