.NET 9 Preview 7

[jamesmontemagno]

We are pleased to announce the release of .NET 9 Preview 7.

This release contains the following improvements:

📚Libraries

• Removal of BinaryFormatter is complete
• Enumerate over ReadOnlySpan<char>.Split() segments
• Debug.Assert now reports assert condition, by default.
• Compression APIs now use zlib-ng
• Guid.CreateVersion7 enables creating GUIDs with a natural sort order
• Interlocked.CompareExchange for more types
• AES-GCM and ChaChaPoly1305 algorithms enabled for iOS/tvOS/MacCatalyst
• Changes to X.509 Certificate Loading
• Support for XPS documents from XPS virtual printer
• Marking Tensor<T> as Experimental
• Full release notes

⏱️Runtime

• ARM64 SVE Support
• Post-Indexed Addressing on ARM64
• Strength Reduction in Loops
• Object Stack Allocation for Boxes
• GC Dynamic Adaptation To Application Sizes
• Full release notes

🛠️ SDK

• Container publishing improvements for insecure registries
• More consistent environment variables for container publishing
• Introduction of Workload Sets for more control over workloads
• Mitigating analyzer mismatch issues aka 'torn SDK'

• Full release notes

C#

• Prioritize better overloads with OverloadResolutionPriority attribute
• Full release notes

🌐 ASP.NET Core

• SignalR supports trimming and Native AOT
• Microsoft.AspNetCore.OpenApi supports trimming and Native AOT
• Improvements to transformer registration APIs in Microsoft.AspNetCore.OpenApi
• Call ProducesProblem and ProducesValidationProblem on route groups
• Construct Problem and ValidationProblem result types with IEnumerable<KeyValuePair<string, object?>> values
• OpenIdConnectHandler support for Pushed Authorization Requests (PAR)
• Data Protection support for deleting keys
• Customize Kestrel named pipe endpoints
• Improved Kestrel connection metrics
• Opt-out of HTTP metrics on certain endpoints and requests
• ExceptionHandlerMiddleware option to choose the status code based on the exception
• Full release notes

📱 .NET MAUI

• Introduction of HybridWebview
• New TitleBar Control and Window.TitleBar for Windows
• CollectionView & CarouselView improvements with a new opt-in handler for iOS and Mac Catalyst
• Ability to bring a Window to the foregrond with ActivateWindow
• BackButtonBehavior OneWay binding mode
• BlazorWebView backward compatibility host address
• Native Embedding improvements
• MainPage is Obsolete
• New Handler Disconnect Policy
• New ProcessTerminated event on WebView Control
• New lifecycle methods for remote notifications on iOS & Mac Catalyst
• Xcode Sync for CLI and Visual Studio Code

• Full release notes

Get started

To get started with .NET 9, install the .NET 9 SDK.

If you're on Windows using Visual Studio, we recommend installing the latest Visual Studio 2022 preview. .NET 9 is now can be installed directly through the Visual Studio installer starting with Visual Studio 2022 17.12 Preview 1.

You can also use Visual Studio Code and the C# Dev Kit extension with .NET 9.

You can find .NET 9 Preview 7 Installer & Binaries on GitHub.

Team Announcements & Discussions

Discuss this release with the product teams on GitHub through the announcements for this release.

• .NET MAUI
• ASP.NET Core
• Libraries & Runtime
• Source-build

Stay up-to-date with .NET 9

You can stay up-to-date with all the features of .NET 9 with:

• What's new in .NET 9
• What's new in ASP.NET Core
• What's new in .NET MAUI
• What's new in EF Core
• Breaking Changes in .NET 9
• .NET 9 Releases

For .NET 9 previews we are utilizing GitHub Discussions to make announcements. For more information read our discussion.

Stay up to date by subscribing to the RSS feed for all release announcements.

