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This article describes what is new in .NET Core 3.0. One of the biggest enhancements is support for Windows desktop applications (Windows only). By using the .NET Core 3.0 SDK component Windows Desktop, you can port your Windows Forms and Windows Presentation Foundation (WPF) applications. To be clear, the Windows Desktop component is only supported and included on Windows. For more information, see the Windows desktop section later in this article.

.NET Core 3.0 adds support for C# 8.0. It's highly recommended that you use Visual Studio 2019 version 16.3 or newer, Visual Studio for Mac 8.3 or newer, or Visual Studio Code with the latest C# extension.

Download and get started with .NET Core 3.0 right now on Windows, macOS, or Linux.

For more information about the release, see the .NET Core 3.0 announcement.

.NET Core RC1 was considered production ready by Microsoft and was fully supported. If you're using a preview release, you must move to the RTM version for continued support.

Language improvements C# 8.0

C# 8.0 is also part of this release, which includes the nullable reference types feature, async streams, and more patterns. For more information about C# 8.0 features, see What's new in C# 8.0.

Language enhancements were added to support the following API features detailed below:

.NET Standard 2.1

.NET Core 3.0 implements .NET Standard 2.1. However, the default template generates a project that still targets .NET Standard 2.0. To target .NET Standard 2.1, edit your project file and change the property to :

If you're using Visual Studio, you need Visual Studio 2019, as Visual Studio 2017 doesn't support .NET Standard 2.1 or .NET Core 3.0.

Compile/Deploy

Default executables

.NET Core now builds framework-dependent executables by default. This behavior is new for applications that use a globally installed version of .NET Core. Previously, only self-contained deployments would produce an executable.

During or , an executable (known as the appHost) is created that matches the environment and platform of the SDK you're using. You can expect the same things with these executables as you would other native executables, such as:

• You can double-click on the executable.
• You can launch the application from a command prompt directly, such as on Windows, and on Linux and macOS.

macOS appHost and notarization

macOS only

Starting with the notarized .NET Core SDK 3.0 for macOS, the setting to produce a default executable (known as the appHost) is disabled by default. For more information, see macOS Catalina Notarization and the impact on .NET Core downloads and projects.

When the appHost setting is enabled, .NET Core generates a native Mach-O executable when you build or publish. Your app runs in the context of the appHost when it is run from source code with the command, or by starting the Mach-O executable directly.

Without the appHost, the only way a user can start a framework-dependent app is with the command. An appHost is always created when you publish your app self-contained.

You can either configure the appHost at the project level, or toggle the appHost for a specific command with the parameter:

• Project file

• Command-line parameter

For more information about the setting, see MSBuild properties for Microsoft.NET.Sdk.

Single-file executables

The command supports packaging your app into a platform-specific single-file executable. The executable is self-extracting and contains all dependencies (including native) that are required to run your app. When the app is first run, the application is extracted to a directory based on the app name and build identifier. Startup is faster when the application is run again. The application doesn't need to extract itself a second time unless a new version was used.

To publish a single-file executable, set the in your project or on the command line with the command:

-or-

For more information about single-file publishing, see the single-file bundler design document.

Assembly linking

The .NET core 3.0 SDK comes with a tool that can reduce the size of apps by analyzing IL and trimming unused assemblies.

Self-contained apps include everything needed to run your code, without requiring .NET to be installed on the host computer. However, many times the app only requires a small subset of the framework to function, and other unused libraries could be removed.

.NET Core now includes a setting that will use the IL linker tool to scan the IL of your app. This tool detects what code is required, and then trims unused libraries. This tool can significantly reduce the deployment size of some apps.

To enable this tool, add the setting in your project and publish a self-contained app:

As an example, the basic "hello world" new console project template that is included, when published, hits about 70 MB in size. By using , that size is reduced to about 30 MB.

It's important to consider that applications or frameworks (including ASP.NET Core and WPF) that use reflection or related dynamic features, will often break when trimmed. This breakage occurs because the linker doesn't know about this dynamic behavior and can't determine which framework types are required for reflection. The IL Linker tool can be configured to be aware of this scenario.

