Question

我已经在这里上传了示例项目：https://www.file-upload.net/download-13252079/WpfApp1.zip.html 这是一个WPF窗口，其中包含多个相同的用户控件。 UserControl具有一个名为“ Text”的依赖项属性，该属性绑定到MainWindowViewModel的属性，并成功显示在UserControl的TextBlock中。但是，如果我双击UserControl并希望它提供其依赖项属性的值，则该值为null。为什么是这样？非常感谢您的帮助！

编辑：对不起，这是一些源代码： 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

这是您的UserControl后面代码的外观。您不需要实现INotifyPropertyChanged。

有关所有详细信息，请参见Custom Dependency Properties。具体来说，您必须从GetValue属性包装程序的getter和setter调用SetValue和Text（并且没有其他方法）。

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); }
    }
}

要在自己的XAML中绑定到UserControl的Text属性，可以使用RelativeSource而不是ElementName来保存生成的无用的类成员：

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

窗口中的WPF用户控件-双击时依赖项属性值为null

1 个答案: