<Button C IsEnabled="{Binding ElementName=mainWindow, Path=PasswordEnabled}" Click="Button_Click" />
        <PasswordBox Name="passwordBox" PasswordChanged="passwordBox_PasswordChanged" />







        public static readonly DependencyProperty PasswordEnabledProperty = DependencyProperty.Register("PasswordEnabled", typeof(bool), typeof(MainWindow));

        // .NET Property wrapper
        public bool PasswordEnabled
        {
            get { return (bool)GetValue(PasswordEnabledProperty); }
            set { SetValue(PasswordEnabledProperty, value); }
        }


        private void passwordBox_PasswordChanged(object sender, RoutedEventArgs e)
        {
            bool prev = PasswordEnabled;
            PasswordEnabled = passwordBox != null && string.IsNullOrEmpty(passwordBox.Password) == false;
            OnPropertyChanged(new DependencyPropertyChangedEventArgs(PasswordEnabledProperty, prev, PasswordEnabled));
        }