Above all else, be sure to test your app after trimming.

For more information about the IL Linker tool, see the documentation or visit the mono/linker repo.

Tiered compilation

Tiered compilation (TC) is on by default with .NET Core 3.0. This feature enables the runtime to more adaptively use the just-in-time (JIT) compiler to achieve better performance.

The main benefit of tiered compilation is to provide two ways of jitting methods: in a lower-quality-but-faster tier or a higher-quality-but-slower tier. The quality refers to how well the method is optimized. TC helps to improve the performance of an application as it goes through various stages of execution, from startup through steady state. When tiered compilation is disabled, every method is compiled in a single way that's biased to steady-state performance over startup performance.

When TC is enabled, the following behavior applies for method compilation when an app starts up:

• If the method has ahead-of-time-compiled code, or ReadyToRun, the pregenerated code is used.
• Otherwise, the method is jitted. Typically, these methods are generics over value types.
  • Quick JIT produces lower-quality (or less optimized) code more quickly. In .NET Core 3.0, Quick JIT is enabled by default for methods that don't contain loops and is preferred during startup.
  • The fully optimizing JIT produces higher-quality (or more optimized) code more slowly. For methods where Quick JIT would not be used (for example, if the method is attributed with MethodImplOptions.AggressiveOptimization), the fully optimizing JIT is used.

For frequently called methods, the just-in-time compiler eventually creates fully optimized code in the background. The optimized code then replaces the pre-compiled code for that method.

Code generated by Quick JIT may run slower, allocate more memory, or use more stack space. If there are issues, you can disabled Quick JIT using this MSBuild property in the project file:

To disable TC completely, use this MSBuild property in your project file:

For more information about configuring compilation at run time, see Run-time configuration options for compilation.

ReadyToRun images

You can improve the startup time of your .NET Core application by compiling your application assemblies as ReadyToRun (R2R) format. R2R is a form of ahead-of-time (AOT) compilation.

R2R binaries improve startup performance by reducing the amount of work the just-in-time (JIT) compiler needs to do as your application loads. The binaries contain similar native code compared to what the JIT would produce. However, R2R binaries are larger because they contain both intermediate language (IL) code, which is still needed for some scenarios, and the native version of the same code. R2R is only available when you publish a self-contained app that targets specific runtime environments (RID) such as Linux x64 or Windows x64.

To compile your project as ReadyToRun, do the following:

1. Add the setting to your project:

2. Publish a self-contained app. For example, this command creates a self-contained app for the 64-bit version of Windows:

Cross platform/architecture restrictions

The ReadyToRun compiler doesn't currently support cross-targeting. You must compile on a given target. For example, if you want R2R images for Windows x64, you need to run the publish command on that environment.

Exceptions to cross-targeting:

• Windows x64 can be used to compile Windows ARM32, ARM64, and x86 images.
• Windows x86 can be used to compile Windows ARM32 images.
• Linux x64 can be used to compile Linux ARM32 and ARM64 images.

Runtime/SDK

Major-version runtime roll forward

.NET Core 3.0 introduces an opt-in feature that allows your app to roll forward to the latest major version of .NET Core. Additionally, a new setting has been added to control how roll forward is applied to your app. This can be configured in the following ways:

• Project file property:
• Run-time configuration file property:
• Environment variable:
• Command-line argument:

One of the following values must be specified. If the setting is omitted, Minor is the default.

• LatestPatch
  Roll forward to the highest patch version. This disables minor version roll forward.
• Minor
  Roll forward to the lowest higher minor version, if requested minor version is missing. If the requested minor version is present, then the LatestPatch policy is used.
• Major
  Roll forward to lowest higher major version, and lowest minor version, if requested major version is missing. If the requested major version is present, then the Minor policy is used.
• LatestMinor
  Roll forward to highest minor version, even if requested minor version is present. Intended for component hosting scenarios.
• LatestMajor
  Roll forward to highest major and highest minor version, even if requested major is present. Intended for component hosting scenarios.
• Disable
  Don't roll forward. Only bind to specified version. This policy isn't recommended for general use because it disables the ability to roll forward to the latest patches. This value is only recommended for testing.

