Announcing .NET 7 Preview 7

Jeremy Likness

Today we released .NET 7 Preview 7. This is the last preview for .NET 7 and the next version will be our first release candidate (RC). The dates for .NET Conf 2022 have been announced! Join us November 8-10, 2022 to celebrate the .NET 7 release!

Visual Studio 2022 17.3 also released today with GA support for .NET Multi-platform App UI (MAUI). Read the .NET MAUI announcement and tune into .NET Conf: Focus on MAUI that is live streaming now!

This preview of .NET 7 includes improvements to System.LINQ, Unix file permissions, low-level structs, p/Invoke source generation, code generation, and websockets.

You can download .NET 7 Preview 7, for Windows, macOS, and Linux.

.NET 7 Preview 7 has been tested with Visual Studio 17.4 Preview 1. We recommend you use the preview channel builds if you want to try .NET 7 with Visual Studio family products. If you’re on macOS, we recommend using the latest Visual Studio 2022 for Mac preview. Now, let’s get into some of the latest updates in this release.

Simplified ordering with System.LINQ


System.Linq now has the methods Order and OrderDescending, which are there to order an IEnumerable according to T.

IQueryable also supports this now.

Note: This change does not introduce a new language feature to System.Linq.Expressions.


Previously, you had to call OrderBy/OrderByDescending by referencing the own value.

var data = new[] { 2, 1, 3 };
var sorted = data.OrderBy(static e => e);
var sortedDesc = data.OrderByDescending(static e => e);

Now, you can write:

var data = new[] { 2, 1, 3 };
var sorted = data.Order();
var sortedDesc = data.OrderByDescending();

Support for Unix file modes

dotnet/runtime PR#69980

Previously, .NET had no built-in support for getting and setting Unix file permissions, which control which users can read, write, and execute files and directories. P/Invoking manually to syscalls isn’t always easy because some are exposed differently on different distros. For example, on Ubuntu you may have to pinvoke to __xstat, on RedHat to stat, and so on. This makes a first-class .NET API important.

In Preview 7, we introduced a new enum:

public enum UnixFileMode
    OtherExecute, OtherWrite, OtherRead,
    GroupExecute, GroupWrite, GroupRead,
    UserExecute, UserWrite, UserRead,

and APIs File.GetUnixFileMode and File.SetUnixFileMode that get and set the file mode on either a path or a handle (file descriptors). As well as a new property on FileInfo and DirectoryInfo named UnixFileMode.

There is also a new overload of Directory.CreateDirectory and a new property on FileStreamOptions to allow you to create a directory or file with a particular mode in one shot. Note that when you use these, umask is still applied, as it would if you created the directory or file in your shell.


// Create a new directory with specific permissions
Directory.CreateDirectory("myDirectory", UnixFileMode.UserRead | UnixFileMode.UserWrite | UnixFileMode.UserExecute);

// Create a new file with specific permissions
FileStreamOptions options = new()
    Access = FileAccess.Write,
    Mode = FileMode.Create,
    UnixCreateMode =  UnixFileMode.UserRead | UnixFileMode.UserWrite,
using FileStream myFile = new FileStream("myFile", options);

// Get the mode of an existing file
UnixFileMode mode = File.GetUnixFileMode("myFile");

// Set the mode of an existing file
File.SetUnixFileMode("myFile", UnixFileMode.UserRead | UnixFileMode.UserWrite | UnixFileMode.UserExecute);

See all new Unix File Mode APIs.

A big thank you goes out to @tmds, a long-term contributor from Red Hat, who proposed, designed, and implemented this feature.

Low-level struct improvements: ref field support

The .NET 7 runtimes now have full support for ref fields within ByRefLike types (that is, ref struct). There was extensive language design behind this much requested feature that users can read about: low-level struct improvements. With this feature, types previously requiring specialized handling in the runtimes (for example, Span<T> and ReadOnlySpan<T>), can now be fully implemented in C#.

LibraryImport P/Invoke source generator


The LibraryImport source generator is now available in a supported manner to all users. The culmination of more than 18 months this source generator is designed to be a drop-in replacement for the majority of DllImport uses, both in the runtime product and in user code. The .NET libraries have all adopted LibraryImport and have been shipping with source generated marshalling code since .NET 7 Preview 1.

