main.cs

using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Linq;
using System.Windows;
using System.Windows.Controls;

namespace WpfApplication12
{
    public partial class MainWindow : INotifyPropertyChanged
    {
        public ObservableCollection<MyVm> MyCollection { get; set; }

        private MyVm _selectedItem;
        public MyVm SelectedItem {
            get { return _selectedItem; }
            set
            {
                _selectedItem = value;
                OnPropertyChanged("SelectedItem");
            }
        }

        public MainWindow()
        {
            InitializeComponent();

            MyGlobals.MainWindow = this;

            MyCollection = new ObservableCollection<MyVm>();
            MyCollection.Add(new MyVm());
            MyCollection.Add(new MyVm());
            MyCollection.Add(new MyVm());

            DataContext = this;
        }

        private void ButtonBase_OnClick(object sender, RoutedEventArgs e)
        {
            SelectedItem = MyCollection.First();
        }

        #region Property Changes

        public void NotifyPropertyChanged(string _property)
        {
            OnPropertyChanged(_property);
        }

        public event PropertyChangedEventHandler PropertyChanged;

        // Create the OnPropertyChanged method to raise the event 
        protected void OnPropertyChanged(string name)
        {
            PropertyChangedEventHandler handler = PropertyChanged;
            if (handler != null)
            {
                handler(this, new PropertyChangedEventArgs(name));
            }
        }

        #endregion
    }

    public static class MyGlobals
    {
        public static MainWindow MainWindow;
    }

    public class MyVm
    {
        
    }

    public class MySelector : DataTemplateSelector
    {
        public DataTemplate NoSelectionTemplate { get; set; }
        public DataTemplate ItemTemplate { get; set; }

        public override DataTemplate SelectTemplate(object item, DependencyObject container)
        {
            if (MyGlobals.MainWindow.SelectedItem != null)
                return ItemTemplate;

            return NoSelectionTemplate;
        }
    }
}

main.xaml

<Window x:Class="WpfApplication12.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:wpfApplication12="clr-namespace:WpfApplication12"
        Title="MainWindow" Height="350" Width="525">
    
    <Window.Resources>
        
        <DataTemplate x:Key="NoSelectionTemplate">
            <TextBlock Text="No selection, I Require access to the entire collection" />
        </DataTemplate>

        <DataTemplate x:Key="ItemTemplate">
            <TextBlock Text="I Represent a single item of the collection" />
        </DataTemplate>

        <wpfApplication12:MySelector x:Key="MySelector" ItemTemplate="{StaticResource ItemTemplate}" NoSelectionTemplate="{StaticResource NoSelectionTemplate}" />
        
    </Window.Resources>
    
    <StackPanel Orientation="Vertical">

        <ContentPresenter Content="{Binding}" ContentTemplateSelector="{StaticResource MySelector}" />
        
        <Button Click="ButtonBase_OnClick" Content="Select Item" Margin="0,10,0,0"></Button>
        
    </StackPanel>
</Window>