.NET 5 Preview 4


The following improvements are new in Preview 4 and not otherwise covered in the earlier highlights section.

F# 5

Building on the F# 5 preview released earlier this year, the update to F# 5 includes support for consuming Default Interface Methods (DIMs) and some big performance improvements. Here’s a sneak peek at the DIMs support in F#:

open CSharp

// You can implement the interface via a class
type MyType() =
    member _.M() = ()

    interface MyDim

let md = MyType() :> MyDim
printfn "DIM from C#: %d" md.Z

// You can also implement it via an object expression
let md' = { new MyDim }
printfn "DIM from C# but via Object Expression: %d" md'.Z

For more information, you can read the F# 5 update for .NET 5 preview 4.

C# Source Generators Update

An update to the C# Source Generators preview is included in this iteration.

  • Besides some bug fixes, it includes support for passing an analyzerconfig, wich is essentially a list of key-value pairs, to a Source Generator.
  • This lets your source generators work differently based on the input they receive.
  • For example, you may want to generate source code differently if a consuming project targets .NET Framework vs. .NET 5.
  • Using an analyzerconfig allows you to pass information like a consuming project's TFM to allow for exactly this scenario.

Support for ICU on Windows

  • The ICU library provides Unicode and Globalization support for applications on Linux and macOS.
  • This library can be used now on Windows, and this change makes the behavior of globalization APIs such as culture-specific string comparison consistent between Windows 10 and other operating systems.

Support for cgroup v2 (for containers)

.NET now has support for cgroup v2, which we expect will become an important container-related API in 2020 and beyond.

  • Docker currently uses cgroup v1 which is already supported by .NET.
  • In comparison, cgroup v2 is simpler, more efficient, and more secure than cgroup v1.
  • You can learn more about cgroup and Docker resource limits from our 2019 Docker update.
  • Linux distros and container runtimes are in the process of adding support for cgroup v2.
  • .NET 5.0 will work correctly in cgroup v2 environments once they become more common.

Reducing the Size of Container Images

Everyone is always looking for opportunities to make .NET container images smaller and easier to use.

  • Preview 4, dramatically reduces the size of the aggregate images you pull in multi-stage-build scenarios which is a very common pattern.
  • We re-based the SDK image on top of the ASP.NET image instead of buildpack-deps.

This change has the following win for multi-stage builds.

Multi-stage build costs with Ubuntu 20.04 Focal:

Pull Image Before After
sdk:5.0-focal 268 MB 232 MB
aspnet:5.0-focal 64 MB 10 KB (manifest only)

Net download savings: 100 MB (-30%)

Multi-stage build costs with Debian 10 Buster:

Pull Image Before After
sdk:5.0 280 MB 218 MB
aspnet:5.0 84 MB 4 KB (manifest only)

Net download savings: 146 MB (-40%)

.NET 5.0 will switch to the dotnet container repo

As part of the move to .NET as the product name, first, they publish .NET 5.0 Preview 4 and later images to the mcr.microsoft.com/dotnet family of repos, instead of mcr.microsoft.com/dotnet/core.

  • Make sure to update your FROM statements and scripts accordingly.
  • .NET Core 3.1 and 2.1 will continue to be published to mcr.microsoft.com/dotnet/core.