The source generator ships with the .NET 7 TFM and is readily available for consumption. In order to get the benefit of the source generated marshalling, replace usages of DllImport with LibraryImport. There are Analyzers and Fixers that can assist with this process.



public static class Native
    [DllImport(nameof(Native), CharSet = CharSet.Unicode)]
    public extern static string ToLower(string str);


public static class Native
    [LibraryImport(nameof(Native), StringMarshalling = StringMarshalling.Utf16)]
    public static partial string ToLower(string str);

There is an analyzer and code-fix to automatically convert your DllImport attributes to LibraryImport. For Preview 7, it is opt-in. Add dotnet_diagnostic.SYSLIB1054.severity = suggestion to your EditorConfig file to enable the conversion analyzer as a diagnostic.

Design documentation and details on marshalling custom types can be found under docs/design/libraries/LibraryImportGenerator.

ClientWebSocket upgrade response details


ClientWebSocket previously did not provide any details about upgrade response. However, the information about response headers and status code might be important in both failure and success scenarios.

In case of failure, the status code can help to distinguish between retriable and non-retriable errors (server doesn’t support web sockets at all vs. just a tiny transient error). Headers might also contain additional information on how to handle the situation.

The headers are also helpful even in case of a successful web socket connect, e.g., they can contain a token tied to a session, some info related to the subprotocol version, or the server can go down soon, etc.


ClientWebSocket ws = new();
ws.Options.CollectHttpResponseDetails = true;
    await ws.ConnectAsync(uri, default);
    // success scenario
    ws.HttpResponseHeaders = null; // clean up (if needed)
catch (WebSocketException)
    // failure scenario
    if (ws.HttpStatusCode != null)
        ProcessFailure(ws.HttpStatusCode, ws.HttpResponseHeaders);

Improvements in CodeGen

Many thanks to JIT community contributors for these community PRs!

Loop Optimizations

In preview 7, we made several improvements on Loop Optimizations.

  • PR #71184 strengthens checking of the loop table for better loop integrity checks as described in #71084.
  • PR #71868 Do not compact blocks around loops
  • PR #71659 Adjust weights of blocks with profile inside loops in non-profiled methods
  • PR #71504 Improvements to loop hoisting
  • PR #70271 optimized multi-dimensional array access. It improved the latency by up to 67% (Performance Link).

In addition, Hot/Cold splitting is enabled for Exception Handling funclets in PR #71236.

Contributor spotlight: Hugh Bellamy

A huge “Thank you” goes out to all our community members. We deeply appreciate your thoughtful contributions. We asked contributor @hughbe to share his thoughts.

Image bellamy jpg

In Hugh’s own words:

The first application I ever developed was as a young teen. Although “Hughser”, a C# wrapping the WebBrowser component, unfortunately did not prove to win the browser wars, .NET was my first window into the world of software development. After a few years developing apps for iOS, I regained interest in .NET when roslyn, corefx and coreclr were open-sourced. I was excited by the opportunity to look inside the black box and see how the SDK I used actually worked. My first PR in November 2015 was to clean up and add tests for various methods on System.String. (Anyone who has taken a look at my contribution history is well aware that cleaning up, improving and adding tests became something of a specialty of mine!). I soon gained confidence contributing and became one of the most active contributors to .NET open source. I’ve now contributed to a variety of .NET Foundation projects, including corefx/coreclr/runtime, roslyn, xunit, mono, wpf and winforms. A highlight of my time contributing was definitely when I was flown out to Seattle for the Microsoft Build Conference 2019 by Microsoft to meet the team and get a shoutout at the .NET Keynote! I’ve also been awarded three consecutive Microsoft MVP Awards for my contributions, which I’m very proud of.

