How to bind an attached property in XAML, WPF

Question

I would like to bind the User.Password property to PasswordBox (TwoWay). Since PasswordBox.Password is not binding, I did AttachedProperties to fix this (one to activate the binding and one to store the actual password). The problem is that they will not bind (GetBindingExpression returns null).

also:

• Work of AttachedProperties. Password and PasswordValue (attached) are set correctly if I enter the password in PasswordBox, but User.Password remains empty.
• AttachedProperty binding also works, but just the opposite: if I bind PasswordValue to TextBlock (TextBlock.Text is target, helper: PasswordValue is source), it works. Only I cannot use this because User properties are not dependency objects.
• User.Password is a binder (User implements INotifyPropertyChanged), and I managed to associate User.Username with TextBox.Text and (Username and password are similar string properties)

Here are the AttachedProperties:

public static bool GetTurnOnBinding(DependencyObject obj) { return (bool)obj.GetValue(TurnOnBindingProperty); } public static void SetTurnOnBinding(DependencyObject obj, bool value) { obj.SetValue(TurnOnBindingProperty, value); } // Using a DependencyProperty as the backing store for TurnOnBinding. This enables animation, styling, binding, etc... public static readonly DependencyProperty TurnOnBindingProperty = DependencyProperty.RegisterAttached( "TurnOnBinding", typeof(bool), typeof(PasswordHelper), new UIPropertyMetadata(false, (d, e) => { var pb = d as PasswordBox; SetPasswordValue(pb, pb.Password); pb.PasswordChanged += (s, x) => SetPasswordValue(pb, pb.Password); })); public static string GetPasswordValue(DependencyObject obj) { return (string)obj.GetValue(PasswordValueProperty); } public static void SetPasswordValue(DependencyObject obj, string value) { obj.SetValue(PasswordValueProperty, value); } // Using a DependencyProperty as the backing store for PasswordValue. This enables animation, styling, binding, etc... public static readonly DependencyProperty PasswordValueProperty = DependencyProperty.RegisterAttached( "PasswordValue", typeof(string), typeof(PasswordHelper), new UIPropertyMetadata(null, (d, e) => { PasswordBox p = d as PasswordBox; string s = e.NewValue as string; if (p.Password != s) p.Password = s; })); 

And the part of XAML with binding:

 <PasswordBox x:Name="passBox" root:PasswordHelper.TurnOnBinding="True" root:PasswordHelper.PasswordValue="{Binding Text, ElementName=passSupport, Mode=TwoWay}"/> 

Answer 1

Sorry, I can’t say what is wrong with your code (IMHO it should work), but since I had a similar problem in Silverlight, and the designer doesn’t really like binding to and from attached properties, I found that they are not are a hassle.
My current preferred method of extending control behavior is using the base class System.Windows.Interactivity.Behavior (see here and here ), and then bind my behavior to the controls.

So, you need a class PasswordBehavior : Behavior<PasswordBox> , which overrides OnAttached() and subscribes to the passwordBox PasswordChanged event and has a DependencyProperty associated with it called PasswordValue. In the case when the handler changes the value of dp, and on dp callback changes the value of the control password.

Answer 2

Give up PasswordBoxAssistant . Just change the properties BoundPassword => BoundPasswordProperty, BindPassword => BindPasswordProperty, UpdatingPassword => UpdatingPasswordProperty to avoid warnings in the XAML view.