CommunityToolkit.Mvvm (core)

Use this skill when authoring or reviewing ViewModels, properties, commands, or validation in apps that use CommunityToolkit.Mvvm 8.x.

Companion skills. Load mvvm-toolkit-messenger for IMessenger pub/sub patterns. Load mvvm-toolkit-di for Microsoft.Extensions.DependencyInjection integration.

Quick recap. [ObservableProperty] on private fields in partial classes; [RelayCommand] on instance methods; inherit from ObservableObject (or ObservableValidator for input forms, ObservableRecipient when using IMessenger).

Package & setup

<ItemGroup>
  <PackageReference Include="CommunityToolkit.Mvvm" Version="8.*" />
</ItemGroup>

Targets: netstandard2.0, netstandard2.1, net6.0+. Works on .NET, .NET Framework, Mono. Source generators ship in the same NuGet — no extra analyzer reference required.

Namespaces:

using CommunityToolkit.Mvvm.ComponentModel;   // ObservableObject, [ObservableProperty]
using CommunityToolkit.Mvvm.Input;             // [RelayCommand], RelayCommand, AsyncRelayCommand

Universal rule. Every type that uses [ObservableProperty] or [RelayCommand] — and every enclosing type, if nested — must be declared partial. Without it, the generators emit MVVMTK0008 / MVVMTK0042.

Source generators cheat sheet

Naming. Field name / _name / m_name → Name. Method LoadAsync → LoadCommand (the Async suffix is stripped; a leading On is also stripped).

See references/source-generators.md for the full attribute reference with generated-code samples.

ViewModel patterns

Simple observable property

public partial class ContactViewModel : ObservableObject
{
    [ObservableProperty]
    private string? name;
}

Hooks: OnXxxChanging / OnXxxChanged

[ObservableProperty]
private string? name;

partial void OnNameChanged(string? value) =>
    Logger.LogInformation("Name changed to {Name}", value);

Both single-arg (value) and two-arg (oldValue, newValue) overloads are available. Implement only the ones you need; unimplemented hooks are elided by the compiler (zero runtime cost).

Dependent properties + dependent commands

[ObservableProperty]
[NotifyPropertyChangedFor(nameof(FullName))]
[NotifyCanExecuteChangedFor(nameof(SaveCommand))]
private string? firstName;

[ObservableProperty]
[NotifyPropertyChangedFor(nameof(FullName))]
[NotifyCanExecuteChangedFor(nameof(SaveCommand))]
private string? lastName;

public string FullName => $"{FirstName} {LastName}".Trim();

Wrapping a non-observable model

public sealed class ObservableUser(User user) : ObservableObject
{
    public string Name
    {
        get => user.Name;
        set => SetProperty(user.Name, value, user, (u, n) => u.Name = n);
    }
}

…