About LINQ statements

 Linq A statement is a set of methods , Allows filtering of collections with minimal code 、 Sorting and grouping operations .Linq Represents language integration query . It is based on the integration of query functions into C# The name of the technology in the language . although Linq Methods are not language integrated , But there is also a declarative 、 Language integrated query snytax.

   Array

   The most basic enumerable items are arrays , It can store a fixed number of typed elements .

  int[] numbers = new int[3] { 1, 2, 3};

   In the above code, you create one that contains 3 An integer array of values , And use value 1、2 and 3 Initialize the array . Assign the array to int[] The variable of type is named numbers. Each element of the array can be accessed by indexing or enumerating values .

   int fistNumber = numbers[0]; //1

  foreach( int number in numbers){
   

     Console.WriteLine(number); // 1,2,3

}

  list

  A common type of enumeration is list , Unlike arrays , The size of the list is variable .

var number = new list<int>{1,2,3};

  In the example above , Created a list of integers , And use three values 1,2,3 Initialize it . Use keywords var To define the type of number . Use var, We let the compiler decide which type to use . When we will List<int> When an object of type is assigned to a number , The type of the variable will be List<int>. These value indexes can also be accessed, and they can also be enumerated .

 var    FirstNumber = number[0];     //1

froeach(var numbers in number){
   Console.WriteLine(numbers); // 1,2,3
}

   aggregate

The collection contains unique elements , for example ,HashSet Class provides high performance collection operations （ for example UnionWith、IntersectWith etc. ）

  var number = new HashSet<int>{1,2,3}

  With the front 2 The difference between an array and a list is , You cannot access the elements of a collection through its index . There is no such concept as element index in hash set . however , You can still enumerate the values stored in the collection .

  foreach( int number in numbers){
   

  Console.WriteLine(number); //1,2,3

}

  Although all enumerable types have different purposes and methods , But they have one thing in common . They all implement the same interface , namely IEnumerable<T>,Net in ,C# There is a declarative language structure in , Make it easy to handle enumeration items , Like the one above foreach loop .

   Other precautions ： Interface IEnumberable<T> It's a universal interface . This means that enumerations are strongly typed , But you can define specific types in your code . Strong typing means that it can only hold elements of the specified type （ Or you can implicitly convert elements of this type ）.T It's the template type , Is the placement order of the specified type .

   Type list <T> It's also a generic type . This list can be used for any type of （<T>）. however , You have to choose a fixed type in the code . for example <int>（ Shaping list ） yes IEnumerable<int>（ Integer enumerable ） You can add... To the list int value . You can also add byte values to the list （ Because they can be automatically converted to int, Invisible transformation , Without losing ）. however , You cannot add strings or long values to the list . The former cannot be replaced with an integer at all , The latter can be converted to an integer , But you may lose data , And it's never done automatically .

    Filtering data

in many instances , Data sets must be filtered , A dataset is a collection of objects （IEnumerable<T>）, A climbing predicate is an expression or function , You can decide whether to keep an item . If the predicate matches , Keep this item . without , You just try to drive it away . The following example ： Keep only positive values from the list of numbers . The first way to filter data is to create a new empty list , Then enumerate the numbers , And add a positive value to the new list .

The above successfully created a filtering method KeepositiveNumbers, In this method , We can pass a list of arbitrary numbers as input , And just get a list of positive numbers . This method accepts any IEnumerable<int>, Therefore, the method is implemented using IEnumerable<T> Array of interfaces 、 Lists or other data structures . To be added ..