Announcing .NET 5 Preview 1

Scott Hunter [MSFT]


At the end of last year, we shipped .NET Core 3.0 and 3.1. These versions added the desktop app models Windows Forms (WinForms) and WPF, ASP.NET Blazor for building single page applications and gRPC for cross-platform, contract-based messaging. We also added templates for building services, rich generation of client code for talking to gRPC, REST API services, and a lot more. We’re excited to see that .NET Core 3 has become the fastest adopted version of .NET ever and we’ve gained another million more users in just the last year.

We also communicated with these releases that this would conclude the porting of the app models from .NET Framework. With .NET Core 3, we have ported all of the most used app models as well as introduced newer cross- platform frameworks to replace the ones we did not port.

As we look forward to the next major release, .NET 5, we will continue to unify .NET into a single platform by including our .NET mobile device app model (Xamarin) in .NET 5. .NET 5 will include ASP.NET Core, Entity Framework Core, WinForms, WPF, Xamarin and ML.NET. For the first time, the entire platform will use a unified BCL (Base Class Libraries) for all the app models. Having a version 5 that is higher than both .NET Core and .NET Framework also makes it clear that .NET 5 is the future of .NET, which is a single unified platform for building any type of application.

We have said this many times, but we will reiterate again; .NET Core and then .NET 5 is the .NET you should build all your NEW applications with. .NET Framework will remain supported as long as Windows itself is supported. We will continue to provide security and bug fixes and keep the networking and crypto API’s up to date. It will remain safe and supported to keep your older applications on .NET Framework.

Install .NET 5.0 Preview 1

Today we are releasing the first preview of .NET 5, which is scheduled to GA (General Availability) later this year in November.

See ASP.NET Core updates in .NET 5 Preview 1 to learn about today’s ASP.NET Core release.

Preview 1 includes support for Windows ARM64 for the first time. Today’s release includes the .NET Core runtime. We expect Preview 2 to include the SDK (ASP.NET Core but not WPF or Windows Forms). A later preview will include WPF and Windows Forms. Support for Windows ARM64 will also be back-ported to .NET Core 3.1. We will share more information on that with the Preview 2 post.

Updating existing projects

You can update existing projects by updating your target framework, as follows:


High-level goals for .NET 5

Let me highlight some of the high-level goals for .NET 5:

  • Unified .NET SDK experience:
    • Single BCL (Base Class Library) across all .NET 5 applications. Today Xamarin applications use the Mono BCL but will move to use the.NET Core BCL, improving compatibility across our application models.
    • Mobile development (Xamarin) is integrated into .NET 5. This means the .NET SDK will support mobile. For example, you can use “dotnet new XamarinForms” to create a mobile application.
  • Native Applications supporting multiple platforms: Single Device project that supports an application that can work across multiple devices for example Window Desktop, Microsoft Duo (Android), and iOS using the native controls supported on those platforms.
  • Web Applications supporting multiple platforms: Single Blazor project that supports an application that can work in browsers, on mobile devices and as a native desktop application (Windows 10x for example)
  • Cloud Native Applications: High performance, single file (.exe) <50MB microservices and support for building multiple project (API, web front ends, containers) both locally and in the cloud.
  • Continuous Improvements, such as: faster algorithms in the BCL, better support for containers in the runtime, support for HTTP3.

Today’s first preview does not contain all the work to support these high-level goals yet, but we will continue to announce more capabilities and features in future previews.

Improvements in Preview 1

The following improvements are in Preview 1:

Regular expression performance improvements

We’ve invested in significant improvements to the Regex engine. On many of the expressions we’ve tried, these improvements routinely result in throughput improvements of 3-6x, and in some cases, much more. We have a blog post coming shortly that will describe these improvements in much more detail.

Code quality improvements in RyuJIT

Every release includes a set of performance improvements to the code that the JIT generates. We refer to these type of improvements as “CQ” or code quality. In most cases, these improvements also apply to the code generated for ready-to-run images.

The following improvements are in preview 1:

Assembly load diagnostics added to event pipe

We have added assembly load information to event pipe. This improvement is the start of making similar diagnostics functionality available as is part of .NET Framework with the Fusion Log Viewer. You can now use dotnet-trace to collect this information, using the following command:

dotnet-trace collect --providers Microsoft-Windows-DotNETRuntime:4:4 --process-id [process ID]

The workflow is described in Trace Assembly Loading with Event Pipe. You can see assembly loading information for a simple test app.

Image trace assemblies loads

Event pipe profiler APIs

Event pipe is a new subsystem and API that we added in .NET Core 2.2 to make it possible to perform performance and other diagnostic investigations on any operating system. In .NET 5.0, the event pipe has been extended to enable profilers to write event pipe events. This scenario is critical for instrumenting profilers that previously relied on ETW to monitor application behavior and performance.

