WPF data binding does not work

Question

This is my data binding from a string (History.current_commad) to a text box (tbCommand):

        history = new History();

        Binding bind = new Binding("Command");

        bind.Source = history;
        bind.Mode = BindingMode.TwoWay;
        bind.Path = new PropertyPath("current_command");
        bind.UpdateSourceTrigger = UpdateSourceTrigger.PropertyChanged;

        // myDatetext is a TextBlock object that is the binding target object
        tbCommand.SetBinding(TextBox.TextProperty, bind);
        history.current_command = "test";

history.current_command is changing but the text box is not being update. What is wrong?

Thanks

What does the History class look like?

dlev
– dlev

2011-08-20 06:06:33 +00:00
Commented Aug 20, 2011 at 6:06 — dlev
– dlev, Commented Aug 20, 2011 at 6:06
public class History { public string current_command; }

Charlie
– Charlie

2011-08-20 06:10:12 +00:00
Commented Aug 20, 2011 at 6:10 — Charlie
– Charlie, Commented Aug 20, 2011 at 6:10

dlev · Accepted Answer · 2011-08-20 06:23:14Z

1

The reason you don't see the change reflected in the TextBlock is because current_command is just a field, so the Binding doesn't know when it's been udpated.

The easiest way to fix that is to have your History class implement INotifyPropertyChanged, convert current_command to a property, and then raise the PropertyChanged event in the setter of your property:

public class History : INotifyPropertyChanged
{

    public event PropertyChangedEventHandler PropertyChanged;

    private void NotifyPropertyChanged(string propName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propName));
        }
    }

    private string _current_command;
    public string current_command
    {
        get
        {
            return _current_command;
        }
        set
        {
            if (_current_command == null || !_current_command.Equals(value))
            {
                // Change the value and notify that the property has changed
                _current_command = value;
                NotifyPropertyChanged("current_command");
            }
        } 
    }
}

Now, when you assign a value to the current_command, the event will fire, and the Binding will know to update its target as well.

If you find yourself with a lot of classes where you'd like to bind to their properties, you should consider moving the event and the helper method into a base class, so that you don't write the same code repeatedly.

edited Aug 20, 2011 at 6:23

answered Aug 20, 2011 at 6:12

dlev

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1 Comment

Charlie Over a year ago

Hi @dlev, can you have a look at this problem? stackoverflow.com/questions/7130143/… The reason I'm doing data binding in code is I could not get the xmlns thing right. Thanks

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