Help with some very basic WPF databinding

Question

I'm new to WPF and trying a simple example of databinding, but it's not working. My window has a TextBlock, which I bound to a property of the window object. I declared the property in code.

When running this, I see the correct value appearing in the TextBlock. There's also a button, which when clicked updates the property, except I don't see this affecting the TextBlock.

I implemented the INotifyPropertyChanged correctly, as far as I'm able to determine. I also see, when debugging, that something has subscribed to the PropertyChanged event, except it doesn't seem to do anything.

I have 2 questions:

1) Why isn't this working as expected?

2) Is there any easy way to debug during run-time what's causing this, without resorting to third-party tools? From my cursory knowledge, it seems to me the debugging support in WPF is sorely lacking.

The XAML is (not including the "standard" XAML window element):

<TextBlock Height="28" Name="label1" VerticalAlignment="Top"
       Text="{Binding Path=TheName}"
       Grid.Row="0"
       ></TextBlock>
<Button Height="23" Name="button1" VerticalAlignment="Stretch" Grid.Row="1"
Click="button1_Click">
    Button
</Button>

The code in the window class is:

    public partial class Window1 : Window
    {
        protected MyDataSource TheSource { get; set; }

        public Window1()
        {
            InitializeComponent();
            TheSource = new MyDataSource();
            TheSource.TheName = "Original";  // This works
            this.label1.DataContext = this.TheSource;
        }

        private void button1_Click(object sender, RoutedEventArgs e)
        {
            TheSource.TheName = "Changed";  // This doesn't work
        }
    }
    public class MyDataSource : INotifyPropertyChanged
    {
        string thename;
        public string TheName 
        {
            get { return thename; }
            set { thename = value; OnPropertyChanged(thename); }
        }
        public event PropertyChangedEventHandler PropertyChanged;
        protected void OnPropertyChanged(string name)
        {
            PropertyChangedEventHandler handler = PropertyChanged;
            if (handler != null)
            {
                handler(this, new PropertyChangedEventArgs(name));
            }
        }
    }
}

Brad Leach Brad Leach · Accepted Answer · 2009-05-16T22:17:02

The problem is in you "TheName" property setter. The OnPropertyChanged method call is passing the value of "thename", not "the name" of "thename". (Sorry if that doesn't make sense - the variable names used for the example conspired against us!)

The correct code would be:

string thename;
public string TheName 
{
  get { return thename; }
  set { thename = value; OnPropertyChanged("TheName"); }
}

Here is an example from MSDN.

Hope this helps!

Help with some very basic WPF databinding

3 Answers