Introduction to ASP.NET Core

By Daniel Roth, Rick Anderson, and Shaun Luttin

ASP.NET Core is a cross-platform, high-performance, open-source framework for building modern, cloud-based, Internet-connected applications. With ASP.NET Core, you can:

  • Build web apps and services, IoT apps, and mobile backends.
  • Use your favorite development tools on Windows, macOS, and Linux.
  • Deploy to the cloud or on-premises.
  • Run on .NET Core or .NET Framework.

Why use ASP.NET Core?

Millions of developers have used (and continue to use) ASP.NET 4.x to create web apps. ASP.NET Core is a redesign of ASP.NET 4.x, with architectural changes that result in a leaner, more modular framework.

ASP.NET Core provides the following benefits:

Build web APIs and web UI using ASP.NET Core MVC

ASP.NET Core MVC provides features to build web APIs and web apps:

Client-side development

ASP.NET Core integrates seamlessly with popular client-side frameworks and libraries, including Angular, React, and Bootstrap. For more information, see Client-side development.

ASP.NET Core targeting .NET Framework

ASP.NET Core 2.x can target .NET Core or .NET Framework. ASP.NET Core apps targeting .NET Framework aren't cross-platform—they run on Windows only. Generally, ASP.NET Core 2.x is made up of .NET Standard libraries. Apps written with .NET Standard 2.0 run anywhere that .NET Standard 2.0 is supported.

ASP.NET Core 2.x is supported on .NET Framework versions compatible with .NET Standard 2.0:

  • .NET Framework 4.7.1 and later is strongly recommended.
  • .NET Framework 4.6.1 and later.

ASP.NET Core 3.0 and later will only run on .NET Core. For more details regarding this change, see A first look at changes coming in ASP.NET Core 3.0.

There are several advantages to targeting .NET Core, and these advantages increase with each release. Some advantages of .NET Core over .NET Framework include:

  • Cross-platform. Runs on macOS, Linux, and Windows.
  • Improved performance
  • Side-by-side versioning
  • New APIs
  • Open source

We're working hard to close the API gap from .NET Framework to .NET Core. The Windows Compatibility Pack made thousands of Windows-only APIs available in .NET Core. These APIs weren't available in .NET Core 1.x.

How to download a sample

Many of the articles and tutorials include links to sample code.

  1. Download the ASP.NET repository zip file.
  2. Unzip the Docs-master.zip file.
  3. Use the URL in the sample link to help you navigate to the sample directory.

Preprocessor directives in sample code

To demonstrate multiple scenarios, sample apps use the #define and #if-#else/#elif-#endif C# statements to selectively compile and run different sections of sample code. For those samples that make use of this approach, set the #define statement at the top of the C# files to the symbol associated with the scenario that you want to run. Some samples require setting the symbol at the top of multiple files in order to run a scenario.

For example, the following #define symbol list indicates that four scenarios are available (one scenario per symbol). The current sample configuration runs the TemplateCode scenario:

#define TemplateCode // or LogFromMain or ExpandDefault or FilterInCode

To change the sample to run the ExpandDefault scenario, define the ExpandDefault symbol and leave the remaining symbols commented-out:

#define ExpandDefault // TemplateCode or LogFromMain or FilterInCode

For more information on using C# preprocessor directives to selectively compile sections of code, see #define (C# Reference) and #if (C# Reference).

Regions in sample code

Some sample apps contain sections of code surrounded by #region and #end-region C# statements. The documentation build system injects these regions into the rendered documentation topics.

Region names usually contain the word "snippet." The following example shows a region named snippet_FilterInCode:

#region snippet_FilterInCode
WebHost.CreateDefaultBuilder(args)
    .UseStartup<Startup>()
    .ConfigureLogging(logging =>
        logging.AddFilter("System", LogLevel.Debug)
            .AddFilter<DebugLoggerProvider>("Microsoft", LogLevel.Trace))
            .Build();
#endregion

The preceding C# code snippet is referenced in the topic's markdown file with the following line:

[!code-csharp[](sample/SampleApp/Program.cs?name=snippet_FilterInCode)]

You may safely ignore (or remove) the #region and #end-region statements that surround the code. Don't alter the code within these statements if you plan to run the sample scenarios described in the topic. Feel free to alter the code when experimenting with other scenarios.

For more information, see Contribute to the ASP.NET documentation: Code snippets.

Next steps

For more information, see the following resources:

Note

We’re testing the usability of a proposed new structure for the ASP.NET Core table of contents. If you have a few minutes to try an exercise of finding 7 different topics in the current or proposed table of contents, please click here to participate in the study.