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I want to bind a TextBlock to a string which takes its value from a txt file. The string is correctly filled but its contents are not displayed.

Class file:

public partial class JokesMessageBox : Window { public JokesMessageBox() { InitializeComponent(); } public string Joke { get; set; } public string path = "data/jokes.txt"; public void ReadFile(string path) { Joke = File.ReadAllText(path); } }

XAML:

<TextBlock HorizontalAlignment="Left" Margin="22,10,0,0" TextWrapping="Wrap" Text="{Binding Joke}" VerticalAlignment="Top" Height="60" Width="309"/>

EDIT:

In the MainWindow class:

 private void btnJokesFirstScreen_Click_1(object sender, RoutedEventArgs e) { JokesMessageBox jkb = new JokesMessageBox(); jkb.Show(); jkb.ReadFile("data/jokes.txt"); }

I spent 3+ hours on google, youtube, MSDN, StackOverflow and still can't get it working. What am I missing?

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  • 1
    How/when is ReadFile called? Commented Mar 26, 2015 at 22:32
  • call readfile in constructor Commented Mar 26, 2015 at 22:33
  • I will recomend you to read a little about DataBinding. Commented Mar 26, 2015 at 22:35
  • I added where the ReadFile() is called. Commented Mar 26, 2015 at 22:41

3 Answers 3

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10

If the you need to update the binding, the property Joke must be a DependencyProperty or the Windows must implement INotifyPropertyChanged interface.

On the view, the binding needs to know Source.

Example #1 (Using DependencyProperty):

public partial class JokesMessageBox : Window { public JokesMessageBox() { InitializeComponent(); ReadFile(Path); //example call } public string Joke { get { return (string)GetValue(JokeProperty); } set { SetValue(JokeProperty, value); } } public static readonly DependencyProperty JokeProperty = DependencyProperty.Register("Joke", typeof(string), typeof(JokesMessageBox), new PropertyMetadata(null)); public const string Path = "data/jokes.txt"; public void ReadFile(string path) { Joke = File.ReadAllText(path); } }

Example #2 (Using INotifyPropertyChanged interface):

public partial class JokesMessageBox : Window, INotifyPropertyChanged { public JokesMessageBox() { InitializeComponent(); ReadFile(Path); //example call } private string _joke; public string Joke { get { return _joke; } set { if (string.Equals(value, _joke)) return; _joke = value; OnPropertyChanged("Joke"); } } public const string Path = "data/jokes.txt"; public void ReadFile(string path) { Joke = File.ReadAllText(path); } //INotifyPropertyChanged members public event PropertyChangedEventHandler PropertyChanged; protected virtual void OnPropertyChanged(string propertyName) { var handler = PropertyChanged; if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName)); } }

And the view (XAML partial):

... <TextBlock HorizontalAlignment="Left" Margin="22,10,0,0" TextWrapping="Wrap" Text="{Binding Joke,RelativeSource={RelativeSource Mode=FindAncestor,AncestorType=Window}}" VerticalAlignment="Top" Height="60" Width="309"/> ...

I hope it helps.

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1 Comment

This worked! I went with the second example. After I had read this: link I thought it would be something quick and easy. Now I looked at the other similar problems other people had posted and I noticed this INotifyPropertyChanged interface. I'll look into it. Thanks!
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When you read the contents of the file, you assign the read string to your Joke property:

Joke = File.ReadAllText(path);

The Text property of the TextBlock is indeed bound to that property (if you have properly set the data context):

Text="{Binding Joke}"

However, what is missing is that the binding cannot possibly have any idea that the property value has changed. You need to issue a notification about the property change.

There are two ways to do this that will be recognized by WPF bindings:

  • You declare your Joke property as a dependency property. This is based on some WPF infrastructure that automatically issues the change notifications.
  • You have your class implement the INotifyPropertyChanged interface. Here, you have to implement a simple interface with a PropertyChanged event, which you have to fire in your property setter while passing the name of the property as a string.
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Your class is not implementing INotifyPropertyChanged interface. So when you change property Joke TextBlock is not updated. I would do something like this:

public partial class JokesMessageBox : Window, INotifyPropertyChanged { public JokesMessageBox() { InitializeComponent(); } public event PropertyChangedEventHandler PropertyChanged; public string Joke { get; set; } public string path = "data/jokes.txt"; public void ReadFile(string path) { Joke = File.ReadAllText(path); OnPropertyChanged("Joke"); } private void OnPropertyChanged(string propertyName) { var handler = PropertyChanged; if (handler != null) { handler(this, new PropertyChangedEventArgs(propertyName)); } } }

I would also suggest you to read about MVVM patern.

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