Announcing .NET 7 Preview 2 – The New, ‘New’ Experience

Today, we are glad to release .NET 7 Preview 2. The second preview of .NET 7 includes enhancements to RegEx source generators, progress moving NativeAOT from experimental status into the runtime, and a major set of improvements to the “dotnet new” CLI experience. The bits are available for you to grab right now and start experimenting with new features like:

Build a specialized RegEx pattern matching engine using source generators at compile-time rather than slower methods at runtime.
Take advantage of SDK improvements that provide an entirely new, streamlined tab completion experience to explore templates and parameters when running dotnet new.
Don’t trim your excitement, just your apps in preparation to try out NativeAOT with your own innovative solutions.

EF7 preview 2 was also released and is available on NuGet. You can also read what’s new in ASP.NET Core Preview 2.

You can download .NET 7 Preview 2, for Windows, macOS, and Linux.

Installers and binaries
Container images
Linux packages
Release notes
Known issues
GitHub issue tracker

We recommend you use the preview channel builds if you want to try .NET 7 with Visual Studio family products. Visual Studio for Mac support for .NET 7 previews isn’t available yet but is coming soon.

Preview 2

The following features are now available in the Preview 2 release.

Introducing the new Regex Source Generator

Have you ever wished you had all of the great benefits that come from having a specialized Regex engine that is optimized for your particular pattern, without the overhead of building this engine at runtime?

We are excited to announce the new Regex Source Generator which was included in Preview 1. It brings all of the performance benefits from our compiled engine without the startup cost, and it has additional benefits, like providing a great debugging experience as well as being trimming-friendly. If your pattern is known at compile-time, then the new regex source generator is the way to go.

In order to start using it, you only need to turn the containing type into a partial one, and declare a new partial method with the RegexGenerator attribute that will return the optimized Regex object, and that’s it! The source generator will fill the implementation of that method for you, and will get updated automatically as you make changes to your pattern or to the additional options that you pass in. Here is an example:


public class Foo
public Regex regex = new Regex(@”abc|def”, RegexOptions.IgnoreCase);

public bool Bar(string input)
bool isMatch = regex.IsMatch(input);
// ..


public partial class Foo // <– Make the class a partial class
[RegexGenerator(@”abc|def”, RegexOptions.IgnoreCase)] // <– Add the RegexGenerator attribute and pass in your pattern and options
public static partial Regex MyRegex(); // <– Declare the partial method, which will be implemented by the source generator

public bool Bar(string input)
bool isMatch = MyRegex().IsMatch(input); // <– Use the generated engine by invoking the partial method.
// ..

And that’s it. Please try it out and let us know if you have any feedback.


Community PRs (Many thanks to JIT community contributors!!)

From @sandreenko

[Jit] Delete Statement::m_compilerAdded . runtime#64506

From @SingleAccretion

Delete GT_DYN_BLK runtime#63026
Address-expose locals under complex local addresses in block morphing runtime#63100
Refactor optimizing morph for commutative operations runtime#63251
Preserve OBJ/BLK on the RHS of ASG runtime#63268
Handle embedded assignments in copy propagation runtime#63447
Do not set GTF_NO_CSE for sources of block copies runtime#63462
Exception sets: debug checker & fixes runtime#63539
Stop using CLS_VAR for “boxed statics” runtime#63845
Tune floating-point CSEs live across a call better runtime#63903
Reverse ASG(CLS_VAR, …) runtime#63957
Fix invalid threading of nodes in rationalization runtime#64012
Add the exception set for ObjGetType runtime#64106
Improve fgValueNumberBlockAssignment runtime#64110
Commutative morph optimizations runtime#64122
Fix unique VNs for ADDRs runtime#64230
Copy propagation tweaking runtime#64378
Introduce GenTreeDebugOperKind runtime#64498
Add support for TYP_BYREF LCL_FLDs to VN runtime#64501
Do not add NRE sets for non-null addresses runtime#64607
Take into account zero-offset field sequences when propagating locals runtime#64701
Another size estimate fix for movs runtime#64826
Mark promoted SIMD locals used by HWIs as DNER runtime#64855
Propagate exception sets for assignments runtime#64882
Account for HWI stores in LIR side effects code runtime#65079

From @Wraith2

Recognize BLSR in “x & (x-1)” Add blsr runtime#63545
Add xarch andn runtime#64350

Dynamic PGO

JIT: simple forward substitution pass runtime#63720


Local heap optimizations on Arm64 runtime#64481
Couple optimization to MultiRegStoreLoc runtime#64857
Implement LoadPairVector64 and LoadPairVector128 runtime#64864
‘cmeq’ and ‘fcmeq’ Vector64.Zero/Vector128.Zero ARM64 containment optimizations runtime#62933
Increase arm32/arm64 maximum instruction group size runtime#65153
Prefer “mov reg, wzr” over “mov reg, #0” runtime#64740
Biggen GC Gen0 for Apple M1 Fix LLC cache issue on Apple M1 runtime#64576
Optimize full memory barriers around volatile reads/writes ARM64: Avoid LEA for volatile IND runtime#64354
Optimize Math.Round(x, MidpointRounding.AwayFromZero/ToEven) runtime#64016

