Announcing .NET Community Toolkit 8.1! Better, faster MVVM source generators, .NET 7 support, and more!

Sergio Pedri

We’re happy to announce the official launch of the 8.1 release of the .NET Community Toolkit! This new version includes highly requested new features, bug fixes, and massive performance improvements to the MVVM Toolkit source generators, to make the developer UX when using them better than ever!

.NET Community Toolkit 8.1.0 Preview 1

Just like in our previous release, we greatly appreciate all the feedback received both by teams here at Microsoft using the Toolkit, as well as other developers in the community, that had and continues to have a huge impact on how we’re designing and prioritizing all the new features and improvements. We’re really thankful to everyone that has contributed and that keeps helping the .NET Community Toolkit get better every day! πŸŽ‰

What’s in the .NET Community Toolkit? πŸ‘€

Just like we did in our previous announcement posts, let’s start with a small recap of what the .NET Community Toolkit includes. It’s composed of several independent libraries:

  • CommunityToolkit.Common
  • CommunityToolkit.Mvvm (aka “Microsoft MVVM Toolkit”)
  • CommunityToolkit.Diagnostics
  • CommunityToolkit.HighPerformance

The libraries are also widely used in several inbox apps that ship with Windows, such as the Microsoft Store and the Photos app! πŸš€

For more details on the history of the .NET Community Toolkit, here is a link to our previous 8.0.0 announcement post.

Here is a breakdown of the main changes that are included in this new 8.1 release of the .NET Community Toolkit.

Custom attributes for [ObservableProperty] πŸ€–

As we mentioned in our 8.1.0 Preview 1 announcement blog post, one of our most requested features (see #208, #217, #228) for the MVVM Toolkit source generator was to support using custom attributes for [ObservableProperty]. There were several proposed designs to support this, and we eventually decided to leverage the existing property: syntax in C# to let developers mark attributes to propagate to generated properties. This gives us several advantages:

  • It leverages built-in C# syntax, making the feature feel “native” and without needing extra attributes
  • It solves the issue of annotating attributes that can only target properties and not fields

That is, with the MVVM Toolkit 8.1, the following scenario is now supported:

[property: JsonPropertyName("responseName")]
[property: JsonRequired]
private string? _name;

This will then generate the following property behind the scenes:

public string? Name
    get => _name;
        if (!EqualityComparer<string?>.Default.Equals(_name, value))

            _name = value;


partial void OnNameChanging(string? value);
partial void OnNameChanged(string? value);

You can see how the generated property has the two attributes we specified! This allows complete flexibility in annotations for generated properties, all while using built-in C# syntax and with no constraints on the types of attributes that this features supports πŸ™Œ

Note: the generated code is slightly different and includes additional performance optimizations not shown here.

You can find all of our docs on the new source generators here, and if you prefer a video version, James Montemagno has also done several videos on them, such as this one.

MVVM Toolkit analyzers πŸ“–

This release of the MVVM Toolkit is also the first one to introduce dedicated analyzers to help developers use the MVVM Toolkit in the best way possible. That is, the MVVM Toolkit will no longer just emit diagnostics for features used incorrectly (as in, in a way that would result in an error), but it will now also show recommendations to improve the code and avoid common errors! 🎯

The first analyzer covers a common mistake when using the [ObservableProperty] attribute. Consider this example:

private string? name;

public async Task LoadUserAsync()
    User user = await _userService.FetchUserAsync();

    name = user.Name; // Whoops! Assigning to the field!

This has been brought up several times by developers accidentally assigning to a field (and not the generated property), and then getting stuck with the UI not reflecting the change, without a clear indication as to why that was the case. The new analyzer will help in these cases, and it will flag all assignments to fields backing an observable property, showing diagnostics to suggest to reference the generated property instead. No more mysterious missing property notifications!

The second new analyzer instead is meant to help reduce binary size in applications using the MVVM Toolkit. As we mentioned in the 8.0.0 announcement post, the MVVM Toolkit includes several attributes (such as [ObservableObject]) that allow the generators to inject all code necessary to implement the INotifyPropertyChanged and INotifyPropertyChanging interfaces (optionally with additional helpers too) into existing classes. This is particularly useful in cases where a class is already inheriting from a different type than ObservableObject: you can use the attribute and still have access to the same helpers without having to reimplement the logic yourself.

