System.Linq Namespace

Provides classes and interfaces that support Language Integrated Query (LINQ), which adds native data querying capabilities to .NET languages.

Summary

The System.Linq namespace is the core of LINQ, offering the fundamental types and operations for querying collections, XML, databases, and other data sources. It introduces extension methods to common collection types, enabling a declarative and expressive way to filter, sort, group, and project data.

Key Concepts

Query Operators: Methods like Where, Select, OrderBy, GroupBy, etc., that transform sequences.
Deferred Execution: Most LINQ queries are not executed until the results are iterated.
Query Syntax vs. Method Syntax: LINQ can be expressed using a SQL-like query syntax or fluent method calls.
IEnumerable<T> and IQueryable<T>: Interfaces representing sequences that LINQ operates on.

Classes

Enumerable Class

Contains static methods that query sequences. It is the foundation for LINQ to Objects.

Key Methods: Where<TSource>(this IEnumerable<TSource>, Func<TSource, bool>), Select<TSource, TResult>(this IEnumerable<TSource>, Func<TSource, TResult>), OrderBy<TSource, TKey>(this IEnumerable<TSource>, Func<TSource, TKey>).
Queryable Class

Provides extension methods that invoke the query operators that must translate to a IQueryable data source.

Key Methods: Similar to Enumerable but designed for IQueryable<T> to enable translation to underlying data sources (e.g., SQL).

Interfaces

IEnumerable<T> Interface

Represents a sequence of items that can be iterated over. This is the primary interface for LINQ to Objects.
IQueryable<T> Interface

Represents a query against a data source. IQueryable<T> allows LINQ providers to translate queries into an expression tree that can be executed against various data sources.
IGrouping<TKey, TElement> Interface

Represents a collection of elements that have a common key.

Example Usage (LINQ to Objects)

Filtering and Projecting

Find all numbers greater than 5 and select their squares.

// C# Example
var numbers = new[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

var results = numbers
.Where(n => n > 5)
.Select(n => n * n);

foreach (var square in results)
{
Console.WriteLine(square); // Output: 36, 49, 64, 81, 100
}

Grouping

Group words by their length.

// C# Example
var words = new[] { "apple", "banana", "kiwi", "orange", "grape", "pear" };

var groupedByLength = words
.GroupBy(w => w.Length);

foreach (var group in groupedByLength)
{
Console.WriteLine($"Words of length {group.Key}:");
foreach (var word in group)
{
Console.WriteLine($" - {word}");
}
}

.NET Documentation