众所周知，WPF 的 PasswordBox 控件因为安全原因，默认不支持MVVM的双向数据绑定，那么我们在MVVM开发的过程中如何解决这个问题呢？

添加附加属性

新建PasswordBoxHelper.cs文件，添加如下的附加属性：

using System.Windows;
using System.Windows.Controls;

namespace PasswordBoxControl.Helpers
{
    public class PasswordBoxHelper
    {
        public static readonly DependencyProperty PasswordProperty = DependencyProperty.RegisterAttached(
            "Password", typeof(string), typeof(PasswordBoxHelper), new PropertyMetadata(string.Empty,OnPasswordPropertyChanged));

        private static void OnPasswordPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            PasswordBox passwordBox=d as  PasswordBox;
            passwordBox.PasswordChanged -= PasswordChanged;
            if (!GetIsUpdating(passwordBox))
            {
                /*从Password往控件方向更新绑定值*/
                passwordBox.Password = e.NewValue.ToString();
            }

            passwordBox.PasswordChanged += PasswordChanged;
        }

        public static void SetPassword(DependencyObject element, string value)
        {
            element.SetValue(PasswordProperty, value);
        }

        public static string GetPassword(DependencyObject element)
        {
            return (string) element.GetValue(PasswordProperty);
        }

        public static readonly DependencyProperty AttachProperty = DependencyProperty.RegisterAttached(
            "Attach", typeof(bool), typeof(PasswordBoxHelper), new PropertyMetadata(false,Attach));

        private static void Attach(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            if (!(d is PasswordBox passwordBox))
            {
                return;
            }

            if ((bool)e.OldValue)
            {
                passwordBox.PasswordChanged -= PasswordChanged;
            }

            if ((bool)e.NewValue)
            {
                /*当控件的值发生变化的时候，更新Password的值*/
                passwordBox.PasswordChanged += PasswordChanged;
            }
        }

        private static void PasswordChanged(object sender, RoutedEventArgs e)
        {
            PasswordBox passwordBox=sender as PasswordBox;
            /*IsUpdating的作用类似一把互斥锁,因涉及到双向绑定更新*/
            SetIsUpdating(passwordBox,true);
            SetPassword(passwordBox,passwordBox.Password);
            SetIsUpdating(passwordBox,false);
        }

        public static void SetAttach(DependencyObject element, bool value)
        {
            element.SetValue(AttachProperty, value);
        }

        public static bool GetAttach(DependencyObject element)
        {
            return (bool) element.GetValue(AttachProperty);
        }

        public static readonly DependencyProperty IsUpdatingProperty = DependencyProperty.RegisterAttached(
            "IsUpdating", typeof(bool), typeof(PasswordBoxHelper), new PropertyMetadata(default(bool)));

        public static void SetIsUpdating(DependencyObject element, bool value)
        {
            element.SetValue(IsUpdatingProperty, value);
        }

        public static bool GetIsUpdating(DependencyObject element)
        {
            return (bool) element.GetValue(IsUpdatingProperty);
        }
    }
}

如何使用呢？

在需要用的PasswordBox中添加上面写的附加属性Password即可：

 <PasswordBox Helper:PasswordBoxHelper.Attach="True"
              Helper:PasswordBoxHelper.Password="{Binding Password,Mode=TwoWay}"/>

这下PasswordBox控件就可以像我们熟悉的MVVM 绑定的方式去使用了！