What is LINQ in C#?

Language-Integrated Query (LINQ) in C# allows data manipulation in the form of queries similar to SQL ones. It enables data in data stores to be treated as objects, which can then be retrieved via queries.

The data stores on which LINQ is applicable include the following: collections, ADO.net, databases, web services, and XML docs.

The flow of data in LINQ

Syntax

The following lines represent how LINQ can be used in C#:

from <var> in <collection>:
<Standard Query Operator><expression>
<Selection Operator><result>

A loop traverses through the data collection, and a range variable is used to track which data record is currently in use. Then, an SQL operator is used to retrieve the data according to some condition that comes in the expression clause.

The result is then returned via a selection operator. The selection operator could be SELECT or GROUPBY.

Example

The following code has a LINQ query in it:

using System;
using System.Linq;
using System.Collections.Generic;
class HelloWorld
{
    static void Main()
    {
        IList<string> data = new List<string>(){
            "Pop",
            "Stack",
            "Queue",
            "List",
            "Data structures"
        };
        var result =  from x in data
                      where x.Contains("a")
                      select x;
        foreach(string res in result){
            Console.WriteLine(res);
        }
    }
}

Explanation

  • Lines 8–14: We create a data collection stored in a List structure with multiple string objects.

  • Lines 15–17: We create a LINQ query to get all the string objects from the list with an "a" in them. The result variable has the objects returned by the LINQ query.

  • Lines 18–20: We create a loop to print the result of the LINQ query.

Different types of LINQ queries

The following are some examples of LINQ queries that we can use in C#:

Anonymous types

We can manipulate anonymous types using LINQ query:

using System;
using System.Linq;
class HelloWorld
{
    class Person{
        public int id;
        public string name;
        public string occupation;
        public string dob;
        public Person(){
            id = 0;
            name = "";
            occupation = "";
            dob = "";
        }
        public Person(int x, string n, string o, string d){
            id = x;
            name = n;
            occupation = o;
            dob = d;
        }
    }
    static void Main()
    {
        Person[] p_array = {
            new Person(1, "John", "Manager", "2001-04-01"),
            new Person(2, "Lenny", "Lawyer", "1997-12-11"),
            new Person(3, "Andrew", "Manager", "1987-02-22"),
            new Person(4, "Peter", "Lawyer", "1936-03-24"),
            new Person(5, "Anna", "Lawyer", "1973-11-18")        
        }; 
        var res =   from person in p_array
                    where person.occupation == "Lawyer"
                    select new { person.name, person.occupation };
        
        Console.WriteLine(string.Join(",",res));
        
        
    }
}

The OrderBy query

The OrderBy clause is also present in the LINQ query, and we can implement it in this way:

using System;
using System.Linq;
using System.Collections.Generic;
class HelloWorld
{
    static void Main()
    {
        IList<string> data = new List<string>(){
            "Pop",
            "Stack",
            "Queue",
            "List",
            "Data structures"
        };
        var result =  (from x in data
                      where x.Contains("a")
                      //OrderBy clause to 
                      //get data according to 
                      //alphabetic order
                      select x).OrderBy(x=>x);
        foreach(string res in result){
            Console.WriteLine(res);
        }
    }
}

The group clause

We also have the group clause in LINQ:

using System;
using System.Linq;
class HelloWorld
{
  class Person{
        public int id;
        public string name;
        public string occupation;
        public string dob;
        public Person(){
            id = 0;
            name = "";
            occupation = "";
            dob = "";
        }
        public Person(int x, string n, string o, string d){
            id = x;
            name = n;
            occupation = o;
            dob = d;
        }
    }
    static void Main(){
     Person[] p_array = {
            new Person(1, "John", "Manager", "2001-04-01"),
            new Person(2, "Lenny", "Lawyer", "1997-12-11"),
            new Person(3, "Andrew", "Manager", "1987-02-22"),
            new Person(4, "Peter", "Lawyer", "1936-03-24"),
            new Person(5, "Anna", "Lawyer", "1973-11-18")        
        }; 
        //LINQ query where groups are made using
        //occupations of the persons
      var groupByOccupation = from person in p_array
                                  group person by person.occupation into g
                                  select g;
        //Printing each group's items 
      foreach (var g in groupByOccupation){
              Console.WriteLine($"Key: {g.Key}");
              foreach (var person in g){
                  Console.WriteLine($"\t{person.name}, {person.occupation}");
              }
      }     
        
    }
}

Advantages

LINQ has the following benefits:

  • Code effortlessly: There is IntelliSense support for LINQ. This increases the productivity of developers.

  • Umbrella syntax: Developers do not need to learn a new SQL language for different types of data collections.

  • Readability: It makes the code much more readable.

  • Data manipulation: Data objects can be shaped into different forms using LINQ queries.

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