Question

I want to learn how to use Dependency Objects and Properties. I have created this class,

    public class TestDependency : DependencyObject
    {
        public static readonly DependencyProperty TestDateTimeProperty =
            DependencyProperty.Register("TestDateTime", 
            typeof(DateTime), 
            typeof(TestDependency), 
            new PropertyMetadata(DateTime.Now));

        public DateTime TestDateTime
        {
            get { return (DateTime) GetValue(TestDateTimeProperty); }
            set { SetValue(TestDateTimeProperty, value); }
        }
    }

The window class is like this

public partial class MainWindow : Window
{
    private TestDependency td;
    public MainWindow()
    {
        InitializeComponent();
        td = new TestDependency();
        td.TestDateTime = DateTime.Now;
    }
}

Now I want to use it to show a the current DateTime in the TextBlock which updates itself every second, by adding this to a grid

<Grid>
    <TextBlock Text="{Binding TestDateTime,ElementName=td}" Width="200" Height="200"/>
</Grid>

I can see the TextBlock, but there is no Date Time value in it at all. What am I doing wrong?

Answer 1

First of all if you want to update the display time once a second your going to need a timer to trigger an update. A DispatchTimer works works well for that.

public class TestDependency : DependencyObject
{
    public static readonly DependencyProperty TestDateTimeProperty =
        DependencyProperty.Register("TestDateTime", typeof(DateTime), typeof(TestDependency),
        new PropertyMetadata(DateTime.Now));

    DispatcherTimer timer;

    public TestDependency()
    {
        timer = new DispatcherTimer(new TimeSpan(0,0,1), DispatcherPriority.DataBind, new EventHandler(Callback), Application.Current.Dispatcher);
        timer.Start();

    }

    public DateTime TestDateTime
    {
        get { return (DateTime)GetValue(TestDateTimeProperty); }
        set { SetValue(TestDateTimeProperty, value); }
    }

    private void Callback(object ignore, EventArgs ex)
    {
        TestDateTime = DateTime.Now;
    }

}

Next we need to modify the XAML so it binds properly to the updated dependency object.

<Window.DataContext>
    <local:TestDependency/>
</Window.DataContext>
<Grid>
    <TextBlock Text="{Binding TestDateTime}" />
</Grid>

Since we set the DataContext in XAML you can actually delete all of the code behind code in the MainWindow constructor.