Does mvvm light v3 unregister work properly?

Question

Is there a problem with the MvvM-Light v3 unregister?

I am registering like this:

 Messenger.Default.Register<DialogResponse>(this,
                                            MessageTypesUI.YesNoQueryResponse,
                                            AnswerResponse);

Then in "AnswerResponse"

 Messenger.Default.Unregister<DialogResponse>(MessageTypesUI.YesNoQueryResponse);

Both methods execute, and AnswerResponse is called, but the second time it is called twice and the third time thrice, etc.

It acts as if my "Unregister" is being ignored.

I tried it both with Unregister() and plain old Unregister()

There was something in the release notes for V4 that made me suspect a problem with the version 3 unregister...

AxelEckenberger AxelEckenberger · Accepted Answer · 2011-08-03T06:01:31

You should modify the call to Unregister to pass in the recipient object, not the message type:

Messenger.Default.Unregister<DialogResponse>(this);

If you look at the MVVM Light code for the messenger all unregister actions use the recipient - as internally the recipient is used for identifying which message handlers will be removed. The most basic method is Unregister(recipient), which de-registers all actions for a given recipient. All additional parameters allow for a more fine grained approach.

If you have multiple actions handling the same message type and you want to unregister only one of them you can use

Messenger.Unregister<DialogMessage>(recipient, action);

to only remove one of the actions.

Edit

The MVVM Light release notes for V. 4 indicate the following changes to the Messenger:

Missing methods where added to. The IMessenger interface.
An additional Unregister method allowing to use tokens for filtering was added.
The Messenger.CleanupList method was made thread safe.

Does mvvm light v3 unregister work properly?

1 Answers