WPF 用户控件和名称范围 [英] WPF user controls and name scoping

查看：37 发布时间：2021/6/4 21:03:20 c# wpf xaml mvvm scope

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问题描述

我一直在玩 WPF 和 MVVM，并注意到一件奇怪的事情.在自定义用户控件上使用 {Binding ElementName=...} 时，用户控件中根元素的名称似乎在使用该控件的窗口中可见.说，这是一个示例用户控件:

I've been playing around with WPF and MVVM and noticed a strange thing. When using {Binding ElementName=...} on a custom user control, the name of the root element within the user control seems to be visible in the window using the control. Say, here is an example user control:

<UserControl x:Class="TryWPF.EmployeeControl"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:TryWPF"
             Name="root">
  <Grid>
    <Grid.ColumnDefinitions>
      <ColumnDefinition Width="*" />
      <ColumnDefinition Width="Auto" />
    </Grid.ColumnDefinitions>
    <TextBlock Grid.Column="0" Text="{Binding}"/>
    <Button Grid.Column="1" Content="Delete"
                Command="{Binding DeleteEmployee, ElementName=root}"
                CommandParameter="{Binding}"/>
  </Grid>
</UserControl>

对我来说看起来很合法.现在，依赖属性 DeleteEmployee 在代码隐藏中定义，如下所示:

Looks pretty legit to me. Now, the dependency property DeleteEmployee is defined in the code-behind, like this:

public partial class EmployeeControl : UserControl
{
    public static DependencyProperty DeleteEmployeeProperty
        = DependencyProperty.Register("DeleteEmployee",
                                      typeof(ICommand),
                                      typeof(EmployeeControl));

    public EmployeeControl()
    {
        InitializeComponent();
    }

    public ICommand DeleteEmployee
    {
        get
        {
            return (ICommand)GetValue(DeleteEmployeeProperty);
        }
        set
        {
            SetValue(DeleteEmployeeProperty, value);
        }
    }
}

这里没有什么神秘的.然后，使用该控件的窗口如下所示:

Nothing mysterious here. Then, the window using the control looks like this:

<Window x:Class="TryWPF.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:TryWPF"
        Name="root"
        Title="Try WPF!" Height="350" Width="525">
  <StackPanel>
    <ListBox ItemsSource="{Binding Employees}" HorizontalContentAlignment="Stretch">
      <ListBox.ItemTemplate>
        <DataTemplate>
          <local:EmployeeControl
            HorizontalAlignment="Stretch"
            DeleteEmployee="{Binding DataContext.DeleteEmployee, ElementName=root}"/>
        </DataTemplate>
      </ListBox.ItemTemplate>
    </ListBox>
  </StackPanel>
</Window>

再说一次，没什么特别的……除了窗口和用户控件具有相同的名称！但我希望 root 在整个窗口 XAML 文件中的含义相同，因此指的是窗口，而不是用户控件.唉，运行的时候会打印如下信息:

Again, nothing fancy... except the fact that both the window and the user control have the same name! But I'd expect root to mean the same thing throughout the whole window XAML file, and therefore refer to the window, not to the user control. Alas, the following message is printed when I run it:

System.Windows.Data 错误:40:BindingExpression 路径错误:在对象"字符串"上找不到DeleteEmployee"属性(HashCode=-843597893)'.BindingExpression:Path=DataContext.DeleteEmployee;DataItem='EmployeeControl' (Name='root');目标元素是'EmployeeControl' (Name='root');目标属性是DeleteEmployee"(输入'ICommand')

System.Windows.Data Error: 40 : BindingExpression path error: 'DeleteEmployee' property not found on 'object' ''String' (HashCode=-843597893)'. BindingExpression:Path=DataContext.DeleteEmployee; DataItem='EmployeeControl' (Name='root'); target element is 'EmployeeControl' (Name='root'); target property is 'DeleteEmployee' (type 'ICommand')

DataItem='EmployeeControl' (Name='root') 让我觉得它把 ElementName=root 视为指代控件本身.它在 string 上查找 DeleteEmployee 的事实证实了这种怀疑，因为 string 正是我设计的 VM 中的数据上下文.为了完整起见，这里是:

DataItem='EmployeeControl' (Name='root') makes me think that it treats ElementName=root as referring to the control itself. The fact that it looks for DeleteEmployee on string confirms that suspicion because string is exactly what the data context is in my contrived VM. Here it is, for the sake of completeness:

class ViewModel
{
    public ObservableCollection<string> Employees { get; private set; }
    public ICommand DeleteEmployee { get; private set; }

    public ViewModel()
    {
        Employees = new ObservableCollection<string>();
        Employees.Add("e1");
        Employees.Add("e2");
        Employees.Add("e3");
        DeleteEmployee = new DelegateCommand<string>(OnDeleteEmployee);
    }

    private void OnDeleteEmployee(string employee)
    {
        Employees.Remove(employee);
    }
}

它在构造函数中被实例化并分配给窗口，这是窗口代码隐藏的唯一内容:

It is instantiated and assigned to the window in the constructor, which is the only thing in code-behind for the window:

    public MainWindow()
    {
        InitializeComponent();
        DataContext = new ViewModel();
    }

这种现象提示以下问题:

This phenomenon prompts the following questions:

这是故意的吗?
如果是这样，使用自定义控件的人如何知道它在内部使用的名称?
如果 Name 根本不应该用于自定义控件?
如果是这样，那么有哪些替代方案?我切换到在 FindAncestor 模式下使用 {RelativeSource}，这工作正常，但有更好的方法吗?
这是否与数据模板定义自己的名称应对这一事实有关?如果我只是重命名它以便名称不会与控件冲突，它并不会阻止我从模板中引用主窗口.

Is this by design?
If so, how is someone using a custom control supposed to know what name it uses internally?
If Name is not supposed to be used in custom control at all?
If so, then what are the alternatives? I switched to using {RelativeSource} in FindAncestor mode, which is working fine, but are there better ways?
Does this have anything to do with the fact that data templates define their own names copes? It doesn't stop me from referring to the main window from within a template if I just rename it so the name doesn't clash with the control.

WPF 用户控件和名称范围 [英] WPF user controls and name scoping

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WPF 用户控件和名称范围 [英] WPF user controls and name scoping

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