This is only meant for when inheriting is not possible though: if that’s not the case you should rather just inherit from ObservableObject and benefit from the reduced binary size, as the compiler won’t have to just copy over those same helpers over and over in each type. Consider this example:

public partial class MyViewModel
    private string? name;

Here, MyViewModel isn’t inheriting from any type, so it should rather inherit from ObservableObject rather than using the [ObservableObject] attribute, to benefit from the binary size improvements. The new analyzer will flag all scenarios like this one, suggesting to use inheritance instead. This will particularly help beginners, who might not understand the nuances of the two different approaches and might not know how to choose. In those cases, the analyzer will now be there to help.

MVVM Toolkit source generator optimizations πŸ›«

As we mentioned, this new release also includes major performance optimizations to the MVVM Toolkit, to improve the developer UX even more especially when working on very large solutions. We spent a lot of time improving the architecture of all our generators and talking with engineers from the Roslyn team to make sure that we were doing everything possible to squeeze as much performance out of them as we could.

Here’s just some of the improvements on this front:

  • Added multi-targeting for Roslyn 4.3 (#428, #462): the MVVM Toolkit source generators will now use the Roslyn 4.3 target if supported, so that they can opt-in into some more optimized APIs if the host supports it. This is automatically enabled when referencing the MVVM Toolkit.
  • Used ForAttributeWithMetadataName<T> (#436): we switched the generators to the new high-level Roslyn API to match attributes, which greatly improves performance for generators triggering over specific attributes. For instance, [ObservableProperty] is now using this.
  • Moved diagnostics to diagnostic analyzers (#433, #434): we moved almost all diagnostics to diagnostics analyzers, which are run out of process and independently from source generators. This significantly reduces overhead when typing, as all the logic for the diagnostics is now run in a separate process and cannot slow IntelliSense down.
  • Stopped using symbols in incremental providers (#435): we updated all our incremental providers to no longer propagate symbols. This can reduce memory use, as propagating symbols can cause Roslyn to unnecessarily root compilation objects.
  • More performance optimizations (#447, #460, #469, #487, #489): we overhauled all our incremental models and incremental pipelines to significantly improve performance and reduce memory allocations.

IObservable<T> messenger extensions πŸ“«

Another requested feature, especially by developers heavily using Reactive-style APIs in their applications, was to have a way to bridge the functionality exposed by the messenger APIs in the MVVM Toolkit. This is now supported, thanks to the new IObservable<T> extensions for the IMessenger interface. They can be used as follows:

IObservable<MyMessage> observable = Messenger.CreateObservable<MyMessage>();

…That’s it! This extension will create an IObservable<T> object that can be used to subscribe to messages and dynamically react to them. Specifying different tokens is also supported, through a separate overloads. Here is another example showing an end-to-end use of the new API:

var token =
    .Subscribe(m => Console.WriteLine($"Hello {m.Username}!"));

.NET 7 and C# 11 support πŸš€

This new release of the .NET Community Toolkit also adds the .NET 7 TFM to the HighPerformance package, and includes several changes to benefit from the new C# 11 language features, specifically ref fields.

The following types are now no longer in preview, and have been updated to leverage the new ref safety rules:

  • Ref<T>
  • ReadOnlyRef<T>
  • NullableRef<T>
  • ReadOnlyNullableRef<T>

An example of where these can be used is as follows:

public static bool TryGetElementRef(out NullableRef<T> reference)
    // Logic here...

That is, the NullableRef<T> type effectively enables a method to have an out ref T parameter, which is not otherwise expressible in C#. We also plan to extend the API surface of these types in the future to allow these types to provide an easy to use alternative to GC-ref arithmetic using the Unsafe type, which can also be visually more similar to traditional pointer arithmetic.

Additionally, all ref struct types that were already not in preview have been updated to use ref fields internally, to benefit from better performance. These include:

  • Span2D<T>
  • ReadOnlySpan2D<T>
  • RefEnumerable<T>
  • ReadOnlyRefEnumerable<T>
  • SpanEnumerable<T>
  • ReadOnlySpanEnumerable<T>

Other changes βš™οΈ

There is much more being included in this new release!

You can see the full changelog in the GitHub release page.

Get started today! πŸŽ‰

You can find all source code in our GitHub repo, some handwritten docs on MS learn, and complete API references in the .NET API browser website. If you would like to contribute, feel free to open issues or to reach out to let us know about your experience! To follow the conversation on Twitter, use the #CommunityToolkit hashtag. All your feedbacks greatly help in shape the direction of these libraries, so make sure to share them!

More resources 🎞️

If you want to learn more about the MVVM Toolkit, you can also check out this video from recent .NET Conf 2022 event, that showcases how you can use the MVVM Toolkit and all the new source generator, and all the new features in 8.1:

There’s a whole ecosystem of Toolkit-s that are available, with tons of useful APIs to build .NET applications! Read more about them on the MS learn docs!

Happy coding! πŸ’»