WPF多绑定中目标元素在不同UserControl时如何指定ElementName？

原本在WPF窗口中，按钮的命令参数通过多绑定关联两个密码框，代码如下：

<Button.CommandParameter>
    <MultiBinding Converter="{converters:Converter_MultipleCommandParameters}">
        <Binding ElementName="passwordBoxSQL" />
        <Binding ElementName="passwordBoxR2" />
    </MultiBinding>
</Button.CommandParameter>

密码框定义：

<PasswordBox
    x:Name="passwordBoxSQL"
    Grid.Row="4"
    Grid.Column="1"
    Margin="0,0,10,0"
    VerticalAlignment="Center" />

将包含密码框的Grid移至名为Test的UserControl（位于XYZ.Control命名空间）后，在窗口中引用该控件：

<Window
    xmlns:local="clr-namespace:XYZ.Control"
....
<local:Test />

程序运行后界面正常，但出现绑定失败错误：XAML Binding Failures Description - Cannot find source: ElementName=passwordBoxSQL。

需要解决如何正确指定Binding的ElementName，或采用其他实现方式。

方法1：在UserControl中暴露依赖属性（推荐，低耦合） #

把密码框的密码值封装成UserControl的依赖属性，窗口直接绑定这些属性，避免跨控件树查找元素。

1. 在Test UserControl中添加两个依赖属性：

public static readonly DependencyProperty SQLPasswordProperty = DependencyProperty.Register(
    "SQLPassword", typeof(string), typeof(Test), new PropertyMetadata(string.Empty));

public string SQLPassword
{
    get => (string)GetValue(SQLPasswordProperty);
    set => SetValue(SQLPasswordProperty, value);
}

public static readonly DependencyProperty R2PasswordProperty = DependencyProperty.Register(
    "R2Password", typeof(string), typeof(Test), new PropertyMetadata(string.Empty));

public string R2Password
{
    get => (string)GetValue(R2PasswordProperty);
    set => SetValue(R2PasswordProperty, value);
}

2. 在UserControl的XAML中，为密码框添加PasswordChanged事件处理：

<PasswordBox
    x:Name="passwordBoxSQL"
    Grid.Row="4"
    Grid.Column="1"
    Margin="0,0,10,0"
    VerticalAlignment="Center"
    PasswordChanged="PasswordBoxSQL_PasswordChanged" />

3. 后台处理事件更新依赖属性：

private void PasswordBoxSQL_PasswordChanged(object sender, RoutedEventArgs e)
{
    if (sender is PasswordBox pb)
        SQLPassword = pb.Password;
}

// 同理处理passwordBoxR2的PasswordChanged事件

4. 窗口中绑定UserControl的依赖属性作为命令参数（记得给UserControl命名）：

<Button.CommandParameter>
    <MultiBinding Converter="{converters:Converter_MultipleCommandParameters}">
        <Binding ElementName="testControl" Path="SQLPassword" />
        <Binding ElementName="testControl" Path="R2Password" />
    </MultiBinding>
</Button.CommandParameter>

<local:Test x:Name="testControl" />

方法2：使用RelativeSource跨控件树查找（不推荐，高耦合） #

如果不想修改UserControl，可以用RelativeSource找到UserControl，再指向内部密码框，但UserControl内部结构变更会导致绑定失效。

1. 修改窗口中的绑定：

<Button.CommandParameter>
    <MultiBinding Converter="{converters:Converter_MultipleCommandParameters}">
        <Binding RelativeSource="{RelativeSource AncestorType={x:Type local:Test}}" Path="passwordBoxSQL.Password" />
        <Binding RelativeSource="{RelativeSource AncestorType={x:Type local:Test}}" Path="passwordBoxR2.Password" />
    </MultiBinding>
</Button.CommandParameter>

2. 把UserControl内部的密码框x:FieldModifier设为public，允许外部访问：

<PasswordBox
    x:Name="passwordBoxSQL"
    x:FieldModifier="public"
    Grid.Row="4"
    Grid.Column="1"
    Margin="0,0,10,0"
    VerticalAlignment="Center" />

方法3：MVVM模式下绑定ViewModel（最佳实践） #

如果采用MVVM，在ViewModel中定义密码属性，UserControl和窗口都绑定到ViewModel：

1. ViewModel中添加属性（需实现INotifyPropertyChanged）：

private string _sqlPassword;
public string SQLPassword
{
    get => _sqlPassword;
    set => SetProperty(ref _sqlPassword, value);
}

private string _r2Password;
public string R2Password
{
    get => _r2Password;
    set => SetProperty(ref _r2Password, value);
}

2. UserControl中处理密码框的PasswordChanged事件更新ViewModel：

private void PasswordBoxSQL_PasswordChanged(object sender, RoutedEventArgs e)
{
    if (DataContext is YourViewModel vm && sender is PasswordBox pb)
        vm.SQLPassword = pb.Password;
}

3. 窗口按钮直接绑定ViewModel的属性：

<Button.CommandParameter>
    <MultiBinding Converter="{converters:Converter_MultipleCommandParameters}">
        <Binding Path="SQLPassword" />
        <Binding Path="R2Password" />
    </MultiBinding>
</Button.CommandParameter>

内容的提问来源于stack exchange，提问作者user26526535