GitHub repo consolidation

As part of the .NET 5 release, we reduced the number of GitHub repos we use to build and package .NET. Repo boundaries have a significant impact on many aspects of a project, including builds and issue management. With .NET Core 1.0, we had over 100 repos across ASP.NET, EF and .NET Core. With this latest release, we can now count the primary repos on one hand. We also moved nearly all repos to the dotnet org.

Check out the new, consolidated, repos:


We hope that you are excited about the work that is happening with .NET 5! The best way to prepare for .NET 5 is to move all of our .NET Core applications to 3.1- we will make the transition from .NET Core 3.1 to .NET 5 as painless as possible. And if you are still building applications on .NET Framework, please feel safe leaving those applications on .NET Framework but think of using .NET Core 3.1 for all your new applications. There are lots of exciting things coming to .NET!


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  • Avatar
    Niall Ginsbourg

    Lot of folks on Twitter asking – can you please clarify the high-level goal :
    “Native Applications supporting multiple platforms: Single Device project that supports an application that can work across multiple devices for example Window Desktop, Microsoft Duo (Android), and iOS using the native controls supported on those platforms.”

    Are you talking about Xamarin (Forms?) + .NET 5? UWP + .NET 5? or is there something brand new on the horizon?

  • Avatar
    Алексей Сморкалов

    Thanks for the announcement. Can you share when Windows Forms Designer for .Net5 Apps will be available to work with custom & 3rd party user controls?

    • Phillip Carter
      Phillip CarterMicrosoft employee

      At this time, there aren’t plans to move Visual Studio to .NET 5. Visual Studio is an immense codebase with many components that take a hard dependency on .NET Framework APIs that aren’t compatible with .NET Core. A .NET 5 version of Visual Studio would force a lot of components (and extensions!) to be unsupported, leaving a lot of customers without the tools that they know and love.

      That being said, in VS 2019 there was an effort to move components to .NET Standard 2.0 and/or .NET Framework 4.7 so that we could begin to do things like share some components between Visual Studio and Visual Studio for Mac. There may be future work more aligned with similar goals.

  • 남정현

    Great news! I’m happy hear about that. ☺️

    I have a question about the high level goals about compilation.

    Cloud Native Applications: High performance, single file (.exe) <50MB microservices and support for building multiple project (API, web front ends, containers) both locally and in the cloud.

    Does this paragraph points out to CoreRT?

  • Avatar
    Piercarlo Schiavo - BIsolution

    Hello Scott,
    I updated my project (Blazor Client + Web Api Server Side) to Core 5.0.
    But I have following problem with DataContext.cs:

    public DataContext(DbContextOptions<DataContext> options) : base(options)

    On instancing on above constructor point I get the following error: cannot load “Microsoft.Bcl.AsyncInterfaces”

    I tried to install the package but it is following with a lot of errors more.

    I needed to downgrade the all to work.

    Some suggestion ?

    Thank you

    • Avatar
      Jos Krause

      Hi Piercarlo,

      From what I understood is that IAsyncEnumerable is contained within the ‘Microsoft.Bcl.AsyncInterfaces’ package for .NET Standard 2.0 scenarios, but is included in ‘System.Runtime’ starting with .NET Standard 2.1. Is it possbile that some of your current project dependencies are targetting a lower framework thus pulling back in this dependency? Perhaps some packages require an update so they can target the newer framework? (#thoughts-out-loud)

      • Avatar
        Piercarlo Schiavo - BIsolution

        Dear Jos,
        I investigated on your suggestion but I think the problem is in somewhere else:

        I updated to Framework 5.0 as follow:


        I updated one per one the all existing packages and all packages are working fine except for following packages:


        Last working version: 3.1.3

        The project is a Blazor Webassembly + WebApi Servers Side

        The problem coming on EntityFramewok and related packages.

        Some suggestions?

  • Daniel Hughes
    Daniel Hughes

    Will dotnet new XamarinForms work on Linux with vscode? the dotnet core tooling has supported Linux from the start, but Xamarin has been Windows and Mac only. With no vscode support.

  • Alexander Baggett
    Alexander Baggett

    I feel the naming is a bit confusing.

    Since .NET 4.5 meant .NET framework.

    Now .NET 5.0 means all of .NETcore, ASP.NET Core, Xamarin, etc, but not .NET framework.

  • Avatar
    Stuart Ballard

    Presuming there’s still no plan to support ASP.NET Web Forms on anything other than .NET Framework, have you considered making the Web Forms codebase open source so that third party developers could try porting it? I’m maintaining a large Web Forms application, and while it’s nice to know that there will be continued security and bugfix support, it’s frustrating to be locked out of all new features, especially now that new versions of the C# language itself are tied to the new platform versions as well!

    • Avatar
      Richard LanderMicrosoft employee

      Correct. ASP.NET Web Forms will not be supported on .NET 5+. We actually tried this a few years ago. It became obvious that it was going to be a challenging project. This is primarily because Web Forms depends on a bunch of APIs not in .NET 5, and to a large degree that we cannot add back. Many APIs were removed from .NET Core because they created really bad dependencies that turned the product back into a monolith. In the case of instance methods, they can be added back as extension methods. Static methods, properties and interfaces cannot be.

