Use of LINQ to query a collection

Question

about min: I come from JAVA and recently I am learning c#, but I have a doubt about the use of LINQ. Many tutorials speak of the wonder that is LINQ but I so far can not understand its usefulness. Look at this code below:

    class Program
{
    static void Main(string[] args)
    {
        Pessoa p1 = new Pessoa() { nome= "Fulano", idade = 10};
        Pessoa p2 = new Pessoa() { nome = "Fulano2", idade = 18 };

        List<Pessoa> listaDePessoa = new List<Pessoa>();
        listaDePessoa.Add(p1);
        listaDePessoa.Add(p2);

        var consulta = from p in listaDePessoa
              where p.idade > 10
              select p;

    }

}

public class Pessoa
{
    public string nome;
    public int idade;
}

Wouldn’t it be easier to do a direct database query? in JAVA I would do this using JDBC.

select * from tb_pessoas
where idade > 10

Maybe I’m just thinking about the database if that’s what gives me some other uses of LINQ.

Answer 1

It would not be easier to make a direct consultation of the bank?

No, because LINQ is not only for formulating SQL queries. It is a much more powerful resource.

LINQ, according to Wikipedia article, "is a component of Microsoft . NET that adds query features in some . NET" programming languages. This means that we can use this query functionality on any object in . NET in any programming language whose code is managed (Managed).

Suppose an object Cerveja:

public class Cerveja 
{
    public String Nome { get; set; }
    public String Tipo { get; set; }
    public decimal Preco { get; set; }
}

Suppose a beer list:

var cervejas = new List<Cerveja> 
{
    new Cerveja { Nome = "Budweiser", Tipo = "American Lager", Preco = 3M },
    new Cerveja { Nome = "Heineken", Tipo = "Pale Lager", Preco = 3.5M },
    new Cerveja { Nome = "Stella Artois", Tipo = "Pilsner", Preco = 3.3M },
};

I add the LINQ feature to my code:

using System.Linq;

I can search the list using a query syntax. For example, if I want to know if there is a beer with the name "Budweiser":

var bud = cervejas.FirstOrDefault(c => c.Nome == "Budweiser");

The search occurs in memory and returns an object. If there was no beer whose name is "Budweiser" in the list, the return would be a null object.

If I want all the beers more expensive than 3 real, I can use:

var cervejasCaras = cervejas.Where(c => c.Preco > 3M).ToList();

About your example

This example does not go to the database. Just like in my example, you only perform a memory search operation. To go to the database, you would need to connect your code to a data source. The resource that does this is called LINQ to SQL. An example of full use is here.

Unlike pure LINQ, LINQ to SQL mounts a query through the accumulation of predicates in an object that implements IQueryable<T>, being T a class representing a database record in the . NET language you are using. This query is executed only when the code asks for the list to materialize (talk about it here).

LINQ to SQL received a first update, called LINQ to Entities, where the database to which the code connects is treated as a context of objects (ObjectContext) and each entity (table, in the case of a relational database) is mapped as a ObjectSet<TEntity>, being TEntity a class whose properties are the columns (or fields) of each data record.

This update has turned into a framework widely used currently called Entity Framework, more mature, optimized, which has additional capabilities to LINQ to Entities, such as the ability to generate a database through code and incremental support for data schema migrations. That is, if you are writing your code and modeling entities, the database tracks these changes in modeling through a resource known as migrations (or Migrations). We have a tag here on the site just talking of this framework.

Answer 2

Linq is a query abstraction language where it focuses on making it easier to write queries.

Utilize Linq aims to standardize all your queries made in c#, and this is its main feature, microsoft provides this content saying some reasons why use Linux not only Linq-to-sql, but as a data query standard.