WPF - DataTemplate - Using element binding within an attached property

Question

I'm working on an application that uses a richly formatted ListBox. One of the things I need is to bind multiple pieces of information to a button that lives within the ListBox's DataTemplate.

This is an over-simplification of my actual code that I made to help you understand the problem.

Here's a chunk of XAML from inside the DataTemplate that works:

<Button Command="local:MediaCommands.StreamVideo"
    CommandParameter="{Binding Path=Folder}" />

When I press the button, it sends the Folder property of the data item that this list is based around (the member of ItemsSource that the current list item is showing). However, I need another piece of data, the Filename property of the current item. To accomplish this, I set up a new class, FileInfo, that contains dependency properties for Folder and Filename. I then replaced the XAML code before with:

<Button Command="local:MediaCommands.StreamVideo">
    <Button.CommandParameter>
        <data:FileInfo Folder="{Binding Path=Folder}"
            Filename="{Binding Path=Filename}" />
    </Button.CommandParameter>
</Button>

However, the only thing that my code is sending me is a blank FileInfo object. Note that if I change the XAML above to include literal values for Folder and Filename, the code works fine, in the sense that it correctly creates the FileInfo object and assigns the correct properties.

For reference, my FileInfo class looks a bit like this:

class FileInfo : DependencyObject {
    public static readonly DependencyProperty FolderProperty;
    public static readonly DependencyProperty FilenameProperty;
    static FileInfo() {
        FolderProperty = DependencyProperty.Register("Folder",
            typeof(string), typeof(FileInfo));
        FilenameProperty = DependencyProperty.Register("Filename",
            typeof(string), typeof(FileInfo));
    }
    public string Folder {
        get { return (string) GetValue(FolderProperty); }
        set { SetValue(FolderProperty, value); }
    }
    public string Filename {
        get { return (string) GetValue(FilenameProperty); }
        set { SetValue(FilenameProperty, value); }
    }
}

Ignoring the fact that, in this case, I could simply pass a reference to the data object itself (in my actual application, I need to draw data from a couple nested ListBoxs, but the issue is the same), can anybody see what is going on here? Have my dependency properties not been declared properly? Do I need to do something quirky with the bindings?

Answer 1

Bindings that don't explicitly declare a Source rely on the DataContext as their Source. You haven't declared a DataContext on the FileInfo instance, which would normally mean that an inherited DataContext would be used. DataContext inheritance depends on both FrameworkElement and the runtime Visual Tree, neither of which come into play when you're using a non-FrameworkElement derived class assigned to a property that doesn't display in the tree.