Besides the Disable setting, all settings will use the highest available patch version.

By default, if the requested version (as specified in for the application) is a release version, only release versions are considered for roll forward. Any pre-release versions are ignored. If there is no matching release version, then pre-release versions are taken into account. This behavior can be changed by setting , in which case all versions are always considered.

Build copies dependencies

The command now copies NuGet dependencies for your application from the NuGet cache to the build output folder. Previously, dependencies were only copied as part of .

There are some operations, like linking and razor page publishing that will still require publishing.

Local tools

.NET Core 3.0 introduces local tools. Local tools are similar to global tools but are associated with a particular location on disk. Local tools aren't available globally and are distributed as NuGet packages.

Local tools rely on a manifest file name in your current directory. This manifest file defines the tools to be available at that folder and below. You can distribute the manifest file with your code to ensure that anyone who works with your code can restore and use the same tools.

For both global and local tools, a compatible version of the runtime is required. Many tools currently on NuGet.org target .NET Core Runtime 2.1. To install these tools globally or locally, you would still need to install the NET Core 2.1 Runtime.

New global.json options

The global.json file has new options that provide more flexibility when you're trying to define which version of the .NET Core SDK is used. The new options are:

• : Indicates whether the SDK resolver should consider prerelease versions when selecting the SDK version to use.
• : Indicates the roll-forward policy to use when selecting an SDK version, either as a fallback when a specific SDK version is missing or as a directive to use a higher version.

For more information about the changes including default values, supported values, and new matching rules, see global.json overview.

Smaller Garbage Collection heap sizes

The Garbage Collector's default heap size has been reduced resulting in .NET Core using less memory. This change better aligns with the generation 0 allocation budget with modern processor cache sizes.

Garbage Collection Large Page support

Large Pages (also known as Huge Pages on Linux) is a feature where the operating system is able to establish memory regions larger than the native page size (often 4K) to improve performance of the application requesting these large pages.

The Garbage Collector can now be configured with the GCLargePages setting as an opt-in feature to choose to allocate large pages on Windows.

Windows Desktop & COM

.NET Core SDK Windows Installer

The MSI installer for Windows has changed starting with .NET Core 3.0. The SDK installers will now upgrade SDK feature-band releases in place. Feature bands are defined in the hundreds groups in the patch section of the version number. For example, 3.0.101 and 3.0.201 are versions in two different feature bands while 3.0.101 and 3.0.199 are in the same feature band. And, when .NET Core SDK 3.0.101 is installed, .NET Core SDK 3.0.100 will be removed from the machine if it exists. When .NET Core SDK 3.0.200 is installed on the same machine, .NET Core SDK 3.0.101 won't be removed.

For more information about versioning, see Overview of how .NET Core is versioned.

Windows desktop

.NET Core 3.0 supports Windows desktop applications using Windows Presentation Foundation (WPF) and Windows Forms. These frameworks also support using modern controls and Fluent styling from the Windows UI XAML Library (WinUI) via XAML islands.

The Windows Desktop component is part of the Windows .NET Core 3.0 SDK.

You can create a new WPF or Windows Forms app with the following commands:

Visual Studio 2019 adds New Project templates for .NET Core 3.0 Windows Forms and WPF.

For more information about how to port an existing .NET Framework application, see Port WPF projects and Port Windows Forms projects.

WinForms high DPI

.NET Core Windows Forms applications can set high DPI mode with Application.SetHighDpiMode(HighDpiMode). The method sets the corresponding high DPI mode unless the setting has been set by other means like or P/Invoke before .

The possible values, as expressed by the System.Windows.Forms.HighDpiMode enum are:

For more information about high DPI modes, see High DPI Desktop Application Development on Windows.

Create COM components

On Windows, you can now create COM-callable managed components. This capability is critical to use .NET Core with COM add-in models and also to provide parity with .NET Framework.

Unlike .NET Framework where the mscoree.dll was used as the COM server, .NET Core will add a native launcher dll to the bin directory when you build your COM component.

For an example of how to create a COM component and consume it, see the COM Demo.

