CommunityToolkit.Mvvm Messenger

Pub/sub messaging for ViewModels (or any objects) without forcing a shared reference graph. Part of CommunityToolkit.Mvvm 8.x.

TL;DR. Default to WeakReferenceMessenger.Default. Register handlers with the (recipient, message) lambda and the static modifier so you never capture this. Inherit from ObservableRecipient and toggle IsActive at activation/deactivation to get automatic register/unregister.

When to use this skill

• Two or more ViewModels need to react to an event (login, theme change, save, navigation) without holding references to each other
• A ViewModel needs to ask another VM for a value (request/reply)
• You're scoping events to a sub-system or window with channel tokens
• Diagnosing "my handler never fires" or weak-reference recipient lifetime problems

For source generators, base classes, and commands see the mvvm-toolkit skill. For DI wiring (registering an IMessenger instance), see mvvm-toolkit-di.

Choose an implementation

ObservableRecipient's parameterless constructor uses WeakReferenceMessenger.Default. Pass a different IMessenger to its constructor to override.

Define a message

The toolkit ships base classes; any class works.

using CommunityToolkit.Mvvm.Messaging.Messages;

// Single-payload broadcast
public sealed class LoggedInUserChangedMessage(User user)
    : ValueChangedMessage<User>(user);

// Custom shape (records are great for this)
public sealed record ThemeChangedMessage(AppTheme NewTheme);

// Empty signal
public sealed record RefreshRequestedMessage;

Register a recipient

Lambda style (recommended)

WeakReferenceMessenger.Default.Register<MyViewModel, ThemeChangedMessage>(
    this,
    static (recipient, message) => recipient.OnThemeChanged(message.NewTheme));

The static modifier prevents accidental closure allocation and keeps this out of the lambda — use the recipient parameter instead.

IRecipient<TMessage> interface style

public sealed class MyViewModel : ObservableRecipient,
    IRecipient<ThemeChangedMessage>,
    IRecipient<RefreshRequestedMessage>
{
    public void Receive(ThemeChangedMessage message) { /* ... */ }
    public void Receive(RefreshRequestedMessage message) { /* ... */ }
}

ObservableRecipient.OnActivated() calls Messenger.RegisterAll(this), which subscribes every IRecipient<T> interface implemented by the type. If you're not using ObservableRecipient, register manually:

WeakReferenceMessenger.Default.RegisterAll(this);

Send a message

WeakReferenceMessenger.Default.Send(new ThemeChangedMessage(AppTheme.Dark));

// Empty payloads use the parameterless overload:
WeakReferenceMessenger.Default.Send<RefreshRequestedMessage>();

Channels (tokens)

Scope messages to a sub-system or window with a token (any equatable value — int, string, Guid):

const int LeftPaneChannel = 1;

WeakReferenceMessenger.Default.Register<MyViewModel, RefreshRequestedMessage, int>(
    this, LeftPaneChannel,
    static (r, _) => r.RefreshLeft());

WeakReferenceMessenger.Default.Send(new RefreshRequestedMessage(), LeftPaneChannel);

Messages sent without a token use the default shared channel — they are not delivered to channel-scoped recipients.

Request / reply

For ask-style scenarios where a recipient provides a value back to the sender, use the RequestMessage<T> family.

Sync request

public sealed class CurrentUserRequest : RequestMessage<User> { }

WeakReferenceMessenger.Default.Register<UserService, CurrentUserRequest>(
    this,
    static (r, m) => m.Reply(r.CurrentUser));

User user = WeakReferenceMessenger.Default.Send<CurrentUserRequest>();

The implicit conversion from CurrentUserRequest to User throws if no

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