mvvm-toolkit-messenger

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 recipient called Reply. Capture the message to check first:

var request = WeakReferenceMessenger.Default.Send<CurrentUserRequest>();
if (request.HasReceivedResponse)
{
    User user = request.Response;
}

Async request

This async CurrentUserRequest is an alternative to the sync definition above. Declare only one of the two in a project, or give the async type a different name if you need both.

public sealed class CurrentUserRequest : AsyncRequestMessage<User> { }

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

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

Collection requests (fan-in)

CollectionRequestMessage<T> and AsyncCollectionRequestMessage<T> collect a Reply from every responding recipient:

public sealed class OpenDocumentsRequest : CollectionRequestMessage<Document> { }

var docs = WeakReferenceMessenger.Default.Send<OpenDocumentsRequest>();
foreach (Document doc in docs) { /* ... */ }

Only recipients registered for OpenDocumentsRequest reply, each calling Reply once per item. When no recipient is registered, the result is empty.

Lifecycle

Even with WeakReferenceMessenger, unregister explicitly when a recipient is being torn down — it trims dead entries and improves performance:

WeakReferenceMessenger.Default.Unregister<ThemeChangedMessage>(this);
WeakReferenceMessenger.Default.Unregister<ThemeChangedMessage, int>(this, LeftPaneChannel);
WeakReferenceMessenger.Default.UnregisterAll(this);

ObservableRecipient.OnDeactivated() does this automatically when IsActive flips to false. Set it from your activation hook:

protected override void OnNavigatedTo(NavigationEventArgs e)
{
    base.OnNavigatedTo(e);
    ViewModel.IsActive = true;
}

protected override void OnNavigatedFrom(NavigationEventArgs e)
{
    ViewModel.IsActive = false;
    base.OnNavigatedFrom(e);
}

Common pitfalls

1. Capturing this in the lambda. (r, m) => OnX(m) implicitly captures this; allocates a closure and confuses lifetime. Always use (r, m) => r.OnX(m) with static.
2. Strong-ref recipients without Unregister. With StrongReferenceMessenger, recipients (and their entire object graph) stay pinned forever. Either inherit from ObservableRecipient (auto-unregisters in OnDeactivated) or call UnregisterAll(this).
3. Inherited message types. A handler registered for BaseMessage is not invoked for DerivedMessage : BaseMessage. Register each concrete type.
4. Wrong messenger instance. Sending via WeakReferenceMessenger.Default and registering via an injected per-window messenger means the message never arrives. Use the same IMessenger everywhere (typically inject it via ObservableRecipient(messenger)).
5. OnActivated never runs. ObservableRecipient only registers IRecipient<T> handlers when IsActive flips from false to true. Set IsActive = true in the view activation hook as shown in Lifecycle, or call Messenger.RegisterAll(this) manually.
6. Cross-thread updates. The messenger is thread-agnostic. If a handler updates UI, marshal manually (DispatcherQueue.TryEnqueue / Dispatcher.BeginInvoke).

Multiple messengers (per-window scoping)

services.AddSingleton<IMessenger>(WeakReferenceMessenger.Default); // app-wide
services.AddScoped<WindowScopedMessenger>();                       // per-window

Inject the appropriate IMessenger into the ViewModel constructor:

public sealed partial class WindowViewModel(IMessenger messenger)
    : ObservableRecipient(messenger) { }

Caution: the scoped line registers only the concrete WindowScopedMessenger type, not IMessenger. With the registrations above, a ViewModel that asks for IMessenger receives the app-wide WeakReferenceMessenger.Default, so broadcasts reach every window. Desktop DI does not create a scope per window for you. To isolate broadcasts to a single window, create one messenger instance per window, register it as IMessenger inside a scope you create for that window, and pass that same instance to every ViewModel of that window. This is useful for multi-window desktop apps such as WinUI 3, WPF, MAUI desktop and Avalonia.

References

External:

• Messenger docs: https://learn.microsoft.com/en-us/dotnet/communitytoolkit/mvvm/messenger
• WeakReferenceMessenger API: https://learn.microsoft.com/en-us/dotnet/api/communitytoolkit.mvvm.messaging.weakreferencemessenger
• Source: https://github.com/CommunityToolkit/dotnet