Windows Native Interop

Windows offers a rich native API in the form of flat C APIs, COM, and WinRT. While .NET Core supports P/Invoke, .NET Core 3.0 adds the ability to CoCreate COM APIs and Activate WinRT APIs. For a code example, see the Excel Demo.

MSIX Deployment

MSIX is a new Windows application package format. It can be used to deploy .NET Core 3.0 desktop applications to Windows 10.

The Windows Application Packaging Project, available in Visual Studio 2019, allows you to create MSIX packages with self-contained .NET Core applications.

The .NET Core project file must specify the supported runtimes in the property:

Linux improvements

SerialPort for Linux

.NET Core 3.0 provides basic support for System.IO.Ports.SerialPort on Linux.

Previously, .NET Core only supported using on Windows.

For more information about the limited support for the serial port on Linux, see GitHub issue #33146.

Docker and cgroup memory Limits

Running .NET Core 3.0 on Linux with Docker works better with cgroup memory limits. Running a Docker container with memory limits, such as with , changes how .NET Core behaves.

• Default Garbage Collector (GC) heap size: maximum of 20 mb or 75% of the memory limit on the container.
• Explicit size can be set as an absolute number or percentage of cgroup limit.
• Minimum reserved segment size per GC heap is 16 mb. This size reduces the number of heaps that are created on machines.

GPIO Support for Raspberry Pi

Two packages have been released to NuGet that you can use for GPIO programming:

The GPIO packages include APIs for GPIO, SPI, I2C, and PWM devices. The IoT bindings package includes device bindings. For more information, see the devices GitHub repo.

ARM64 Linux support

.NET Core 3.0 adds support for ARM64 for Linux. The primary use case for ARM64 is currently with IoT scenarios. For more information, see .NET Core ARM64 Status.

Docker images for .NET Core on ARM64 are available for Alpine, Debian, and Ubuntu.

Security

TLS 1.3 & OpenSSL 1.1.1 on Linux

.NET Core now takes advantage of TLS 1.3 support in OpenSSL 1.1.1, when it's available in a given environment. With TLS 1.3:

• Connection times are improved with reduced round trips required between the client and server.
• Improved security because of the removal of various obsolete and insecure cryptographic algorithms.

When available, .NET Core 3.0 uses OpenSSL 1.1.1, OpenSSL 1.1.0, or OpenSSL 1.0.2 on a Linux system. When OpenSSL 1.1.1 is available, both System.Net.Security.SslStream and System.Net.Http.HttpClient types will use TLS 1.3 (assuming both the client and server support TLS 1.3).

The following C# 8.0 example demonstrates .NET Core 3.0 on Ubuntu 18.10 connecting to https://www.cloudflare.com:

Cryptography ciphers

.NET 3.0 adds support for AES-GCM and AES-CCM ciphers, implemented with System.Security.Cryptography.AesGcm and System.Security.Cryptography.AesCcm respectively. These algorithms are both Authenticated Encryption with Association Data (AEAD) algorithms.

The following code demonstrates using cipher to encrypt and decrypt random data.

Cryptographic Key Import/Export

.NET Core 3.0 supports the import and export of asymmetric public and private keys from standard formats. You don't need to use an X.509 certificate.

All key types, such as RSA, DSA, ECDsa, and ECDiffieHellman, support the following formats:

• Public Key

  • X.509 SubjectPublicKeyInfo

• Private key

  • PKCS#8 PrivateKeyInfo
  • PKCS#8 EncryptedPrivateKeyInfo

RSA keys also support:

• Public Key

• Private key

The export methods produce DER-encoded binary data, and the import methods expect the same. If a key is stored in the text-friendly PEM format, the caller will need to base64-decode the content before calling an import method.

PKCS#8 files can be inspected with System.Security.Cryptography.Pkcs.Pkcs8PrivateKeyInfo and PFX/PKCS#12 files can be inspected with System.Security.Cryptography.Pkcs.Pkcs12Info. PFX/PKCS#12 files can be manipulated with System.Security.Cryptography.Pkcs.Pkcs12Builder.

