Binding to a usercontrols dependencyproperties

I have a WPF UserControl project named FormattedTextBox that contains a TextBox and a WPF window project in the same solution.

My user control has two dependency properties registered like this:

public static readonly DependencyProperty NumberProperty =  
    DependencyProperty.Register("Number", 
        typeof(double), 
        typeof(FormattedTextBox), 
        new FrameworkPropertyMetadata());  

public static readonly DependencyProperty NumberFormatStringProperty =
    DependencyProperty.Register("NumberFormatString", 
        typeof(string), 
        typeof(FormattedTextBox),
        new FrameworkPropertyMetadata());

I make an instance of my usercontrol in the main window. The main window inplements INotifyPropertyChanged and has a property named MyNumber. In the XAML of the main window I try to bind to MyNumber like this:

Number="{Binding Path=MyNumber,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"

The binding doesn't work - I never get into the get or set on the Number property in the user control. Can anybody help?

When a dependency property is set in XAML (or by binding or animation etc.), WPF directly accesses the underlying DependencyObject and DependencyProperty without calling the CLR wrapper. See XAML Loading and Dependency Properties, Implications for Custom Dependency Properties.

In order to get notified about changes of the Number property, you have to register a PropertyChangedCallback:

public static readonly DependencyProperty NumberProperty =  
    DependencyProperty.Register("Number", 
        typeof(double), 
        typeof(FormattedTextBox), 
        new FrameworkPropertyMetadata(NumberPropertyChanged));

private static void NumberPropertyChanged(
    DependencyObject obj, DependencyPropertyChangedEventArgs e)
{
    var textBox = obj as FormattedTextBox;
    ...
}