Setting to DataContext not reflecting in Listbox

I have a ViewModel and i am setting an instance of it to DataContext.In xaml i have binded the Listbox itemsource to an observable collection in ViewModel.But when i run the program nothing appears in listbox.What may be the Reason.

My ViewModel class:

public class ViewModel 
    { 
        public ObservableCollection<Data> _collectionData = new ObservableCollection<Data>(); 
        ----- 
        ----- 
     } 

Xaml:

ListBox Name="myListBox" Margin="8,113,8,8" ItemsSource="{Binding _collectionData}"> 
                <ListBox.ItemTemplate> 
                    <DataTemplate> 
                        <StackPanel Orientation="Horizontal"   > 
                            <Border BorderThickness="3" BorderBrush="#A5FFFFFF" Width="80" Margin="0,20,0,20" Height="60"> 
                                <Image Source="{Binding ImageUrl, Mode=OneWay}"  VerticalAlignment="Stretch" Margin="0,0,0,0" Width="80" Height="60" Stretch="Fill" /> 
    开发者_StackOverflow中文版                        </Border> 
                            <TextBlock TextWrapping="Wrap" Text="{Binding Title}" FontSize="40" FontWeight="Normal" VerticalAlignment="Center" Margin="30,0,0,0" /> 
                        </StackPanel> 
                    </DataTemplate> 
                </ListBox.ItemTemplate> 
              </ListBox> 

Constructor of main page:

ViewModel vm = new ViewModel(); 
this.DataContext = vm; 

But when i give myListBox.ItemsSource = vm._collectionSplashData; it works .what may be the reason?

Make property instead of field

see Binding Sources Overview

Applying both ItemsSource and Datacontext not needed.