General Optimizations

Security: Add JIT support for control-flow guard on x64 and arm64 runtime#63763
Testing: Spmi replay asmdiffs mac os arm64 runtime#64119


Support Metrics UpDownCounter instrument runtime#63648

static Meter s_meter = new Meter(“MyLibrary.Queues”, “1.0.0”);
static UpDownCounter<int> s_queueSize = s_meter.CreateUpDownCounter<int>(“Queue-Size”);
static ObservableUpDownCounter<int> s_pullQueueSize = s_meter.CreateObservableUpDownCounter<int>(“Queue-Size”, () => s_pullQueueSize);


Logging source generator improvements

Logging source generator should gracefully fail when special parameters incorrectly get passed as template parameter runtime#64310
Logging Source Generator fails to compile when in parameter modifier is present runtime#62644
Logging Source Generator fails to compile using keyword parameters with @ prefixes runtime#60968
Logging Source Generator fails ungracefully with overloaded methods runtime#61814
Logging Source Generator fails to compile due to CS0246 and CS0265 errors if type for generic constraint is in a different namespace runtime#58550

SDK Improvements

[Epic] New CLI parser + tab completion #2191

For 7.0.100-preview2, the dotnet new command has been given a more consistent and intuitive interface for many of the subcommands that users already use. In addition, support for tab completion of template options and arguments has been massively updated, now giving rapid feedback on valid arguments and options as the user types.

Here’s the new help output as an example:

❯ dotnet new –help
Template Instantiation Commands for .NET CLI.

dotnet new [<template-short-name> [<template-args>…]] [options]
dotnet new [command] [options]

<template-short-name> A short name of the template to create.
<template-args> Template specific options to use.

-?, -h, –help Show command line help.

install <package> Installs a template package.
uninstall <package> Uninstalls a template package.
update Checks the currently installed template packages for update, and install the updates.
search <template-name> Searches for the templates on
list <template-name> Lists templates containing the specified template name. If no name is specified, lists all templates.

New Command Names

Specifically, all of the commands in this help output no longer have the — prefix that they do today. This is more in line with what users expect from subcommands in a CLI application. The old versions (–install, etc) are still available to prevent breaking user scripts, but we hope to add obsoletion warnings to those commands in the future to encourage migration.

Tab Completion

The dotnet CLI has supported tab completion for quite a while on popular shells like PowerShell, bash, zsh, and fish (for instructions on how to enable that, see How to enable TAB completion for the .NET CLI). It’s up to individual dotnet commands to implement meaningful completions, however. For .NET 7, the new command learned how to provide tab completion for

Available template names (in dotnet new <template-short-name>)

❯ dotnet new angular
angular grpc razor viewstart worker -h
blazorserver mstest razorclasslib web wpf /?
blazorwasm mvc razorcomponent webapi wpfcustomcontrollib /h
classlib nugetconfig react webapp wpflib install
console nunit reactredux webconfig wpfusercontrollib list
editorconfig nunit-test sln winforms xunit search
gitignore page tool-manifest winformscontrollib –help uninstall
globaljson proto viewimports winformslib -? update

Template options (the list of template options in the web template)

❯ dotnet new web –dry-run
–dry-run –language –output -lang
–exclude-launch-settings –name –type -n
–force –no-https -? -o
–framework –no-restore -f /?
–help –no-update-check -h /h

Allowed values for those template options (choice values on an choice template argument)

❯ dotnet new blazorserver –auth Individual
Individual IndividualB2C MultiOrg None SingleOrg Windows

There are a few known gaps in completion – for example, –language doesn’t suggest installed language values.

Future work

In future previews, we plan to continue filling gaps left by this transition, as well as make enabling completions either automatic or as simple as a single command that the user can execute. We hope that this will make improvements in tab completion across the entire dotnet CLI more broadly used by the community!

What’s next

dotnet new users – go enable tab completion and try it for your templating use! Template authors – try out tab completion for the options on your templates and make sure you’re delivering the experiences you want your users to have. Everyone – raise any issues you find on the dotnet/templating repo and help us make .NET 7 the best release for dotnet new ever!

NativeAOT Update

We previously announced that we’re moving the NativeAOT project out of experimental status and into mainline development in .NET 7. Over the past few months, we’ve been heads down doing the coding to move NativeAOT out of the experimental dotnet/runtimelab repo and into the dotnet/runtime repo.

That work has now been completed, but we have yet to add first-class support in the dotnet SDK for publishing projects with NativeAOT. We hope to have that work done shortly, so you can try out NativeAOT with your apps. In the meantime, please try trimming your app and ensure there are no trim warnings. Trimming is a requirement of NativeAOT. If you own any libraries there are also instructions for preparing libraries for trimming.

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.


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 Current 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.

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:

ASP.NET Core 7 and Blazor Roadmap
EF 7 Roadmap


We appreciate and thank you for your all your support and contributions to .NET. Please give .NET 7 Preview 2 a try and tell us what you think!

The post Announcing .NET 7 Preview 2 – The New, ‘New’ Experience appeared first on .NET Blog.

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