Microsoft's .NET Framework is a development platform that includes a Common Language Runtime (CLR) that manages the execution of code, and provides a rich library of classes to build applications.

Microsoft designed .NET Framework to have the possibility of being cross-platform, but Microsoft put their implementation effort into making it work best with Windows.

Practically speaking, .NET Framework is Windows-only, and a legacy platform.

Third parties developed a .NET implementation named the Mono project that you can read more about at the following link:http://www.mono-project.com/

Mono is cross-platform, but it fell well behind the official implementation of .NET Framework. It has found a niche as the foundation of the Xamarin mobile platform.

Microsoft purchased Xamarin in 2016 and now gives away what used to be an expensive Xamarin extension for free with Visual Studio 2017. Microsoft renamed the Xamarin Studio development tool to Visual Studio for Mac, and has given it the ability to create ASP.NET Core Web API services. Xamarin is targeted at mobile development and building cloud services to support mobile apps.

Today, we live in a truly cross-platform world. Modern mobile and cloud development has made Windows a much less important operating system. So, Microsoft has been working on an effort to decouple .NET from its close ties with Windows.

While rewriting .NET to be truly cross-platform, Microsoft has taken the opportunity to refactor .NET to remove major parts that are no longer considered core.

This new product is branded as .NET Core, which includes a cross-platform implementation of the CLR known as CoreCLR, and a streamlined library of classes known as CoreFX.

Scott Hunter, Microsoft Partner Director Program Manager for .NET, says, "Forty percent of our .NET Core customers are brand-new developers to the platform, which is what we want with .NET Core. We want to bring new people in."

The following table shows when important versions of .NET Core were released, and Microsoft's schedule for the next major release:

.NET Core is much smaller than the current version of .NET Framework because a lot has been removed.

For example, Windows Forms and Windows Presentation Foundation (WPF) can be used to build graphical user interface (GUI) applications, but they are tightly bound to Windows, so they have been removed from .NET Core. The latest technology used to build Windows apps is Universal Windows Platform (UWP), and UWP is built on a custom version of .NET Core. You will learn about it in Chapter 17, Building Windows Apps Using XAML and Fluent Design.

ASP.NET Web Forms and Windows Communication Foundation (WCF) are old web application and service technologies that fewer developers choose to use for new development projects today, so they have also been removed from .NET Core. Instead, developers prefer to use ASP.NET MVC and ASP.NET Web API. These two technologies have been refactored and combined into a new product that runs on .NET Core, named ASP.NET Core. You will learn about ASP.NET Core MVC in Chapter 15, Building Web Sites Using ASP.NET Core MVC, ASP.NET Core Razor Pages in Chapter 14, Building Web Sites Using ASP.NET Core Razor Pages, and ASP.NET Core Web API and Single Page Applications (SPAs) such as Angular and React in Chapter 16, Building Web Services and Applications Using ASP.NET Core.

The Entity Framework (EF) 6 is an object-relational mapping technology to work with data stored in relational databases such as Oracle and Microsoft SQL Server. It has gained baggage over the years, so the cross-platform version has been slimmed down and named Entity Framework Core. You will learn about it in Chapter 11, Working with Databases Using Entity Framework Core.

In addition to removing large pieces from .NET Framework to make .NET Core, Microsoft has componentized .NET Core into NuGet packages: small chunks of functionality that can be deployed independently.

The situation with .NET today is that there are three forked .NET platforms, all controlled by Microsoft:

Each have different strengths and weaknesses because they are designed for different scenarios. This has led to the problem that a developer must learn three platforms, each with annoying quirks and limitations.

So, Microsoft defined .NET Standard 2.0: a specification for a set of APIs that all .NET platforms must implement. You cannot install .NET Standard 2.0 in the same way that you cannot install HTML5. To use HTML5, you must install a web browser that implements the HTML5 specification. To use .NET Standard 2.0, you must install a .NET platform that implements the .NET Standard 2.0 specification.

.NET Standard 2.0 is implemented by the latest versions of .NET Framework, .NET Core, and Xamarin. .NET Standard 2.0 makes it much easier for developers to share code between any flavor of .NET.

For .NET Core 2.0, this adds many of the missing APIs that developers need to port old code written for .NET Framework to the cross-platform .NET Core. However, some APIs are implemented, but throw an exception to indicate to a developer that they should not actually be used! This is usually due to differences in the operating system on which you run .NET Core. You will learn how to handle these exceptions in Chapter 2,Speaking C#.

The following diagram summarizes how the three variants of .NET (sometimes known as App Models) will share the common .NET Standard 2.0 and infrastructure:

The first edition of this book focused on .NET Core, but used .NET Framework when important or useful features had not yet been implemented in .NET Core. Visual Studio 2015 was used for most examples, with Visual Studio Code shown only briefly.

The second edition was almost completely purged of all .NET Framework code examples.

The third edition completes the job. It has been rewritten so that all of the code is pure .NET Core and can be written with Visual Studio 2017, Visual Studio for Mac, or Visual Studio Code on any supported operating system. The only exceptions are the last two chapters. In Chapter 17,Building Windows Apps Using XAML and Fluent Design, you will use .NET Core for UWP, which requires Visual Studio 2017 running on Windows 10, and in Chapter 18,Building Mobile Apps Using XAML and Xamarin.Forms, you will use Xamarin instead of .NET Core.

Another .NET initiative is .NET Native. This compiles C# code to native CPU instructions ahead-of-time (AoT), rather than using the CLR to compile intermediate language (IL) code just-in-time (JIT) to native code later.

.NET Native improves execution speed and reduces the memory footprint for applications. It supports the following:

The following table summarizes and compares .NET technologies: