C# .NET Standard Library: Complete Overview

C# (.NET) Standard Library: Complete Overview

The .NET Standard Library (Base Class Library, BCL) is a fundamental set of classes, interfaces, and value types that are part of the .NET platform. It provides C# developers with ready-made components for solving common tasks: from working with collections and files to network interaction and asynchronous programming. In this article, we will take a detailed look at the key namespaces and classes that every C# developer should know.

1. System Namespace — The Foundation of Everything

The root namespace System contains fundamental types:

Base Data Types: Object, String, Int32, Double, Boolean, etc.
Utilities: Console (input/output), Math (mathematical functions), Random (random number generation).
Error Handling: Exception and its descendants (ArgumentException, NullReferenceException, etc.).

Usage example:

using System;

class Program{    static void Main()    {        Console.WriteLine("Hello, .NET!");        double result = Math.Sqrt(144);        Console.WriteLine($"Square root of 144: {result}");    }}

2. System.Collections — Working with Collections

A namespace for storing and managing groups of objects. It is divided into generic and non-generic collections.

Main Classes:

List<T> — a dynamic array with fast index-based access.
Dictionary<TKey, TValue> — an associative array (hash table).
Queue<T> — a queue (FIFO).
Stack<T> — a stack (LIFO).
HashSet<T> — a set of unique elements.

Example:

using System.Collections.Generic;

var numbers = new List<int> { 10, 20, 30 };numbers.Add(40);

var lookup = new Dictionary<string, int>{    ["one"] = 1,    ["two"] = 2};

Console.WriteLine(lookup["two"]); // 2

3. System.Linq — Language Integrated Query

LINQ allows you to execute SQL-like queries against collections, databases, and XML. Main extension methods:

Where() — filtering.
Select() — projection.
OrderBy() — sorting.
GroupBy() — grouping.
Aggregate() — aggregate functions (Sum, Count, Average).

Example:

using System.Linq;

int[] scores = { 85, 92, 78, 95, 88 };var highScores = scores    .Where(s => s >= 90)    .OrderByDescending(s => s)    .ToList();

Console.WriteLine(string.Join(", ", highScores)); // 95, 92

4. System.IO — Working with Files and Streams

A namespace for data input/output. Key classes:

File / FileInfo — creating, reading, writing files.
Directory / DirectoryInfo — working with directories.
StreamReader / StreamWriter — reading/writing text streams.
FileStream — low-level file operations.

Example of writing and reading:

using System.IO;

string path = "example.txt";File.WriteAllText(path, "Hello, .NET!");

string content = File.ReadAllText(path);Console.WriteLine(content); // Hello, .NET!