Datepicker Watermark

On my current project, we make use of the WPF Toolkit. One of the things we use is the Datepicker. A nice usefull control. We wanted to set the watermark, but the control doesn't support setting the watermark with a public property and we didn't want to go into the control's source itself.

On Codeplex, we found a forum topic with a solution (thnx Spud). Derive from the Datepicker control and override the OnRender method. This works and sets the watermark correctly.

But after a date is selected, the value was cleared and the control loses the focus, the default watermark returns again. I fixed this by using the LostFocus event on the textbox. The result is this:

 
public class SottjeDatePicker : DatePicker
{
    private FieldInfo _textboxFieldInfo;
    private DatePickerTextBox _dateTextBox;

    /// 

    /// Overrides the OnRender methods
    /// 
    /// 
    protected override void OnRender(DrawingContext drawingContext)
    {
        base.OnRender(drawingContext);
        _textboxFieldInfo = typeof(DatePicker).GetField("_textBox"
            , BindingFlags.Instance | BindingFlags.NonPublic);

        if (_textboxFieldInfo != null)
        {
            _dateTextBox = (DatePickerTextBox)_textboxFieldInfo.GetValue(this);

            // Add event to update watermark when needed on lostfocus.
            _dateTextBox.LostFocus += OnDateTextBoxLostFocus;

            UpdateWatermark();
        }
    }

    /// 

    /// Called when textbox loses focus.
    /// 
    /// The sender.
    /// The instance containing the event data.
    private void OnDateTextBoxLostFocus(object sender
                    , System.Windows.RoutedEventArgs e)
    {
        if (_dateTextBox != null && string.IsNullOrEmpty(_dateTextBox.Text))  
        {
            UpdateWatermark();
        }
    }

    /// 

    /// Updates the watermark.
    /// 
    private void UpdateWatermark()
    {
        PropertyInfo watermarkPropertyInfo = 
            typeof(DatePickerTextBox).GetProperty("Watermark"
                , BindingFlags.Instance | BindingFlags.NonPublic);

        if (watermarkPropertyInfo != null)
        {
            string watermark = Properties.Resources.DatePickerWatermark;
            watermarkPropertyInfo.SetValue(_dateTextBox, watermark, null);
        }
    }
}

Have fun!