MVVM and Updating a Selected Count

Here is one way to do what you require, without having to connect alot of events from every single myObject. It goes a little outside the MVVM box, but despite people talking best practise and all, it does not ruin the application to deviate a little once in a while. 

Personally, i prefer this method from filling up the object class.

The sample is made out of a messed up solution, so it might be a little difficult to make out some things, but i am sure you will get the idea.

XAML

<Window x:Class="WpfApplication1.Window2"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:vm="clr-namespace:WpfApplication1"
        Title="Window2" Height="300" Width="300"><Window.DataContext><vm:ViewModel/></Window.DataContext><Window.Resources><vm:Converter x:Key="SelectedCountConverter"/></Window.Resources><Grid><Grid.RowDefinitions><RowDefinition/><RowDefinition Height="auto"/></Grid.RowDefinitions><DataGrid ItemsSource="{Binding Objects}" AutoGenerateColumns="True" CurrentCellChanged="DataGrid_CurrentCellChanged_1"/><Label Content="{Binding Objects, Converter={StaticResource SelectedCountConverter}, UpdateSourceTrigger=PropertyChanged}" Grid.Row="1"/></Grid></Window>

Window codebehind

    public partial class Window2 : Window
    {
        public Window2()
        {
            InitializeComponent();
        }

        private void DataGrid_CurrentCellChanged_1(object sender, EventArgs e)
        {
            (DataContext as ViewModel).NotifyPropertyChanged("Objects");
        }
    }

ViewModel, Object & Converter

    public class ViewModel : INotifyPropertyChanged
    {
        public ViewModel()
        {
            Objects = new List<myObject>();

            for (int i = 0; i < 10; ++i)
                Objects.Add(new myObject { Selected = true });
        }

        private List<myObject> _objects;
        public List<myObject> Objects
        {
            get { return _objects; }
            set
            {
                _objects = value;
                NotifyPropertyChanged("Objects");
            }
        }
        private int _count;
        public int Count
        {
            get { return _count; }
            set
            {
                _count = value;
                NotifyPropertyChanged("Count");
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;
        public void NotifyPropertyChanged(string Name)
        {
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs(Name));
        }
    }
    public class myObject
    {
        public bool Selected { get; set; }
    }

    public class Converter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            return ((List<myObject>)value).Where(o => o.Selected).Count();
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            return null; 
        }
    }

Hope this helps.

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