In the past, my contributions to .NET helped me secure an internship as a Junior Developer as a teenager. I currently work as an Management Consultant specializing in Financial Services Data & Analytics. Although I have less time nowadays than I did at school and university, I still follow the .NET open source ecosystem keenly and maintain several .NET open source projects on GitHub under the username @hughbe. Coding and open source has always been a hobby of mine, but also something I’ve enjoyed integrating with my work. . As a Politics undergrad, I’m interested in how technology can be used in a political and social context. For example, as an intern at an NGO in Cambodia, I developed a Facebook scraping platform in .NET, parts of which I open sourced. The platform was used to provide insights into public political engagement to journalists, opposition parties and civil society. At the moment, I’m developing a prototype platform leveraging .NET to scrape public media sources, including Telegram, to assist in the gathering of Open Source Intelligence (OSINT) relating to the Ukrainian War.

Contributing to .NET has been an incredible experience – as a teenager I was remotely working with people who I would never have otherwise crossed paths with, on an almost daily basis. The team at Microsoft, as well as the rest of the open source community, are extremely accommodating, insightful and happy to be challenged. There are so many ways to get started, be it submitting or reviewing issues, documentation and PRs, so get started today!

Targeting .NET 7

To target .NET 7, you need to use a .NET 7 Target Framework Moniker (TFM) in your project file. For example:


The full set of .NET 7 TFMs, including operating-specific ones follows.

  • net7.0
  • net7.0-android
  • net7.0-ios
  • net7.0-maccatalyst
  • net7.0-macos
  • net7.0-tvos
  • net7.0-windows

We expect that upgrading from .NET 6 to .NET 7 should be straightforward. Please report any breaking changes that you discover in the process of testing existing apps with .NET 7.


.NET 7 is a Standard Term Support (STS) release, meaning it will receive free support and patches for 18 months from the release date. It’s important to note that the quality of all releases is the same. The only difference is the length of support. For more about .NET support policies, see the .NET and .NET Core official support policy.

We recently changed the “Current” name to “Standard Term Support (STS)”. We’re in the process of rolling out that change.

Breaking changes

Trimming and NativeAOT: All assemblies trimmed by default

To better align with user expectations and produce smaller and more efficient apps, trimming now trims all assemblies in console apps by default. This change only affects apps that are published with PublishTrimmed=true, and it only affects apps that had existing trim warnings. It also only affects plain .NET apps that don’t use the Windows Desktop, Android, iOS, WASM, or ASP.NET SDK.

Previous behavior

Previously, only assemblies that were opted-in with <IsTrimmable>true</IsTrimmable> in the library project file were trimmed.

New behavior

Starting in .NET 7, trimming trims all the assemblies in the app by default. Apps that may have previously worked with PublishTrimmed may not work in .NET 7. However, only apps with trim warnings will be affected. If your app has no trim warnings, the change in behavior should not cause any adverse affects, and will likely decrease the app size.

For example, an app that uses Newtonsoft.Json or System.Text.Json without source generation to serialize and deserialize a type in the user project may have functioned before the change, because types in the user project were fully preserved. However, one or more trim warnings (warning codes ILxxxx) would have been present. Now, types in the user project are trimmed, and serialization may fail or produce unexpected results.

If your app did have trim warnings, you may see changes in behavior or exceptions. For example, an app that uses Newtonsoft.Json or System.Text.Json without source generation to serialize and deserialize a type in the user project may have functioned before the change, because types in the user project were fully preserved. However, one or more trim warnings (warning codes ILxxxx) would have been present. Now, types in the user project are trimmed, and serialization may fail or produce unexpected results.

The best resolution is to resolve all the trim warnings in your application. To revert to the previous behavior, set the TrimMode property to partial, which is the pre-.NET 7 behavior.


The default .NET 7+ value is full:


Other breaking changes

You can find the most recent list of breaking changes in .NET 7 by reading the Breaking changes in .NET 7 document. It lists breaking changes by area and release with links to detailed explanations.

To see what breaking changes are proposed but still under review, follow the Proposed .NET Breaking Changes GitHub issue.


Releases of .NET include products, libraries, runtime, and tooling, and represent a collaboration across multiple teams inside and outside Microsoft. You can learn more about these areas by reading the product roadmaps:


We appreciate and thank you for your all your support and contributions to .NET. Please give .NET 7 Preview 7 a try and tell us what you think!


Discussion is closed. Login to edit/delete existing comments.