[OwnageIsMagic]

Prioritize better overloads with OverloadResolutionPriority attribute link broken

[jamesmontemagno]

Thanks, fixed https://github.com/dotnet/core/blob/main/release-notes/9.0/preview/preview7/csharp.md#prioritize-better-overloads-with-overloadresolutionpriority-attribute

[wstaelens]

THANK YOU @edwardneal ! 😄 🥳 #xps

[Firuz2006]

Go go go!

[fMichaleczek]

dotnet workload install maui fails with :

Workload installation failed: One or more errors occurred. (Version 17.5.9232-net9-p7 of package microsoft.ios.sdk.net9.0_17.5.msi.x64 is not found in NuGet feeds

@rolfbjarne Sorry to bother you, but it looks like Preview 3 and 4.
#9318 (comment)

[jamesmontemagno]

@davidortinau as well

[dalexsoto]

Do you happen to have this feed in your nuget sources?

<add key="dotnet9" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet9/nuget/v3/index.json" />

If so please remove it and try again, you can confirm this with dotnet nuget list source

[rolfbjarne]

@fMichaleczek can you do:

dotnet --info
``

and

dotnet worklod install maui -v:diagnostic

and attach the output here?

[fMichaleczek]

@rolfbjarne
I installed the Preview 7 SDK, updated VS to the latest preview, then it was impossible to go out the error loop. (with repair, etc..)
After removing the Preview 7 SDK and Keep the VS Preview 7 SDK, this was not resolving my problem, so I remove also C:\Program Files\dotnet\sdk-manifests\9.0.100-preview.7 and force a repair.
It's now working.
It seems a conflict problem between VS SDK and SDK.

[nathan130200]

I'm mad to use C# extension types asap 👁👄👁

[tivik7]

I am getting an exception when using the preview 7 SDK in a WPF project using the new fluent.xaml. The pasted lines (line number 17-19 in the code) throws an exception:

        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="pack://application:,,,/PresentationFramework.Fluent;component/Resources/Fluent.xaml" />
        </ResourceDictionary.MergedDictionaries>

Set property 'System.Windows.ResourceDictionary.Source' threw an exception.' Line number '82' and line position '18'.

with the inner exception:

IOException: Cannot locate resource 'resources/fluent.xaml'.

Workloads, Visual Studio Preview, NuGets and dotnet preview 7 all installed without errors

[jamesmontemagno]

@singhashish-wpf ^

[jeffhandley]

I created an issue for this, @singhashish-wpf: dotnet/wpf#9580

[pchaurasia14]

@tivik7 @jeffhandley - Thank you for pointing this out. I've fixed the documentation as well.

[IanMarteens]

Neither semi-automatic properties, nor extensions, it seems. Am I wrong?

[KennethHoff]

Nor anything in regards to collection expressions or primary constructors, both of which were left partially unfinished in 8 🫠

params collections, while certainly an improvement, is effectively a non-feature now that we have collection expressions, and it's probably the biggest feature coming in C# 13. There are a few ref struct improvements, but those are barely used in application code.

[jamesmontemagno]

There was a roll up blog last month with an update in and around C# 13: https://devblogs.microsoft.com/dotnet/csharp-13-explore-preview-features/

These release notes are just specific to this release.

[KennethHoff]

Considering neither params collection nor any ref struct improvements were referenced in these release notes - and those were the things I mentioned by name - it's fairly obvious that we've already read those

[jamesmontemagno]

If you are looking for status and discussion of features check https://github.com/dotnet/roslyn/blob/main/docs/Language%20Feature%20Status.md

[KennethHoff]

This is out of date though, isn't it? ref struct interfaces are currently planned for C# 13.

There was a brief period where they were considered delayed/dropped, but not anymore

[Firuz2006]

Will dotnet 9 be able to dotnet new winui?

[sirinath]

Looking for the performance improvement blog post.

[richlander]

You won't have to wait long.

[david-nyaoso]

After upgrade to dotnet 9 preview 7, can not deploy Maui App targeting dotnet 9 and building for Windows. This means that code which depended on Application Identity, for example 'Windows.ApplicationModel.Package.Current' throws an exception. Is this by design?

