应用场景
我要做一个系统登录功能，需要传用户名和密码到ViewModel中，可是PasswordBox传值到ViewModel中好像跟TextBox等控件不一样。这里需要用到附加属性。
附加属性：一个属性本来不属于某个对象，但是由于某种需求而被后来附加上，即把对象放入一个特定的环境后才具有的属性。 作用就是将属性与数据类型（宿主）解耦，让数据类型设计更加灵活。
本质还是依赖属性，二者仅在包装器和注册上有点区别。
实现步骤
现在我写了个附加属性类，如下：
（1）针对PasswordBox写一个帮助类(PasswordBoxHelper)，如下：
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls; namespace TLAgent.SecurityManager.WPF
{
/// <summary>
/// 为PasswordBox控件的Password增加绑定功能
/// </summary>
public static class PasswordBoxHelper
{
public static readonly DependencyProperty PasswordProperty =
DependencyProperty.RegisterAttached("Password",
typeof(string), typeof(PasswordBoxHelper),
new FrameworkPropertyMetadata(string.Empty, OnPasswordPropertyChanged));
public static readonly DependencyProperty AttachProperty =
DependencyProperty.RegisterAttached("Attach",
typeof(bool), typeof(PasswordBoxHelper), new PropertyMetadata(false, Attach));
private static readonly DependencyProperty IsUpdatingProperty =
DependencyProperty.RegisterAttached("IsUpdating", typeof(bool),
typeof(PasswordBoxHelper)); public static void SetAttach(DependencyObject dp, bool value)
{
dp.SetValue(AttachProperty, value);
}
public static bool GetAttach(DependencyObject dp)
{
return (bool)dp.GetValue(AttachProperty);
}
public static string GetPassword(DependencyObject dp)
{
return (string)dp.GetValue(PasswordProperty);
}
public static void SetPassword(DependencyObject dp, string value)
{
dp.SetValue(PasswordProperty, value);
}
private static bool GetIsUpdating(DependencyObject dp)
{
return (bool)dp.GetValue(IsUpdatingProperty);
}
private static void SetIsUpdating(DependencyObject dp, bool value)
{
dp.SetValue(IsUpdatingProperty, value);
}
private static void OnPasswordPropertyChanged(DependencyObject sender,
DependencyPropertyChangedEventArgs e)
{
PasswordBox passwordBox = sender as PasswordBox;
passwordBox.PasswordChanged -= PasswordChanged;
if (!(bool)GetIsUpdating(passwordBox))
{
passwordBox.Password = (string)e.NewValue;
}
passwordBox.PasswordChanged += PasswordChanged;
}
private static void Attach(DependencyObject sender,
DependencyPropertyChangedEventArgs e)
{
PasswordBox passwordBox = sender as PasswordBox;
if (passwordBox == null)
return;
if ((bool)e.OldValue)
{
passwordBox.PasswordChanged -= PasswordChanged;
}
if ((bool)e.NewValue)
{
passwordBox.PasswordChanged += PasswordChanged;
}
}
private static void PasswordChanged(object sender, RoutedEventArgs e)
{
PasswordBox passwordBox = sender as PasswordBox;
SetIsUpdating(passwordBox, true);
SetPassword(passwordBox, passwordBox.Password);
SetIsUpdating(passwordBox, false);
}
}
}
（2）给这个PasswordBox添加附加属性,如下绑定：
<PasswordBox x:Name="txtPassword" Height="23" Helper:PasswordBoxHelper.Attach="True" Helper:PasswordBoxHelper.Password="{Binding Path=AuthUser.Password,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}" HorizontalAlignment="Left" Margin="101,92,0,0" VerticalAlignment="Top" Width="178" TabIndex="2">
<i:Interaction.Triggers>
<i:EventTrigger EventName="KeyDown">
<Helper:InteractiveCommand Command="{Binding EnterLoginCommand}" CommandName="EnterLoginCommand" CommandParameter="{Binding ElementName=txtUserName}"/>
</i:EventTrigger>
</i:Interaction.Triggers>
</PasswordBox>如此简单，这样就解决了。