TextBlock文本属性在更新其源Binding属性时未更新吗？

Question

I have the following code snippet in XAML.

<Grid>

    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="01*"/>
        <ColumnDefinition Width="03*"/>
        <ColumnDefinition Width="01*"/>
    </Grid.ColumnDefinitions>

    <Button Name="btnPrevious" Grid.Column="0" Content="Previous" Click="btnPrevious_Click"/>
    <TextBlock Name="txtBlockName" Grid.Column="1" Text="{Binding SelectedName, UpdateSourceTrigger=PropertyChanged}" FontSize="30" VerticalAlignment="Center" HorizontalAlignment="Center"/>
    <Button Name="btnNext" Grid.Column="2" Content="Next"  Click="btnNext_Click"/>

</Grid>

This generates the following output as in Figure1.

The code behind this is shown below.

public partial class MainWindow : Window, INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;
    public List<string> namesList = new List<string>();

    public string SelectedName
    {
        get
        {
            return namesList[1];
        }
        set
        {
            if (value != namesList[1])
            {
                namesList[1] = value;
                NotifyPropertyChanged("SelectedName");
            }
        }
    }

    public MainWindow()
    {
        InitializeComponent();

        namesList.Add("ABC");
        namesList.Add("DEF");
        namesList.Add("GHI");


        this.DataContext = this;
    }

    private void btnPrevious_Click(object sender, RoutedEventArgs e)
    {
        string str = namesList[0];
        namesList[0] = namesList[1];
        namesList[1] = str;

        this.IsEnabled = false;
    }

    private void btnNext_Click(object sender, RoutedEventArgs e)
    {
        string str = namesList[2];
        namesList[2] = namesList[1];
        namesList[1] = str;

        this.IsEnabled = false;
    }



    protected void NotifyPropertyChanged(String propertyName)
    {
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }

}

The Text property of TextBlock is bound to the SelectedName property. And it is not being updated on updating the SelectedName property. Although my class implements the INotifyPropertyChanged interface and defines its NotifyPropertyChanged behavior, it still doesn't work.

While debugging, I put some break points to watch SelectedName property value and I observe that it is updating as per requirement but the Text property of TextBlock is not updating. As shown in Figure2.

I have seen a lot of questions and solutions on Internet, but nothing solves my problem. Any help would be appreciated.

Answer 1

Of the two answers given so far, the only one that comes even close to what I think is a reasonable alternative is the second option presented in this answer (ie after "if you want to 'Update' SelectedName you could" ).

That said, it seems to me that you would do well to change your data structure so that it more closely models what is actually happening in the user interface. Your code will be a lot easier to write and understand, and will be simpler, if you take the time to do so.

In this particular example, that means that, since (it appears) you want to be able to navigate through three different text values, you should use the property simply to present the current text value, and use an index variable to keep track of which value is selected. For example:

public partial class MainWindow : Window, INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    // Don't make fields public. If you do want to expose the list
    // use a public read-only property. And even there, think carefully
    // about whether you want callers to be able to modify the list; you
    // can return a `ReadOnlyCollection<string>` that wraps your list if
    // you only want callers to be able to examine the contents, rather than
    // modify it.
    //
    // Also, make any field that you never change "readonly".
    private readonly List<string> namesList = new List<string>();

    // Here's the index that keeps track of which string is selected
    private int _index;

    public string SelectedName
    {
        get { return namesList[_index]; }
    }

    public MainWindow()
    {
        InitializeComponent();

        namesList.Add("ABC");
        namesList.Add("DEF");
        namesList.Add("GHI");

        this.DataContext = this;
    }

    private void btnPrevious_Click(object sender, RoutedEventArgs e)
    {
        if (_index > 0)
        {
            _index--;
            NotifyPropertyChanged(nameof(SelectedName));
        }

        // I don't know what you expect this to do. You are setting the window's
        // "IsEnabled" property to false, which doesn't seem useful. More likely,
        // you intend to set the "Previous" button's enabled state, but even there
        // you really only want to set it to false if the _index value is 0. If the
        // _index value had been 2 and was just set to 1, you still want the
        // "Previous" button enabled. This would actually be an excellent place for
        // you to learn how to implement an ICommand, to have its CanExecute() method
        // return a value based on the _index value (where the "Previous" ICommand
        // object would return true unless _index is 0, and the "Next" ICommand
        // object would return true unless _index is namesList.Count - 1). Then you
        // can bind the button's "Command" property to the appropriate ICommand
        // object for the button and WPF will automatically deal with enabling
        // or disabling the button according to the command's state
        this.IsEnabled = false;
    }

    private void btnNext_Click(object sender, RoutedEventArgs e)
    {
        if (_index < namesList.Count - 1)
        {
            _index++;
            NotifyPropertyChanged(nameof(SelectedName));
        }

        // See comment above.
        this.IsEnabled = false;
    }

    protected void NotifyPropertyChanged(String propertyName)
    {
        PropertyChanged?.DynamicInvoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

Note that with any of the changes proposed in either of the two other answers, you are still left with bugs related to the navigation between each text value. The initial click of a button may work, but after that your data structure would be incoherent and you would not get the results it appears that you want.

The example in this answer addresses all of those issues, along with your original concern.

Answer 2

You are never setting "SelectedName", you could've made it readonly.

PropertyChanged is never raised.

If you want the binding to notice it, you should do

NotifyPropertyChanged("SelectedName");

after you have changed the index #1 of _namesLIst,

public partial class MainWindow : Window , INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    private readonly List<string> _namesList = new List<string>();

    public string SelectedName
    {
        get { return _namesList[1]; }            
    }

    public MainWindow()
    {
        InitializeComponent();

        _namesList.Add("ABC");
        _namesList.Add("DEF");
        _namesList.Add("GHI");

        DataContext = this;
    }

    private void btnPrevious_Click(object sender, RoutedEventArgs e)
    {
        var str = _namesList[0];
        _namesList[0] = _namesList[1];
        _namesList[1] = str;
        NotifyPropertyChanged(nameof(SelectedName));
        IsEnabled = false;
    }

    private void btnNext_Click(object sender, RoutedEventArgs e)
    {
        var str = _namesList[2];
        _namesList[2] = _namesList[1];
        _namesList[1] = str;
        NotifyPropertyChanged(nameof(SelectedName));

        IsEnabled = false;
    }

    private void NotifyPropertyChanged(string propertyName)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }

}

if you want to 'Update' SelectedName you could

    public string SelectedName
    {
        get { return _namesList[1]; }
        set
        {
            if (value == _namesList[1]) return;
            _namesList[1] = value;
            NotifyPropertyChanged("SelectedName");
        }
    }

    private void btnPrevious_Click(object sender, RoutedEventArgs e)
    {
        var str = _namesList[0];
        _namesList[0] = _namesList[1];
        SelectedName = str;            
        IsEnabled = false;
    }

    private void btnNext_Click(object sender, RoutedEventArgs e)
    {
        var str = _namesList[2];
        _namesList[2] = _namesList[1];
        SelectedName = str;            
        IsEnabled = false;
    }

Answer 3

SelectedName Setter property has never been invoked. So you can update the TextBlock.Text by using BindingExpression.UpdateSource() method like below,

 private void btnPrevious_Click(object sender, RoutedEventArgs e)
    {
        string str = namesList[0];
        namesList[0] = namesList[1];
        namesList[1] = str;
        BindingExpression be = txtBlockName.GetBindingExpression(TextBlock.TextProperty);
        be.UpdateSource();

        this.IsEnabled = false;
    }