Base Class Library (BCL)

The Base Class Library (BCL) is a collection of foundational classes provided by .NET. It includes APIs for collections, file I/O, data types, networking, and more — essential for building any .NET application.

What is the BCL?

The BCL is part of the larger .NET Framework Class Library (FCL). It provides the basic building blocks that all .NET applications depend on.

System – core types like Object, String, Int32
System.IO – file and stream operations
System.Collections – lists, dictionaries, stacks, queues
System.Net – networking utilities
System.Threading – parallelism and concurrency

Common BCL Examples

Working with Strings

string name = "Alice";
string greeting = name.ToUpper(); // "ALICE"

Reading from a File

using System.IO;

string content = File.ReadAllText("data.txt");
Console.WriteLine(content);

Using a List

using System.Collections.Generic;

List<string> fruits = new() { "Apple", "Banana" };
fruits.Add("Cherry");
fruits.ForEach(f => Console.WriteLine(f));

Why the BCL Matters

Reduces the need to write boilerplate code
Provides consistent, battle-tested APIs
Improves productivity with built-in features
Cross-platform and available in all .NET runtimes

Exploring the BCL with IntelliSense

Use Visual Studio’s IntelliSense to explore what’s available in BCL. Type System. or Console. to browse methods, properties, and documentation hints.

The Base Class Library is your toolbox for almost everything in .NET. With it, you can handle files, manage collections, process strings, and so much more. Next up, we’ll look under the hood at how .NET compiles your code.

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