I can't Data Bind to a local variable in WPF/XAML

Question

I want a textbox to display the value of a variable when I click it (an iteration of 1 to 100), I do not know what I am doing Wrong:

When I run the project nothing i

Answer 1

You need to make the property tell the binding that it has updated. The standard way to do this is via:

1. Implementing INotifyPropertyChanged
2. Making the myText property a DependencyProperty
3. Another maybe less used way is to raise the event manually, like this:

public void Button_Click_1(object sender, RoutedEventArgs e)
{
    myText = "Clicked";
    BindingOperations.GetBindingExpressionBase(myTextBox, TextBlock.TextProperty).UpdateTarget();
}

Note that your TextBlock has the confusing name myTextBox

Answer 2

Your current myText property has no way of notifying the WPF binding system when its value has changed, so the TextBlock wont be updated.

If you make it a dependency property instead it automatically implements change notification, and the changes to the property will be reflected in the TextBlock.

So if you replace public string myText { get; set; } with all of this code it should work:

public string myText
{
    get { return (string)GetValue(myTextProperty); }
    set { SetValue(myTextProperty, value); }
}

// Using a DependencyProperty as the backing store for myText.  This enables animation, styling, binding, etc...
public static readonly DependencyProperty myTextProperty =
    DependencyProperty.Register("myText", typeof(string), typeof(Window1), new PropertyMetadata(null));

Answer 3

You should implement INotifyPropertyChanged in your "MainWindow" so your "myTextBlock" can automatically pick up changes from your data and update.

So your "MainWindow" should look like:

public partial class MainWindow : Window, INotifyPropertyChanged
{
    public MainWindow()
    {
        InitializeComponent();
    }
    private string _myText;

    public string myText { 
      get{return _myText;}
      set{_myText = value;
         if(PropertyChanged!=null) PropertyChanged(this, new PropertyChangedEventArgs("myText")) ;
      }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    etc.....
}

Answer 4

Try this:

 public partial class MainWindow : Window, INotifyPropertyChanged
    {
        public MainWindow()
        {
            InitializeComponent();
            this.DataContext = this;
        }

        public string myText { get; set; }

        public void Button_Click_1(object sender, RoutedEventArgs e)
        {
            BackgroundWorker bw = new BackgroundWorker();
            bw.DoWork += delegate
            {
                int i = 0;
                for (i = 0; i < 100; i++)
                {
                    System.Windows.Threading.Dispatcher.CurrentDispatcher.Invoke((Action)(() => { myText = i.ToString(); OnPropertyChanged("myText"); }));                    
                    Thread.Sleep(100);
                }
            };

            bw.RunWorkerAsync();
        }

        public event PropertyChangedEventHandler PropertyChanged;

        protected void OnPropertyChanged(string name)
        {
            PropertyChangedEventHandler handler = PropertyChanged;
            if (handler != null)
            {
                handler(this, new PropertyChangedEventArgs(name));
            }
        }
    }

XAML file:

  <Grid>
            <Button Content="Button" HorizontalAlignment="Left" Height="106" Margin="71,95,0,0" VerticalAlignment="Top" Width="125" Click="Button_Click_1"/>
            <TextBlock x:Name="myTextBox" 
                       HorizontalAlignment="Right" Height="106" Margin="0,95,46,0" 
                       TextWrapping="Wrap" VerticalAlignment="Top" Width="187" 
                       Text= "{Binding myText}" />

        </Grid>

Answer 5

implement INotifyPropertyChanged:

public partial class MainWindow : Window, INotifyPropertyChanged
    {
        public MainWindow()
        {
            this.InitializeComponent();
        }

        private string _txt;
        public string txt
        {
            get
            {
                return _txt;
            }
            set
            {
                if (_txt != value)
                {
                    _txt = value;
                    OnPropertyChanged("txt");
                }
            }
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            txt = "changed text";
        }

        public event PropertyChangedEventHandler PropertyChanged;

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

XAML:

<TextBox Text="{Binding txt}"/>
<Button Click="Button_Click">yes</Button>

and don't forget about adding the DataContext property of your window:

<Window ... DataContext="{Binding RelativeSource={RelativeSource Self}}"/>