[mattleibow]

Is this an existing app or is it a new one from the template?

[david-nyaoso]

Both new from template and existing app behave the same.

[mattleibow]

The new app template was switched to unpackaged by default for preview 7, so that is expected. However, for an existing app, this should be unchanged. The "switch" was just a msbuild property in the csproj in the template, so it should have no way of affecting your existing app.

Please open a new issue in dotnet/maui with a sample project that used to work in net8.0 but now does not in net9.

[david-nyaoso]

https://github.com/david-nyaoso/maui-win-package-bug

[BeepBeepBopBop]

Hello, after updating a .NET MAUI app from .net8 to the newest version (.net9 preview 7), I get the following error message when attempting to compile the Windows app:

Improper project configuration: no AppxManifest is specified, but WindowsPackageType is not set to $(WindowsPackageType).

I have not found any solution out there and I have absolutely no idea how to move forward with this, any help would be greatly appreciated! Thanks.

[nathan130200]

Add compiler preprocessor define to detect if current working platform is big or little endian. #if PLATFORM_LITTLE_ENDIAN and #if PLATFORM_BIG_ENDIAN so we can compile code directly for big/little endian instead of using BitConverter.IsLittleEndian at runtime.

[KennethHoff]

I don't think this would work; The platform you compile on, and the platform you run the code on isn't necessarily the same thing.

[martincostello]

I think the JIT will optimise the branches away anyway?

[amanasifkhalid]

Correct, the JIT should optimize away conditionals that can be evaluated at compile-time. Consider the following example:

public static void Main()
{
    if (BitConverter.IsLittleEndian)
    {
        Console.WriteLine("IsLittleEndian");
    }
    else
    {
        Console.WriteLine("IsBigEndian");
    }
}

When optimizing, the JIT produces the following linear code:

G_M000_IG01:                ;; offset=0x0000
       sub      rsp, 40

G_M000_IG02:                ;; offset=0x0004
       mov      rcx, 0x28A800068B0
       call     [System.Console:WriteLine(System.String)]
       nop

G_M000_IG03:                ;; offset=0x0015
       add      rsp, 40
       ret

[baronfel]

Also in general we've gone away from compiler defines where possible because they cause rebuilds - in general .NET compilations can be done in platform independent ways, and then tools/tech like the trimmer and linker specialize that platform-independent binary for the target platform's characteristics.

[nathan130200]

Oh that's right, I hadn't remembered that the compiler would optimize these calls at compile-time.

[HugoVG]

Is there any plans for Microsoft.AspNetCore.OpenAPI to support XML doc comments like the previous swagger implementation did? The attributes don't feel that intuitive to use, and some things like adding comments to why/when it returns the response type/code cannot be mapped yet. Is there any plans to add that in the future?

I rather have

/// <summary>
/// An experimental endpoint to showcase some OpenAPI problem
/// </summary>
/// <remarks>This also shows in Swagger UI</remarks>
/// <param name="experiment">Experimental message</param>
/// <returns>A Hello world message</returns>
/// <response code="200">Everything is okay :) </response>
/// <response code="400">Something broke</response>
[HttpPost("Experiment")]
[ProducesResponseType(StatusCodes.Status200OK)]
[ProducesResponseType(StatusCodes.Status400BadRequest)]
public ActionResult<string> Test(string experiment)

over

[HttpPost("Experiment")]
[EndpointSummary("This sadly doesn't feel intuitive")]
[EndpointDescription("This sadly doesn't feel intuitive")]
[Produces(typeof(string))]
[ProducesErrorResponseType(typeof(ProblemDetails))]
[ProducesResponseType(StatusCodes.Status200OK)]
[ProducesResponseType(StatusCodes.Status400BadRequest)]
public ActionResult<string> Test([Description("Experimental message")]string experiment)

[martincostello]

See dotnet/aspnetcore#39927 (comment).