LINQ Simple to use

• Simple to use
• From sub-query
• Associating multiple data sources
• Group
• Let clause

• The Query object (the list variable in the example above) is a ienumerable<t> or iqueryable<t> type
• Query return result is also ienumerable<t> or iqueryable<t> type

LINQ is divided into five categories: LINQ to Objects, LINQ to DataSets, LINQ to SQL, LINQ to Entities, and LINQ to XML.

Simple to use

Type query variable = from                temporary variable in collection object or database object               [where Condition expression]               [order by condition]               the value queried in the Select temporary variable or temporary variable               [GROUP by]

① General Enquiry:

Data source int[] arr = new int[] {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};//LINQ query var result = from     item in ARR    //Declaration TEMP variable Item, and describes its array of elements in Arr, item    where item > 4    Select item;        Add the query result (item) to result//iterate over the result of the LINQ query, which is actually a LINQ query that is executed with foreach, that is, the LINQ query does not execute a foreach (int i in result) before the foreach execution {    Console.Write (i);   Output 56789}

② Query and sort:

int[] arr = new int[] {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};var result = from     item in ARR    where item > 4    Item Descending//descending order    Select Item;foreach (int i in result) {    console.write (i);       Output: 98765}

③ Type conversions:

int[] arr = new int[] {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};var result = from    item in ARR    where item > 4    Select String. Format ("value: {0}", item);   Convert to String type and format foreach (string i in result) {   //variable i to String type    console.write (i);           Output value: 5 Value: 6 Value: 7 Value: 8 Value: 8 "}

④ Query Single value:

int[] arr = new int[] {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};int result0 = (from item in arr where item > 4 Select Item). Count ();   The entire statement is picked up by a bunch of () statistics int result1 = (from item in arr where item > 4 Select Item). Max ();   Query maximum value Console.WriteLine (RESULT0); Output: 5console.writeline (RESULT1); Output: 9

From sub-query

Class program{    static void Main (string[] args)    {        //create a generic variable like a two-dimensional array arr, used to store student information        list arr = new List             {                 new Student {name = "Li ning", scores = new list{11, new                Student {name = "Artie", scores = new list{11, Student}},                new {name = "Nike", scores = new list{18, +},            };        Query for a record with a student score greater than 15        var result = from                stu in arr             //Declaration TEMP variable stu from                    sc in stu.scores   //Declaration TEMP Variable SC for " Subquery ",                where SC >                select new {name = Stu.name, score = SC};    "Project" The query result into a new object that has the name and score two properties.        foreach (var s in result) {            Console.WriteLine (s.name + ":" + S.score);}}    } Class student{public    string name;         Student name public    List scores;    Student scores, a student can have multiple scores}

Output Result:

Li Ning: 34 Artie: 16 Artie: 34 Nike: 18

In the above example, it is true that each student has multiple associations with their scores, and the results are stored in a new object (...} In

Associating multiple data sources

Define data source char[] Upper = {' A ', ' B ', ' C '};char[] lower = {' x ', ' y ', ' z '};//associated two data sources var result0 = from up in     upper
   
    from Lo in Lower    select new {upper = up, Lower=lo};foreach (var item in result0) {    Console.WriteLine (Item.upper + "" + Item.lower);} Associate two data sources, and filter by criteria var RESULT1 = from the    upper from the    Lo    in lower where up! = ' B '    select new {upper = up, Lower = Lo};foreach (var item in RESULT1) {    Console.WriteLine (item.upper + "+ item.lower);}
   

The two output results were:

Group

① Simple Grouping

Class program{static void Main (string[] args) {//define data source list<product> arr = new List<produc t> {New product{name = "shirt", Price = 13.9m, cid = 1}, new product{name = "Shorts" , Price = 199.2m, cid = 1}, new product{name = "Audi", Price = 1.6m, cid = 2}, new Product{nam E = "Mercedes", Price = 2.7m, cid = 2}, New product{name = "J Ten", Price = 82.3m, cid = 3}, new Prod        Uct{name = "broadcast", Price = 91.3m, cid = 3},};        var result = from P in ARR group p by P.cid;            Traverse Group foreach (var gp in result) {Console.WriteLine ("==============");            Traverse each group of members foreach (Var pd in GP) {Console.WriteLine ("{0}-{1}-{2}", Pd.name, Pd.price, pd.cid);    }} console.read ();     }}class product{public string name;   Commodity name: decimal price; Commodity price public intCid Product Category}

