MVVM Light Messenger executing multiple times

Question

I am using MVVM Light and am using Messages to communicate between ViewModels to let a ViewModel know when it is ok to execute something. My problem is that I register for

Answer 1

really old but thought I would answer just in case somebody needs it. I was fairly new to silverlight at the time and the issue ended up being a memory leak as the viewModel, which had multiple instances, was still in memory.

Answer 2

I have seen this issue before. It had to do with the Messenger.Default.Register being called more than once. The MVVMLight Messenger class will register the same item 'x' number of times. This is why when you call the Send you get it many times.

Anyone know how to prevent MVVMLight from registering multiple times?

Answer 3

Very old question but I solved the problem by doing this:

static bool isRegistered = false;

and then, in the constructor:

if( !isRegistered )
{
   Messenger.Default.Register<MyMessage>(this, OnMessageReceived);
   isRegisterd = true;
}

Answer 4

Make sure you unregister your message handlers once you do not need them anymore. The Messenger keeps a reference to the registered methods and this prevents them from being garbage collected.

Therefore, for ViewModels: make sure that you call Cleanup once you done (or implement IDisposable and call Cleanup from there).

For Views (Controls, Windows, or similar) call Messenger.Unregister in an event that occurs on the teardown of the view, e.g. the Unloaded event.

This is a known behaviour of the MVVM and has been discussed in several places.

Answer 5

As other contributors mentioned, the same message is being registered multiple times. I have noticed this behavior taking place while navigating to View X then navigating back to View Z where the message is registered in the constructor of the Z ViewModel. One solution is to set the NavigationCacheMode property to Required

<Page
  ........
  ........
  NavigationCacheMode="Required">