Access extended properties in XAML

I am writing an app for Windows 8.1 that has a feature that displays a horizontal series of images that are contained within a wrapper class that I have extended from a previous API that I wrote.

Basically the set up is as follows:

I have base class A that exists within an external library that I am referencing.

I then have class B inside of my current project that extends A but add a WriteableBitmap property:

public class B:A
{
    private WriteableBitmap imageBitmap;

    public WriteableBitmap ImageBitmap
    {
        get { return this.imageBitmap; }
        set
        {
            if (Equals(value, imageBitmap)) return;
            this.imageBitmap = value;
            OnPropertyChanged();
        }
    }    
}

I am then trying to display the images using a ItemsControl in Xaml with its ItemSource bound to a "ImageList" which is a list of my wrapper class (which in the XAML file is apparently being linked only to class A).

<ItemsControl ItemsSource="{Binding ImageList}">
    <ItemsControl.ItemsPanel>
        <ItemsPanelTemplate>
            <StackPanel Orientation="Horizontal"/>
        </ItemsPanelTemplate>
    </ItemsControl.ItemsPanel>
    <ItemsControl.ItemTemplate>
        <DataTemplate>
              <Image Source = "{Binding **trying to bind to the ImageBitmap property**}"/>
        </DataTemplate>
    </ItemsControl.ItemTemplate>
</ItemsControl>

As you can see above, when I am trying to bind the Image, the only properties I have access to are the ones originally declared in the base class A. If I try to cast the binding somehow to to the newer subclass B like this:

<Image Source = "{Binding (ex:B.ImageBitmap)}"/>

That throws errors saying its an invalid binding.

For reference, ex is the name I gave for the namespace where B exists in the project at he beginning of the Xaml file.

Basically I am trying to figure out the proper way to cast a binding to a particular type that you have declared inside of the current project that extends a class that exists in an outside library that you are also referencing.

Any ideas?

Edit: To provide more detail as to what I am trying to do exactly, let me explain a little more and try to trim down the unnecessary details.

I have an external assembly, we will call it ExternalLib. Inside ExternalLib are the following classes:

• Class A which you saw above, which has various properties already declared in the external library.

• Another wrapper class called ImagePackage which contains the following property(s):

  List<A> ImageList;

  String PackageName;

Then in my current project I have class B which extends A and adds an additional WriteableBitmap property.

This all comes together in my app where I am trying to use nested ItemsControl to display a vertical list of ImagePackage, showing its PackageName as the header. Inside each vertical list is then a horizontal list that displays each image inside that list (which is obtained from the WriteableBitmap declared in B). To imagine this, think if how Netflix is laid out (vertical and horizontal lists together).

To accomplish this, I have a global data context that I use and I have the following Xaml:

<ItemsControl x:Name="ImageRoot" Height="861" Width="656" DataContext="{StaticResource GlobalDataContext}" ItemsSource="{Binding MainImageInfo.ImagePackages}" DisplayMemberPath="PackageName">
    <ItemsControl.ItemsPanel>
        <ItemsPanelTemplate>
            <StackPanel/>
        </ItemsPanelTemplate>
    </ItemsControl.ItemsPanel>    
    <ItemsControl.ItemTemplate>
        <DataTemplate>
            <ItemsControl ItemsSource="{Binding ImageList}">
                <ItemsControl.ItemsPanel>
                    <ItemsPanelTemplate>
                        <StackPanel Orientation="Horizontal"/>
                    </ItemsPanelTemplate>
                </ItemsControl.ItemsPanel>
                <ItemsControl.ItemTemplate>
                    <DataTemplate>
                         <Image Source = "{Binding ImageBitmap}"/>                        
                    </DataTemplate>
                </ItemsControl.ItemTemplate>
            </ItemsControl>
        </DataTemplate>
    </ItemsControl.ItemTemplate>
</ItemsControl>

Trying to declare the source of the Image as ImageBitmap causes Intellisense to throw a fit, but it still compiles and runs. The issue is that when the binding actually tries to update the entire app crashes (even though I have it set to break on exceptions) saying:

MyAppName has exited with code -1073741189 (0xc000027b).

Finally, the actual warning that Intellisense is providing on that line is:

Cannot resolve property "ImageBitmap" in data context of type "ExternalLib.A"