The Evolution of .NET

When .NET shipped in 2002, it was a very specific thing: a managed runtime, a giant standard library, and a few languages — all running on Windows, all owned by Microsoft, all closed-source.

Twenty-plus years later, .NET looks completely different. It runs on Linux and macOS. Its code is on GitHub. It is developed in the open with thousands of outside contributors. Most of its users are not Microsoft customers in the old sense at all.

This page tells that arc, because it shapes the C# you will write today.

The original .NET Framework (2002–2019)

The original product was called .NET Framework. It was:

• Windows-only. No Linux, no macOS.
• Closed-source. Microsoft owned and shipped it.
• Shipped with Windows. You generally couldn't install two major versions side-by-side easily, and upgrades came tied to Windows itself.

For its first decade this was fine — .NET Framework was hugely successful in enterprises that already ran Windows everywhere.

But two big shifts were happening in the wider world:

1. The cloud. Companies were renting servers from Amazon (AWS), Google, and later Microsoft itself (Azure). Most of those servers ran Linux, not Windows.
2. Mobile and macOS. Developers increasingly worked on Macs. iPhones and Androids dominated mobile.

A Windows-only .NET could not credibly serve either trend.

Mono: a community runs ahead

A determined developer named Miguel de Icaza had already seen this coming. In 2004 he led a project called Mono — an open-source, cross-platform implementation of .NET that ran on Linux and macOS. Mono kept the .NET ecosystem alive on non-Windows platforms for years, and was later used as the foundation for:

• Xamarin — using C# to build iOS and Android apps.
• Unity — the most popular game engine in the world, where you write your gameplay logic in C#.

Mono proved that .NET could run anywhere. The question was whether Microsoft would embrace that direction.

.NET Core: Microsoft starts over (2016)

Around 2014, under new leadership (Satya Nadella became CEO in 2014, with a famous "Microsoft loves Linux" stance), the company made a decision that would have been unthinkable a decade earlier: it would build a new, cross-platform, open-source implementation of .NET, and shift the entire ecosystem onto it.

The new product was first called .NET Core. It was:

• Cross-platform. Windows, Linux, macOS — all first-class.
• Open source. Developed in public on GitHub.
• Side-by-side installable. You could have .NET 6 and .NET 8 on the same machine and pick a different one per project.
• Modular and faster. Smaller deploy footprints; major performance improvements every release.

In 2020, Microsoft dropped "Core" from the name and shipped .NET 5, unifying the old .NET Framework and .NET Core into a single product line. From that point on, the rhythm has been one major release per year:

.NET Framework still exists and still ships security patches. But all new C# development happens on modern .NET. When this course says ".NET", it means modern .NET — the cross-platform open-source one.

C# kept evolving in lockstep

C# has had its own steady release cadence, and in modern .NET each new language version ships alongside a new runtime version. A few features you will actually use:

The thread connecting all these additions: let the programmer write less ceremony around the same ideas. Compare an old-style "hello world" with a modern one.

Old style (C# 1.0 era):

using System;

namespace HelloApp
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Hello, World!");
        }
    }
}

Modern style (C# 9+ "top-level statements"):

Both compile to the same thing. The second is just less in your way. That is the spirit of modern C#.

Where C# is used today

Because .NET now runs everywhere, C# has spread far beyond its original enterprise-Windows roots:

You may not work in all of these areas. But it is useful to know that the same language — sometimes the same standard library — can take you across this whole landscape.

What "modern C#" assumes

Throughout this course, when we say "modern C#" we mean:

• You're using .NET 6 or later (we test on the latest LTS).
• You can write top-level statements without a Main method.
• You have record, var, string interpolation, and nullable reference types available.
• You think of the runtime as the cross-platform .NET, not the legacy .NET Framework.

The browser playground in this course runs on the .NET WebAssembly runtime, so every example you see should run in your browser.

Test your understanding