WPF MVVM如何将UserControl内部的SelectedItem绑定到外部TextBox

• PersonViewModel的SelectedPerson属性未实现INotifyPropertyChanged通知，属性值变化时无法同步到UI
• PersonView构造函数中硬编码创建了独立的PersonViewModel实例并赋值给自身DataContext，导致和主窗口的数据源实例不统一，自然无法拿到内部选中的值
• PersonModel的ID属性未实现属性变更通知，若后续需要修改ID会无法同步显示

1. 改造PersonViewModel，添加属性变更通知 #

首先让PersonViewModel实现INotifyPropertyChanged接口，给SelectedPerson加变更通知：

public class PersonViewModel : INotifyPropertyChanged
{
    public PersonViewModel()
    {
        Persons = new ObservableCollection<PersonModel>();
        LoadPersons();
    }

    public ObservableCollection<PersonModel> Persons { get; set; }

    private PersonModel _selectedPerson;
    public PersonModel SelectedPerson
    {
        get => _selectedPerson;
        set
        {
            _selectedPerson = value;
            RaisePropertyChanged(nameof(SelectedPerson));
        }
    }

    public void LoadPersons()
    {
        ObservableCollection<PersonModel> persons = new ObservableCollection<PersonModel>();
        persons.Add(new PersonModel { ID = 1, Forname = "Thomas", Surname = "Sogen" });
        persons.Add(new PersonModel { ID = 2, Forname = "Seth", Surname = "Xu" });
        persons.Add(new PersonModel { ID = 3, Forname = "Derik", Surname = "Mayers" });
        persons.Add(new PersonModel { ID = 4, Forname = "Daisy", Surname = "Doros" });
        Persons = persons;
    }

    public event PropertyChangedEventHandler PropertyChanged;
    private void RaisePropertyChanged(string property)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(property));
        }
    }
}

2. 移除PersonView内部硬编码的DataContext赋值 #

删掉PersonView.xaml.cs构造函数里的DataContext设置行，UserControl的数据源由父容器提供：

public partial class PersonView : UserControl
{
    public PersonView()
    {
        InitializeComponent();
        // 删掉原有的DataContext赋值行
        // this.DataContext = new MVVMExample.ViewModel.PersonViewModel();
    }
}

3. 给MainWindow设置统一的数据源 #

在MainWindow的构造函数中实例化PersonViewModel，赋值给自身的DataContext，这样主窗口的控件和内部的PersonView会共享同一个数据源实例：

public MainWindow()
{
    InitializeComponent();
    this.DataContext = new MVVMExample.ViewModel.PersonViewModel();
}

4. 给MainWindow的TextBox添加正确绑定 #

修改MainWindow.xaml中的三个TextBox的绑定路径：

<TextBox Grid.Row="0" Text="{Binding SelectedPerson.ID}"/>
<TextBox Grid.Row="1" Text="{Binding SelectedPerson.Forname}"/>
<TextBox Grid.Row="2" Text="{Binding SelectedPerson.Surname}"/>

可选优化：给PersonView中DataGrid的SelectedItem绑定加上双向绑定声明，确保选中变化能及时同步到ViewModel：

SelectedItem="{Binding SelectedPerson, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"

完全不需要把主窗口的TextBox放到PersonView内部，只要保证所有绑定控件共享同一个ViewModel实例、需要通知的属性都正确实现了INotifyPropertyChanged接口，就能实现跨控件的属性同步。

内容的提问来源于stack exchange，提问作者FloSu