一、概述

眾所周知，繫結的源既可以是依賴屬性也可以是普通的CLR屬性，而繫結的目標只能是依賴屬性。

控制元件PasswordBox的Password屬性不是依賴屬性，不可以作為繫結的目標與後臺資料進行繫結，而在MVVM模式中，前臺和後臺的繫結是經常需要的，為了達到這種目的，我們可以藉助附加屬性來實現PasswordBox的Password屬性的繫結。

二、繫結思路

思路如下：

1）定義一個PasswordBoxHelper類，在類中定義PasswordProperty、AttachProperty和IsUpdatingProperty三個附加屬性以及相應的屬性改變事件；

2）在AttachProperty的OnAttachPropertyChanged事件中新增PasswordBox的PasswordChanged事件處理程式

3）PasswordChanged事件處理函式執行的時候，把控制元件中的資訊賦值給PasswordBoxHelper類中的依賴屬性PasswordProperty；

三、Demo

 1 using System.Windows;
 2 
 3 namespace PasswordBinding
 4 {
 5     /// <summary>
 6     /// Interaction logic for MainWindow.xaml
 
 7     /// </summary>
 8     public partial class MainWindow : Window
 9     {
10         //private string password;
11 
12         //public string Password
13         //{
14         //    get { return password; }
15         //    set { password = value; }
16         //}
17 
18         public string Password//依賴屬性具有自動通知的能力，不需要實現INotifi介面
 

19         {
20             get { return (string)GetValue(PasswordProperty); }
21             set { SetValue(PasswordProperty, value); }
22         }
23 
24         // Using a DependencyProperty as the backing store for Password.  This enables animation, styling, binding, etc...
25         public static readonly DependencyProperty PasswordProperty =
26             DependencyProperty.Register("Password", typeof(string), typeof(MainWindow), new PropertyMetadata(""));
27 
28 
29         public MainWindow()
30         {
31             InitializeComponent();
32             this.DataContext = this;
33         }
34 
35         private void Button_Click(object sender, RoutedEventArgs e)
36         {
37             //tbl.Text = password;
38             tbl.Text = PasswordBoxHelper.GetPassword(pwd);
39         }
40     }
41 }

 1 <Window x:Class="PasswordBinding.MainWindow"
 2         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
 3         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
 4         xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
 5         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
 6         xmlns:local="clr-namespace:PasswordBinding"
 7         mc:Ignorable="d"
 8         Title="MainWindow" Height="447.125" Width="525">
 9     <Grid>
10         <Grid.RowDefinitions>
11             <RowDefinition Height="134*"/>
12             <RowDefinition Height="101*"/>
13         </Grid.RowDefinitions>
14         <Grid.ColumnDefinitions>
15             <ColumnDefinition Width="60*"/>
16             <ColumnDefinition Width="457*"/>
17         </Grid.ColumnDefinitions>
18         <TextBlock Margin="10 50" Text="密碼："></TextBlock>
19         <PasswordBox Grid.Row="0" Grid.Column="1" Margin="10,50,10,157" BorderBrush="Red" local:PasswordBoxHelper.Attach ="True" Name="pwd"
20                      local:PasswordBoxHelper.Password ="{Binding Password, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
21         <TextBlock Margin="10,30,10,10" Text="顯示：" Grid.Row="1" Grid.Column="0"></TextBlock>
22         <TextBlock Grid.Row="1" Grid.Column="1" Margin="10,70,10,72" Background="AliceBlue" Foreground="#FF4EB24E" Name="tbl" />
23         <TextBlock Grid.Column="1" Margin="10,10,10,135" Background="AliceBlue" Foreground="#FF4EB24E" Grid.Row="1" Text="{Binding Password, Mode=TwoWay}"  />
24         <Button Grid.Row="1" Grid.Column="1" Margin="370,135,0,0" Content="顯示密碼" Click="Button_Click" ></Button>
25 
26     </Grid>
27 </Window>

 1 using System.Windows;
 2 using System.Windows.Controls;
 3 
 4 namespace PasswordBinding
 5 {
 6 
 7     public static class PasswordBoxHelper
 8     {
 9 
10         public static readonly DependencyProperty PasswordProperty = DependencyProperty.RegisterAttached("Password", typeof(string), typeof(PasswordBoxHelper), 
11                                                                                                           new FrameworkPropertyMetadata(string.Empty, OnPasswordPropertyChanged));
12         public static readonly DependencyProperty AttachProperty = DependencyProperty.RegisterAttached("Attach", typeof(bool), typeof(PasswordBoxHelper), new PropertyMetadata(false, OnAttachPropertyChanged));
13 
14         private static readonly DependencyProperty IsUpdatingProperty = DependencyProperty.RegisterAttached("IsUpdating", typeof(bool), typeof(PasswordBoxHelper));
15 
16 
17         public static void SetAttach(DependencyObject dp, bool value)
18         {
19             dp.SetValue(AttachProperty, value);
20         }
21 
22         public static bool GetAttach(DependencyObject dp)
23         {
24             return (bool)dp.GetValue(AttachProperty);
25         }
26 
27         public static string GetPassword(DependencyObject dp)
28         {
29             return (string)dp.GetValue(PasswordProperty);
30         }
31 
32         public static void SetPassword(DependencyObject dp, string value)
33         {
34             dp.SetValue(PasswordProperty, value);
35         }
36 
37         private static bool GetIsUpdating(DependencyObject dp)
38         {
39             return (bool)dp.GetValue(IsUpdatingProperty);
40         }
41 
42         private static void SetIsUpdating(DependencyObject dp, bool value)
43         {
44             dp.SetValue(IsUpdatingProperty, value);
45         }
46 
47         private static void OnPasswordPropertyChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)
48         {
49             PasswordBox passwordBox = sender as PasswordBox;
50             passwordBox.PasswordChanged -= PasswordChanged;
51             if (!(bool)GetIsUpdating(passwordBox))
52             {
53                 passwordBox.Password = (string)e.NewValue;
54             }
55             passwordBox.PasswordChanged += PasswordChanged;
56         }
57 
58         private static void OnAttachPropertyChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)
59         {
60             PasswordBox passwordBox = sender as PasswordBox;
61             if (passwordBox == null)
62             {
63                 return;
64             }               
65             if ((bool)e.OldValue)
66             {
67                 passwordBox.PasswordChanged -= PasswordChanged;
68             }
69             if ((bool)e.NewValue)
70             {
71                 passwordBox.PasswordChanged += PasswordChanged;
72             }
73         }
74 
75         private static void PasswordChanged(object sender, RoutedEventArgs e)
76         {
77             PasswordBox passwordBox = sender as PasswordBox;
78             SetIsUpdating(passwordBox, true);
79             SetPassword(passwordBox, passwordBox.Password);
80             SetIsUpdating(passwordBox, false);
81         }
82     }
83 }