  • Marko Stupar 0

    .net 7 preview 7

    System.Security.VerificationException: ‘Method System.Text.RegularExpressions.Symbolic.SymbolicRegexMatcher`1+FullNullabilityHandler[TSet].System.Text.RegularExpressions.Symbolic.SymbolicRegexMatcher.INullabilityHandler.IsNullableAt: type argument ‘TStateHandler’ violates the constraint of type parameter ‘TStateHandler’.’

    .net 7 preview 6

    Method System.Text.RegularExpressions.Symbolic.SymbolicRegexMatcher`1+FullNullabilityHandler[TSet].IsNullableAt: type argument ‘TStateHandler’ violates the constraint of type parameter ‘TStateHandler’.

    .net 7 preview 5

    • Stephen Toub - MSFTMicrosoft employee 0

      Can you share a repro?

      • Marko Stupar 0


        Type x = Type {FullName = “System.Text.RegularExpressions.Symbolic.SymbolicRegexMatcher`1+FullNullabilityHandler”, GenericTypeParameters{TSet}, GenericTypeArguments{}, IsGenericTypeDefinition = true};

        Type x2 = Type {Name = “INullabilityHandler”, FullName = null, DeclaringType = Type{FullName = “System.Text.RegularExpressions.Symbolic.SymbolicRegexMatcher`1”, IsGenericTypeDefinition = true} }


        • Marko Stupar 0


          using System.Reflection;
          namespace net7
              internal static class Program
                  static void Main()
                      Assembly a = Assembly.Load("System.Text.RegularExpressions");
                      Type x = a.GetType("System.Text.RegularExpressions.Symbolic.SymbolicRegexMatcher`1+FullNullabilityHandler");
                      Type x2 = x.GetInterface("INullabilityHandler");
          • Stephen Toub - MSFTMicrosoft employee 0

            Thanks. Exception aside, why are you using private reflection to access these internal implementation details?

          • Marko Stupar 0

            framework recursive reflection pass and test, when it comes out new.

            comparing with previous versions, is it growing. (in natural speaking “pushing every button”, “small code recursively pushes every button”)

            i haven’t expected that there would be any error, and first noticed it in preview 6. i thought it is normal because it is preview and it would be fixed in next version. in preview 7 here it is again, should i post about it ? and i posted.

            when error occurred i was looking where the recursion went it. because of that i needed time to understand, by debugging, what it operated on and generate realCode.

  • Beau Gosse 0

    Does the breaking change “Trimming and NativeAOT: All assemblies trimmed by default” only apply to projects with PublishAot true ?

    • Michal StrehovskyMicrosoft employee 0

      It applies to both PublishTrimmed and PublishAot (PublishAot just turns on PublishTrimmed implicitly and it’s not possible to turn it off).

  • Holger Schwichtenberg ( 0

    The sample show in the section “Support for Unix file modes” does not compile with the .NET 7 Preview 7 SDK.
    The enum UnixFileMode is not in System.IO. The methods File.GetUnixFileMode() and File.SetUnixFileMode() are not found.
    Which extra package is needed for this?

  • Holger Schwichtenberg ( 0

    This sample does not compile in .NET 7 Preview 7 SDK:

    public static class Native
        [LibraryImport(nameof(Native), StringMarshalling = StringMarshalling.Utf16)]
        public static partial string ToLower(string str);

    Error: SYSLIB1050 Method ‘ToLower’ is contained in a type ‘Native’ that is not marked ‘partial’. P/Invoke source generation will ignore method ‘ToLower’.

    Shouldn’t your sample be like that?

    public static partial class Native
        [LibraryImport(nameof(Native), StringMarshalling = StringMarshalling.Utf16)]
        public static partial string ToLower(string str);
  • Blake Mitchell 0

    What is LibraryImport and how does it differ from DllImport? The example shows basically no difference and the description doesn’t say. Where is the documentation for it? Also, partial is a C# language feature and is not in F#, so does LibraryImport support F#?

    • Kirsan 0

      +1 about LibraryImport above DllImport benefits.

    • Yegor Stepanov 0

      As I understand, DllImportGenerator(experimental compile-time DllImport) has been renamed to LibraryImport.

      It eliminates generating stubs on the first invoke and doesn’t use Reflection.

      F# doesn’t support it because F# doesn’t support SourceGenerators yet.

