New Post: Constructor of the ModelViews being called twice

I haven't used MVVM Light's IoC, we use Unity but the ViewModelLocator will return a new instance of the viewmodel unless you explicitly register it as a singleton in the IoC.

Our app is pretty large and we have many separate modules and use Prism for the modularity side of things and MVVM LIght for all the viewmodel communication and messaging between modules.

We tend to register the module main parent view in the Ioc and when the child viewmodels get instantiated using a view 1st approach with a viewmodel locator they get the parent viewmodel from the Ioc in their constructor and set a reference to the child vm in the parent so essentially the child view sets the reference, I've checked this with a memory profiler and this works as expected (also releases ok from memory assuming you remove all your handlers etc)

Parent view (code behind cs file)

public DemoMainView(DemoMainViewModel vm)
        {

            IUnityContainer container = ServiceLocator.Current.GetInstance<IUnityContainer>();
            container.RegisterInstance<DemoMainViewModel>(vm, new ContainerControlledLifetimeManager());

            InitializeComponent();
            this.DataContext = vm;

        }

public void Cleanup(object sender, RoutedEventArgs e)
        {
            // Clear vm from memory
            DemoMainViewModel vm = (DemoMainViewModel)this.DataContext;
            if (vm != null)
            {
                //// Remove instance from IoC and allow Prism to unload the module
                if (vm.KeepAlive == false)
                {

                    this.DataContext = null;

                    // Remove parent class from IoC
                    IUnityContainer container = ServiceLocator.Current.GetInstance<IUnityContainer>();
                    var registration = container.Registrations.First(r => r.RegisteredType == typeof(DemoMainViewModel));
                    registration.LifetimeManager.RemoveValue();
                }
                vm = null;
                

            }

        }

In the child view

public class NotificationViewModel : GalaSoft.MvvmLight.ViewModelBase, ICleanup
    {

        
        public NotificationViewModel()
        {
            // Set reference to parent view model so it can alter child vm properties
            DemoMainViewModel parent = ServiceLocator.Current.GetInstance<DemoMainViewModel>();
            parent.Notifications = this;
            

ViewModelLocator

class DemoViewModelLocator
    {
        private IUnityContainer _container;
        private IUnityContainer Container
        {
            get
            {
                if (_container == null)
                    _container = ServiceLocator.Current.GetInstance<IUnityContainer>();
                return _container;
            }
        }

        public NotificationViewModel NotificationDemo
        {
            get { return Container.Resolve<NotificationViewModel>(); }
        }

        public ThreadingViewModel ThreadingDemo
        {
            get { return Container.Resolve<ThreadingViewModel>(); }
        }

In the child view:

<UserControl x:Class="VShips.Framework.Demo.View.ThreadingView"
             x:Name="ucThreadingView"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:vm="clr-namespace:CompanyName.ApplicationName.Demo.ViewModel"
             xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
             xmlns:common="clr-namespace:CompanyName.ApplicationName.Common.Model;assembly=VShips.Framework.Common"
             xmlns:mvvm="clr-namespace:GalaSoft.MvvmLight.Command;assembly=GalaSoft.MvvmLight.Extras.WPF45"
             xmlns:telerik="http://schemas.telerik.com/2008/xaml/presentation"
             xmlns:scm="clr-namespace:System.ComponentModel;assembly=WindowsBase"
             mc:Ignorable="d" 
             d:DesignHeight="300" 
             d:DesignWidth="300">

    <UserControl.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="pack://application:,,,/CompanyName.Application.Resource;Component/SharedResources.xaml"></ResourceDictionary>
            </ResourceDictionary.MergedDictionaries>

            <vm:DemoViewModelLocator x:Key="ViewModelLocator" />
        </ResourceDictionary>
    </UserControl.Resources>

    <UserControl.DataContext>
        <Binding Path="ThreadingDemo" Source="{StaticResource ViewModelLocator}" />
    </UserControl.DataContext>

I think if your register your vm with a key in simple IoC you should be able to get the existing instance rather than a new one, Laurent discusses it here.

http://stackoverflow.com/questions/7297014/does-the-mvvm-light-simpleioc-support-singletons

Hope this is of some assistance,

Taz.