.NET Core 3.0 API changes

Ranges and indices

The new System.Index type can be used for indexing. You can create one from an that counts from the beginning, or with a prefix operator (C#) that counts from the end:

There's also the System.Range type, which consists of two values, one for the start and one for the end, and can be written with a range expression (C#). You can then index with a , which produces a slice:

For more information, see the ranges and indices tutorial.

Async streams

The IAsyncEnumerable<T> type is a new asynchronous version of IEnumerable<T>. The language lets you over to consume their elements, and use to them to produce elements.

The following example demonstrates both production and consumption of async streams. The statement is async and itself uses to produce an async stream for callers. This pattern (using ) is the recommended model for producing async streams.

In addition to being able to , you can also create async iterators, for example, an iterator that returns an that you can both and in. For objects that need to be disposed, you can use , which various BCL types implement, such as and .

For more information, see the async streams tutorial.

IEEE Floating-point

Floating point APIs are being updated to comply with IEEE 754-2008 revision. The goal of these changes is to expose all required operations and ensure that they're behaviorally compliant with the IEEE spec. For more information about floating-point improvements, see the Floating-Point Parsing and Formatting improvements in .NET Core 3.0 blog post.

Parsing and formatting fixes include:

• Correctly parse and round inputs of any length.
• Correctly parse and format negative zero.
• Correctly parse and by doing a case-insensitive check and allowing an optional preceding where applicable.

New System.Math APIs include:

• BitIncrement(Double) and BitDecrement(Double)
  Corresponds to the and IEEE operations. They return the smallest floating-point number that compares greater or lesser than the input (respectively). For example, would return .

• MaxMagnitude(Double, Double) and MinMagnitude(Double, Double)
  Corresponds to the and IEEE operations, they return the value that is greater or lesser in magnitude of the two inputs (respectively). For example, would return .

• ILogB(Double)
  Corresponds to the IEEE operation that returns an integral value, it returns the integral base-2 log of the input parameter. This method is effectively the same as , but done with minimal rounding error.

• ScaleB(Double, Int32)
  Corresponds to the IEEE operation that takes an integral value, it returns effectively , but is done with minimal rounding error.

• Log2(Double)
  Corresponds to the IEEE operation, it returns the base-2 logarithm. It minimizes rounding error.

• FusedMultiplyAdd(Double, Double, Double)
  Corresponds to the IEEE operation, it performs a fused multiply add. That is, it does as a single operation, thereby minimizing the rounding error. An example is , which returns . The regular returns .

• CopySign(Double, Double)
  Corresponds to the IEEE operation, it returns the value of , but with the sign of .

.NET Platform-Dependent Intrinsics

APIs have been added that allow access to certain perf-oriented CPU instructions, such as the SIMD or Bit Manipulation instruction sets. These instructions can help achieve significant performance improvements in certain scenarios, such as processing data efficiently in parallel.

Where appropriate, the .NET libraries have begun using these instructions to improve performance.

For more information, see .NET Platform-Dependent Intrinsics.

Improved .NET Core Version APIs

Starting with .NET Core 3.0, the version APIs provided with .NET Core now return the information you expect. For example:

Fast built-in JSON support

.NET users have largely relied on Newtonsoft.Json and other popular JSON libraries, which continue to be good choices. uses .NET strings as its base datatype, which is UTF-16 under the hood.

The new built-in JSON support is high-performance, low allocation, and works with UTF-8 encoded JSON text. For more information about the System.Text.Json namespace and types, see the following articles:

HTTP/2 support

The System.Net.Http.HttpClient type supports the HTTP/2 protocol. If HTTP/2 is enabled, the HTTP protocol version is negotiated via TLS/ALPN, and HTTP/2 is used if the server elects to use it.

The default protocol remains HTTP/1.1, but HTTP/2 can be enabled in two different ways. First, you can set the HTTP request message to use HTTP/2:

Second, you can change HttpClient to use HTTP/2 by default:

Many times when you're developing an application, you want to use an unencrypted connection. If you know the target endpoint will be using HTTP/2, you can turn on unencrypted connections for HTTP/2. You can turn it on by setting the environment variable to or by enabling it in the app context:

Next steps