WPF C# binding code - why doesn't this simple example work?

Question

Hi,

I've attached some WPF C# binding code - why doesn't this simple example work? (just trying to understanding binding to a custom object). That is when clicking on the button to increase the counter in the model, the label isn't updated.

<Window x:Class="testapp1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Button  Height="23" HorizontalAlignment="Left" Margin="20,12,0,0" Name="testButton" VerticalAlignment="Top" Width="126" Click="testButton_Click" Content="Increase Counter" />
        <Label Content="{Binding Path=TestCounter}" Height="37" HorizontalAlignment="Right" Margin="0,12,122,0" Name="testLabel2" VerticalAlignment="Top" BorderThickness="3" MinWidth="200"  />
    </Grid>
</Window>


namespace testapp1
{
    public partial class MainWindow : Window
    {
        public TestModel _model;

        public MainWindow()
        {
            InitializeComponent();

            InitializeComponent();
            _model = new TestModel();
            _model.TestCounter = 0;
            this.DataContext = _model;
        }

        private void testButton_Click(object sender, RoutedEventArgs e)
        {
            _model.TestCounter = _model.TestCounter + 1;
            Debug.WriteLine("TestCounter = " + _model.TestCounter);
        }
    }

    public class TestModel : DependencyObject
    {
        public int TestCounter { get; set; }
    }

}

thanks

Answer 1

TestCounter needs to be a DepenencyProperty

    public int TestCounter
    {
        get { return (int)GetValue(TestCounterProperty); }
        set { SetValue(TestCounterProperty, value); }
    }

    // Using a DependencyProperty as the backing store for TestCounter.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty TestCounterProperty =
        DependencyProperty.Register("TestCounter", typeof(int), typeof(TestModel), new UIPropertyMetadata(0));

Answer 2

For this simple example, consider using INotifyPropertyChanged and not DependencyProperties!

UPDATE If you do want to use DPs, use the propdp snippet in VS2010 or Dr WPF's snippets for VS2008?

Answer 3

One easiest way to do this is to use INotifyPropertyChanged Interface

implement it as below

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

    public MainWindow() 
    { 
        InitializeComponent(); 

        InitializeComponent(); 
        _model = new TestModel(); 
        _model.TestCounter = 0; 
        this.DataContext = _model; 
    } 

    private void testButton_Click(object sender, RoutedEventArgs e) 
    { 
        _model.TestCounter = _model.TestCounter + 1; 
         OnpropertyChanged("TestCounter")
        Debug.WriteLine("TestCounter = " + _model.TestCounter); 
    } 

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

Answer 4

You can implement the INotifyPropertyChanged interface in the System.ComponentModel namespace. I usually implement a Changed method that can take a number of property names and check for the event not being set. I do that because sometimes I have multiple properties that depend on one value and I can call one method from all of my property setters.

For instance if you had a Rectangle class with Width and Height properties and an Area read-only property that returns Width * Height, you could put Changed("Width", "Area"); in the property setter for Width.

public class TestModel : INotifyPropertyChanged
{
    int m_TestCounter;
    public int TestCounter {
        get {
            return m_TestCounter;
        }
        set {
            m_TestCounter = value;
            Changed("TestCounter");
        }
    }

    #region INotifyPropertyChanged Members

    public event PropertyChangedEventHandler PropertyChanged;

    #endregion

    void Changed(params string[] propertyNames)
    {
        if (PropertyChanged != null)
        {
            foreach (string propertyName in propertyNames)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }
        }
    }
}