Q:

How to call another view models constructor in MVVM Light

I've got a method that authenticates a users login.  If the user correctly logs in I'd like to call two view models contructors just in case the user has logged in.  Below is my sample code.  What is the best way to acheive this?
Sending view model:
        if (UserName == userName && Password == password)
        {
            ProjectManager.Instance.IsAdmin = true;
            isLoggedIn = true;
            IsLoggedIn = true;
            ValidLoginImage();
            LoginStatus = "Admin Logged In";
            MessengerInstance.Send(true);

        }

Receiving View Model: 
    private void RegisterForMessages()
    {
        MessengerInstance.Register<bool>(this, UpdateEnabled);
    }

    private void UpdateEnabled(bool b)
    {
        IsLoggedIn = b;
    }

The registerformessage method never gets called.

A:

Since you're already using MvvmLight, I'd just send a message containing a boolean that calls a method in the view model to update it.  Register for the messages in the view model that needs updating.  Then it will wait for the "ok i'm logged in" boolean, then run some method that updates your IsEnabled property.  Put this in the view model that needs to be updated:
/// <summary>
        /// Listen for messages from other ViewModels
        /// </summary>
        private void RegisterForMessages()
        {
            MessengerInstance.Register<bool>(this, UpdateMyStuff);
        }

private void UpdateMyStuff(bool b)
        {
            IsEnabled=b;
        }

Then send the message when the user logs in this with this:
//logged in successfully code here...
MessengerInstance.Send(true);  //or your IsLoggedIn property

you can send false if this view has logout or whatever.
This will let your view model know the status of whether the user is logged in, and allow you to update accordingly.  obviously, you need to make sure you implement INotifyPropertyChanged etc.  This is all part of MVVMLight already.  If your viewmodel implements MvvmLight's ViewModelBase, it will already be ready for you.