      I believe this is the source:

      • Avatar
        Stuart Ballard

        Last time I’d searched I wasn’t able to find the Web Forms code in the reference source – the github history seems to suggest those files were missing originally and only got added to the repo in August. Glad to see they’re available now!

      • Avatar
        Jon Miller

        One other thing worth noting. There are often breaking changes between version. You could have moved Web Forms forward and left out the functionality that wasn’t available on .NET Core. It would have been better than dropping it altogether. Instead, you would rather force developers onto a less functional stack. Blazor should include UI controls by default. Things like a grid control that supports sorting, filtering, paging, inline editing, etc. Application developers shouldn’t have to roll their own on that stuff. It should be there out of the box. Things should be getting easier. It’s been 15 years. MVC was a step backwards. Web development has not improved for at least that long. Things are getting worse, not better. Also, had Microsoft actually developed controls like a grid component, maybe it would uncover issues in the underlying design that need to be addressed. The for-for pay Blazor controls that I’ve seen aren’t as good as they are in Web Forms. And also, in this day and age, that should just be built into the platform.

  • Avatar
    Vitaliy Korney

    Hello Scott! Great news, but I want to complain regarding xamarin team. Miguel de Icaza told a year ago on twitter that Catalyst feature will be implemented and now managers at xamarin denies that they commited to implement it and this feature is not only implemented to comply to ios 13, but nobody at xamarin even cares about it. Miguel’s tweet was hidden from public recently. It’s an obvious feature compared to surface duo android feature. Developers want to publish their ipad apps to mac appstore. Seems like you have poor management at xamarin since they do not respond to people and say in twitter that we don’t need this feature saying they collect feedback. React native already implemented this feature. Can you make them at least to provide answer at github regarding plans or whatever? How come that they do features to support surface duo but completely deny catalyst feature?
    Should we demand xamarin to make them comply to xcode 11?

    • Avatar
      David OrtinauMicrosoft employee

      Hi Vitaliy,

      We absolutely care and would love to deliver support for Catalyst at the right time. That is determined by understanding value to customers and balancing that with other impactful priorities. We continue to listen and observe to determine when that time might be. At this time we are not working on enabling Catalyst support. I would be happy to discuss this more in depth with you and learn about your business needs.

      The best path to macOS for .NET developers today is using Xamarin.Mac and Xamarin.Forms to maximize code share.

      We support Surface Duo and Surface Neo because they are amazing, category changing first-party devices! Not only that, but they run Android and Windows which are already supported in .NET. In fact, Xamarin.Forms was able to support them without making any core changes. What better value than enabling .NET developers to target Microsoft’s flagship devices!

      • Avatar
        Vitaliy Korney

        Hi, David! I don’t believe in marketing only time will show if those devices are competitive or not. I don’t feel excitement when another android vendor appears especially when I have to write additional code to support it. Catalyst was exciting to developers.
        I’ve made a research and it will take half a year to port my app from ios to xamarin.mac. while it may look the same it’s a completely different platform, they have similar controls, similar classes, similar events, but even enums are different. Listening means understanding what customers wants. I don’t want to spend half a year doing monkey job porting app to another platform I would prefer spending few weeks making sure my app will work fine both on ipad and mac. I’m pretty sure I won’t be able to implement my app using xamarin forms. Anyway it will take another half a year. What you suggest is to invest in another half a year of development. And I’m just one customer. You offer spending money to all customers on development, because you have different priorities. Maybe it’s another mistake made by me to invest in dead platform like silverlight, winphone, windows store app. But I hope it was not a mistake. A friend of mine has diagramming app on ipad (grapholite) and he also don’t want to spend money on mac development, since it’s a lot of work with ui. He does not like Xamarin Studio and was happy when Catalyst was announced 2 years ago. I have database app for uwp, android and ios. And I keep getting requests of a mac app.

      • Avatar
        Vitaliy Korney

        This is what your people say at github: “it’s the top request from the new iOS 13 features, so it will be prioritized accordingly.” It is completely different from what you say here. Maybe you take too much responsibility by freezing this feature, don’t you? Seems like your developers’ point of view resonates with my point of view and with others who want this feature. I’m writing here since I haven’t lost hope for this feature yet and I got your point of view, which contradicts with me. I wanted to escalate this problem since today our dialog is not constructive at all. You don’t make it and wait. What should happen for this feature to be planned in the next iteration, who make decisions? Best time to implement this feature is 3 month ago. Maybe you should hire developers if you have deficit of the workforce.

        • Avatar

          Sorry Vitaliy, but you didn’t use email with your follow up comments to David as requested. So instead of your pleas being discarded and ignored via email, they will be subsequently discarded and ignored here instead.