双击时，窗口依赖属性值中的WPF用户控件为null

Question

我已经在这里上传了我的示例项目：https://www.file-upload.net/download-13252079/WpfApp1.zip.html它是一个WPF窗口，其中有几个相同的用户控件。UserControl有一个名为"Text"的依赖属性，该属性绑定到MainWindowViewModel的属性，并成功显示在UserControl的TextBlock中。但是，如果双击UserControl并希望它提供其依赖属性的值，则该值为null。为什么会这样？非常感谢你的帮助！

编辑：对不起，这里有一些源代码：UserControl的XAML：

<UserControl x:Class="WpfApp1.UserControl1"
...
x:Name="UC1">    
<StackPanel Orientation="Vertical">
<TextBlock Margin="5" Text="Test" FontSize="20"></TextBlock>
<TextBlock Margin="5" Text="{Binding ElementName=UC1, Path=Text}" FontSize="20"></TextBlock>
</StackPanel>
</UserControl>

UserControl的代码：

public partial class UserControl1 : UserControl, INotifyPropertyChanged
{
public UserControl1()
{
InitializeComponent();
}

string text;
public string Text
{
get { return text; }
set { SetProperty(ref text, value); }
}
public static readonly DependencyProperty TextProperty = DependencyProperty.Register(
"Text", typeof(string), typeof(UserControl1));


public event PropertyChangedEventHandler PropertyChanged;
protected bool SetProperty<T>(ref T storage, T value, [CallerMemberName] String propertyName = null)
{
if (Equals(storage, value))
{
return false;
}
storage = value;
OnPropertyChanged(propertyName);
return true;
}
protected void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}

主窗口的XAML：

<Window x:Class="WpfApp1.MainWindow"
...>
<Window.DataContext>
<local:MainWindowViewModel />
</Window.DataContext>
<StackPanel>
<local:UserControl1 Text="{Binding Values[0]}"
MouseDoubleClick="UserControl1_MouseDoubleClick">    
</local:UserControl1>
<local:UserControl1 Text="{Binding Values[1]}"
MouseDoubleClick="UserControl1_MouseDoubleClick"> 
</local:UserControl1>
<local:UserControl1 Text="{Binding Values[2]}"
MouseDoubleClick="UserControl1_MouseDoubleClick"> 
</local:UserControl1>
<local:UserControl1 Text="{Binding Values[3]}"
MouseDoubleClick="UserControl1_MouseDoubleClick"> 
</local:UserControl1>
</StackPanel>
</Window>

主窗口后面的代码：

private void UserControl1_MouseDoubleClick(object sender, MouseButtonEventArgs e)
{
if (sender is UserControl1)
{
// why is (sender as UserControl1).Text null?
MessageBox.Show("Text is: " + (sender as UserControl1).Text);
}
}

主窗口的视图模型：

class MainWindowViewModel : BindableBase
{
public MainWindowViewModel()
{
Values = new ObservableCollection<string>();
Values.Add("first string");
Values.Add("second string");
Values.Add("third string");
Values.Add("fourth string");
}
#region Values
private ObservableCollection<string> values;
public ObservableCollection<string> Values
{
get { return values; }
set { SetProperty(ref values, value); }
}
#endregion
}

Answer 1

下面是您的UserControl代码隐藏的样子。您不需要实现INotifyPropertyChanged。

有关所有详细信息，请参阅自定义依赖项属性。具体来说，您必须从Text属性包装器的getter和setter调用GetValue和SetValue(而不是其他(。

public partial class UserControl1 : UserControl
{
public UserControl1()
{
InitializeComponent();
}
public static readonly DependencyProperty TextProperty =
DependencyProperty.Register(
nameof(Text), typeof(string), typeof(UserControl1));
public string Text
{
get { return (string)GetValue(TextProperty); }
set { SetValue(TextProperty, value); }
}
}

对于在自己的XAML中绑定到UserControl的Text属性，可以使用RelativeSource而不是ElementName来保存无用的生成类成员：

<UserControl x:Class="WpfApp1.UserControl1" ...>    
...
<TextBlock Text="{Binding Text,
RelativeSource={RelativeSource AncestorType=UserControl}}" .../>
...
</UserControl>