RelativeSource Binding Tooltip to UserControl DependencyProperty Error

I have a custom usercontrol that does a little animation on a DispatcherTimer tick, as well as update a DependencyProperty of that usercontrol :

public partial class EggCounter : UserControl
{
  DispatcherTimer eggTimer;

  public EggCounter()
  {
     // Required to initialize variables
     InitializeComponent();

     eggTimer = new DispatcherTimer();
     eggTimer.Interval = TimeSpan.FromSeconds(5);
     eggTimer.Tick += new EventHandler(eggTimer_Tick);
     eggTimer.Start();

     Eggs = 0;
  }

  void eggTimer_Tick(object sender, EventArgs e)
  {
  开发者_开发百科   Eggs += 4;
     Pop.Begin();
     mePop.Play();
  }

  private void mePop_MediaEnded(object sender, RoutedEventArgs e)
  {
     mePop.Position = TimeSpan.FromSeconds(0);
  }

  /// <summary>
  /// The <see cref="Eggs" /> dependency property's name.
  /// </summary>
  public const string EggsPropertyName = "Eggs";
  /// <summary>
  /// Gets or sets the value of the <see cref="Eggs" />
  /// property. This is a dependency property.
  /// </summary>
  public int Eggs
  {
     get
     {
        return (int)GetValue(EggsProperty);
     }
     set
     {
        SetValue(EggsProperty, value);
     }
  }
  /// <summary>
  /// Identifies the <see cref="Eggs" /> dependency property.
  /// </summary>
  public static readonly DependencyProperty EggsProperty = DependencyProperty.Register(EggsPropertyName, typeof(int), typeof(EggCounter), new UIPropertyMetadata(0));
}

The XAML for this code is irrelevant. Then, I place this control on my MainPage, like this :

<my:EggCounter ToolTipService.ToolTip="{Binding RelativeSource={RelativeSource Self}, Path=Eggs, StringFormat='{} Our chickens have laid {0} eggs since you have been here.'}"/>

The page loads fine, but as soon as the timer fires, I get this error :

"The given key was not present in the dictionary."

in the Eggs property's setter in my usercontrol, i.e. on this line :

SetValue(EggsProperty, value);

I've also tried an ElementBinding on the control, but get the same error. Am I doing something wrong with the dependency property?

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Your code contains UIPropertyMetaData. Silverlight does not have this class it uses just PropertyMetaData instead.

Having said that the failure mode you describe seems to indicate that your code compiles, I don't understand how even got that far.

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If use your code I don't get the error.

But I didn't had the lines

Pop.Begin();
mePop.Play();

in the TimerCallback. So propably you should seek the error in these two methods..