Language-Integrated Query (LINQ) is the name for a set of technologies based on the integration of query capabilities directly into the C# language.

Traditionally, queries against data are expressed as simple strings without type checking or IntelliSense support at compile time. Also, you need to know a different query language for each data source: SQL databases, XML documents, web applications, and so on. In LINQ, the query consists of preliminary language structures, such as classes, methods, and events.

When writing queries, the most visible "language-integrated" part of LINQ is the query specification. Question words are written in declarative question syntax. Using query syntax you perform filtering, sorting, and grouping over data sources with minimal code. We use the same query definition pattern to query and modify data from any data source.

Example

The following example shows a complete query function. The entire process includes creating the data source, defining the query definition, and executing the query in the foreach statement.

csharp
📄
using System;
using System.Collections.Generic;
using System.Linq;

public class example
{
    public static void Main(string[] args)
    {
        // Specify the data source.
        int[] scores = new int[] { 97, 92, 81, 60 };
        
        // Define the query expression.
        IEnumerable<int> scoreQuery =
            from score in scores
            where score > 80
            select score;

        // Execute the query.
        foreach (var i in scoreQuery)
        {
             Console.Write(i + " ");
        }
        // Output: 97 92 81
    }
}

Key concepts in LINQ:

1. Query Syntax: This is the SQL-like syntax in C# for writing LINQ queries.
2. Method syntax: Also called "fluent syntax", it uses.Where(), .Select(), .OrderBy(), and other methods to get the same result as query syntax.
3. Deferred execution: LINQ queries are not executed when they are defined but when they are repeated (e.g., when a foreach is used).
4. Standard query operators: LINQ provides many methods for filtering, organizing, organizing, and merging data.

How LINQ work?

Data Source: LINQ can query various types of data sources, such as:

• Collections (like arrays, and lists)
• Databases (via LINQ to SQL, Entity Framework)
• XML (LINQ to XML)
• In-memory data

Query Expression: The query expression defines the logic to retrieve the data from the data source.

Execution: Once the query is defined, it can be executed by using loops (foreach) or other operators.

Example

The following example illustrates the LINQ Method Syntax.

csharp
📄
using System;
using System.Linq;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        // Data source
        List<int> scores = new List<int>() { 97, 92, 81, 60 };

        // Define the query using method syntax
        IEnumerable<int> scoreQuery = scores.Where(score => score > 80);

        // Execute the query
        foreach (int score in scoreQuery)
        {
            Console.WriteLine(score);
        }
    }
}

Common LINQ Methods.

The following are the common Methods of the LINQ:

• Where: It filters elements based on a condition.

csharp
📄
var filtered = collection.Where(item => item > 10);

• Select: Projects or transforms elements into a new form.

csharp
📄
var projected = collection.Select(item => item * 2);

• OrderBy: Orders elements based on a key.

csharp
📄
var ordered = collection.OrderBy(item => item);

• GroupBy: Groups elements based on a key.

csharp
📄
var grouped = collection.GroupBy(item => item % 2);

• Join: Joins two collections based on matching keys

csharp
📄
var joined = collection1.Join(collection2, item1 => item1.Key, item2 => item2.Key, (item1, item2) => new { item1, item2 });

LINQ is a powerful C# tool widely used to query and manipulate data with minimal complex code.