freeCodeCamp/guide/english/csharp/linq/index.md

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LINQ

LINQ (Language Integrated Query)

LINQ (Language Integrated Query) is a Microsoft programming model and methodology that essentially adds formal query capabilities into Microsoft .NET-based programming languages. LINQ offers a compact, expressive, and intelligible syntax for manipulating data. The real value of LINQ comes from its ability to apply the same query to an SQL database, a DataSet, a list, a dictionary, etc.

Example

LINQ can be used to filter, transform, search data and a lot more complex tasks. Let's say we have the following list of objects:

var fruits = new List<Fruit>() {
    new Fruit() { Id = 1, Name = "Orange",     Color = "Orange", Quantity: 3   },
    new Fruit() { Id = 2, Name = "Strawberry", Color = "Red",    Quantity: 12  },
    new Fruit() { Id = 3, Name = "Grape",      Color = "Purple", Quantity: 25  },
    new Fruit() { Id = 4, Name = "Pineapple",  Color = "Yellow", Quantity: 1   },
    new Fruit() { Id = 5, Name = "Apple",      Color = "Red",    Quantity: 5   },
    new Fruit() { Id = 6, Name = "Mango",      Color = "Yellow", Quantity: 2   }
}

Then we can do things like:

// Get the name of the first fruit
var firstName = fruits.Select(f => f.Name).First(); // Orange

// Count how many fruits are red
var qntRed = fruits.Where(f => f.Color == "Red").Count(); // 2

// Create a list of yellow fruits
var yellowFruits = fruits.Where(f => f.Color == "Yellow").ToList(); // { Pineapple, Mango }

// Orders list by quantity from most to least
var orderedFruits = fruits.OrderByDescending(f => f.Quantity).ToList(); // {Grape, Strawberry, Orange, Apple, Mango, Pineapple}

// Sum the quantity of fruits
var quantity = fruits.Sum(f => f.Quantity); // 53

// Check if there are any green fruits
var hasGreen = fruits.Any(f => f.Color == "Green"); // false

// Group fruits by color into a dictionary
var fruitsByColor = fruits.GroupBy(g => g.Color).ToDictionary(k => k.Key, v => v.ToList()); // Dictionary of list of fruits by color

// linq operations can be concatenated and are not performed as long as data is needed
var logs = new List<Log>;

if (filterBySeverity)
    logs = logs.Where(p => p.Severity == severity);
    //IQueryable

if (filterByUser)
    logs = logs.Where(p => p.User == user);
    //IQueryable
    
    result = logs.ToList();
    //List<log>