The groupings here are not groupings in SQL, and groupings in SQL are used primarily for grouping statistics. And the group here is the original "one-dimensional data" in accordance with a certain rules of the composition of "two-dimensional data", the output of the results:

② Handling of Group objects

    static void Main (string[] args) {//define data source list arr = new List {New Produc T{name = "Shirt", Price = 13.9m, cid = 1}, new product{name = "Shorts", Price = 199.2m, cid = 1}, n                EW product{name = "Audi", Price = 1.6m, cid = 2}, new product{name = "Mercedes", Price = 2.7m, cid = 2},            New Product{name = "J Ten", Price = 82.3m, cid = 3}, new product{name = "broadcast", Price = 91.3m, cid = 3},        }; List classlist = new List {new pclass{cname= "clothes", cid = 1}, new pclass{cname= "car        ", CID = 2}, new Pclass{cname=" Airplane ", cid = 3},};            var result = from P in arr//g is actually a group, formatted as <vey, values> format//key for that group of CID  Values for the set of elements group p by p.cid intoG from CLS in classlist where G.key = = CLS.CID//association group G and classlist collection select new {Cnam        E = cls.cname, cid = cls.cid, gplist = G};            foreach (var gp in result) {Console.WriteLine ();            Console.WriteLine ("group name: {0}, Category ID:", gp.cname, Gp.cid);            foreach (var pd in Gp.gplist) {Console.WriteLine ("{0}-{1}-{2}", Pd.name, Pd.price, pd.cid);         }}}}class pclass{public int cid;    Category ID public string cname;     Category name}class product{public string name;   Commodity name: decimal price;         Commodity price public int CID; Product Category}

The key to the above code is to understand the "into G", the query part of the execution process is:
The first step is to perform the grouping, to separate a group (such as the Cid=1 Group) and save the temporary variable G; then perform the second step, associating the group G with the Classlist collection, storing the eligible results in the select specified format. After the first packet is processed, follow the steps above to process the second grouping (cid=2), and so on.

Program execution output results:

③ to delete a grouped object

static void Main (string[] args) {    list sts = new list        {            new student{id= 1, name= "Li ning", cid = 1},            new Student {id= 2, name= "NA", cid = 2},            new student{id= 3, name= "Nike", cid = 3},            new Student{id= 4, Name= "Artie", CID = 2},            n EW student{id= 5, Name= "Mercedes", CID = 1},            new student{id= 6, name= "Registration", CID = 3},        };        var result = from        s in STS        Group S by s.cid to G        Select new {             id = g.key,             list = (                //Exclude ID Students for even numbers from                m in G                where (m.id% 2 = 0)                Select M            )        };    foreach (var item in result) {        Console.WriteLine ("Cid:{0}", item.id);        foreach (var s in item.list) {            Console.WriteLine ("{0}-{1}-{2}", S.id, S.name, s.cid);}}}    

Let clause

Used to create a new temporary variable in the query

static void Main (string[] args) {    //define Data source    List sts = new list        {            new student{id= 1, name= "Li ning", cid = 1},
   new student{id= 2, name= "Li Na", cid = 2},            new student{id= 3, name= "Nike", cid = 3},            new Student{id= 4, Name= "Artie", CI D = 2},            new student{id= 5, Name= "Mercedes", CID = 1},            new student{id= 6, name= "Registration", CID = 3},        };    var result = from        s in sts let        fullname = s.id + s.name        Select String. Format ("id:{0}, Full name: {1}", S.id, FullName);    foreach (string i in result) {        Console.WriteLine (i);    }}

LINQ Simple to use