    • Michael Taylor 0

      That feature is talked about at the top of the article under LibraryImport P/Invoke source generator. The gist of it is that it is used as an indicator for source generators. Today if you use DllImport then nothing happens until runtime outside the compiler flagging the method as external (I believe). When the method is actually called there is overhead as the attribute values are translated into the actual call (e.g. should we search for an exact name or do a wildcard search, ANSI or Unicode, etc). This slows the call down.

      Source generators are a newer feature of C# and allow the compiler to auto-generate code on the fly. They are becoming heavily used in boilerplate code like implementing INotifyPropertyChanged and logging. You should read up on them if you’re interested. But for a source generator to work it generally needs some sort of indicator to look for, attributes are common. Hence the LibraryImport is an indicator that tells the source generator to generate, at compile time, the necessary code to make the native call at runtime. This increases your code size a little bit but puts a lot of the heavy lifting into the compilation process so the runtime call is fast. Rather than picking the attribute apart at runtime it does so at compile time. The net effect is faster runtime code in many cases.

      They could have used the existing DllImport attribute (hence why they work similar) but then the compiler would be auto-generating code that you might not wanted (and might break things if a particular scenario isn’t working in the generator). So a new attribute avoids problems. In theory you could find all references to DllImport and convert them and then verify your code still works. The compiler just won’t do it automatically.

    • Steve 0

      A DllImport will be compiled to an IL stub, and when the first time a DllImport method is called, the runtime generates code for marshalling data. This works fine if code does not get trimmed. But if trimming is enabled, some types for marshalling will be trimmed away because the compiler does not know whether a type will be referenced by the marshalling code which is generated at the runtime.
      LibraryImport is a source generator that produces all marshalling code at compile time, so types won’t be trimmed by the compiler accidentally.

      • Jeremy KoritzinskyMicrosoft employee 0

        As others have mentioned, LibraryImport is an attribute that a built-in source generator recognizes to generate interop code at compile-time instead of at runtime. This provides a few benefits:

        • The marshalling code is generated at compile-time, so it can be seen with Go-to-Definition in Visual Studio. Additionally, it can be stepped into, unlike marshalling code generated when using DllImport.
        • The marshalling code versions with the TFM you compile against, not the TFM you run against. This makes it easier to know what your code will do and limits our concerns around breaking changes as any new changes in behavior will be part of a new target framework version that users will have to explicitly opt into by compiling against it.

        Additionally, the flexibility of the new model enables us to both add support for newer .NET types in interop and allowing developers like yourself to write your own interop support for either your types or built-in types with specific behaviors for your scenarios without having to allocate or box or having limitations around value types. For example, LibraryImport-based P/Invokes support Span<T> and ReadOnlySpan<T> parameters and return values similar to arrays. Additionally, our new model also enabled us to easily support marshalling jagged arrays, which is not supported in the DllImport system.

  • Jonathan Vella 0

    The below paragraph is written twice in this section.

    For example, an app that uses Newtonsoft.Json or System.Text.Json without source generation to serialize and deserialize a type in the user project may have functioned before the change, because types in the user project were fully preserved. However, one or more trim warnings (warning codes ILxxxx) would have been present. Now, types in the user project are trimmed, and serialization may fail or produce unexpected results.

  • William Liu 2

    There could be a type error in the demo of simplified-ordering-with-system-linq

    The new code might be OrderDescending.

    var data = new[] { 2, 1, 3 };
    var sorted = data.Order();
    var sortedDesc = data.OrderDescending();
  • Ion Robu 0


    MaxInteger[T](System.Collections.Generic.IEnumerable`1[T])' violates the constraint of type parameter 'T' exception

    Automapper issue, I used myget version (11.0.2) and it worked. Is a workaround until issue will be solved with a new nuget package.

  • Rolf Kristensen 1

    Is there a way to mark an assembly, that it should not be trimmed?

    Will the NET7 linker recognize this assembly-attribute:

    [AssemblyMetadata(“IsTrimmable”, “False”)]

  • Rolf Kristensen 0

    Think .NET7 should perform full-trim by default, but when detecting assemblies with warnings, then it should only perform partial triming of the assembly, unless the assembly is explictly marked


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