DependencyObject DateTime Format Change - wpf

Question

I am using a custom datepicker and i am trying to change the dateformat to show Year only via a dependency property. After some experiments i got a Nullable object must have a value exception. I am using the code below:
public bool YearOnly
{
get { return (bool)this.GetValue(YearOnlyProperty); }
set { this.SetValue(YearOnlyProperty, value); }
}
public static readonly DependencyProperty YearOnlyProperty =
DependencyProperty.Register("YearOnly",
typeof(bool),
typeof(CustomizableDatePicker),
new FrameworkPropertyMetadata(false, new PropertyChangedCallback(OnYearOnlyChanged)));
private static void OnYearOnlyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
CustomizableDatePicker control = (CustomizableDatePicker)d;
control.SelectedDate = Convert.ToDateTime(control.SelectedDate.Value.ToString("yyyy"));
}
to change the date format and obviously it doesn't work.
I am using it from the DatePicker as: YearOnly="True".
On the other hand using the Style below formats the DatePicker great:
<Style TargetType="{x:Type DatePickerTextBox}">
<Setter Property="Control.Template">
<Setter.Value>
<ControlTemplate>
<TextBox x:Name="PART_TextBox"
Text="{Binding Path=SelectedDate, StringFormat='yyyy',
RelativeSource={RelativeSource AncestorType={x:Type DatePicker}}}" />
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
Is there a way to make this work? Thanks.

Answer 1

After hours of testing possible solutions, i came to the conclusion that a DatePicker would never accept a DateTime that only has Year. I (short of) implemented the solution that #Clemens suggested hoping to find a solution at some point for a DatePicker that accepts YearOnly DateTimes. Anyway the code i used is bellow for anyone interested. If anyone has another solution for my problem please add an Answer to this Question.
private void CustomizableDatePicker_SelectedDateChanged(object sender, SelectionChangedEventArgs e)
{
if (!this.YearOnly)
return;
this.SetValue(SelectedYearProperty, SelectedDate.Value.Year);
Binding bnd = new Binding
{
Path = new PropertyPath("SelectedDate"),
StringFormat = ("yyyy"),
RelativeSource = new RelativeSource(RelativeSourceMode.FindAncestor, typeof(CustomizableDatePicker), 1)
};
this.innerTextBox.SetBinding(TextBox.CustomizableTextBox.TextProperty, bnd);
}
.................
#region YearOnly DependencyProperty
public bool YearOnly
{
get { return (bool)this.GetValue(YearOnlyProperty); }
set { this.SetValue(YearOnlyProperty, value); }
}
public static readonly DependencyProperty YearOnlyProperty =
DependencyProperty.Register("YearOnly",
typeof(bool),
typeof(CustomizableDatePicker),
new FrameworkPropertyMetadata(false));
#endregion
#region SelectedYear DependencyProperty
public int SelectedYear
{
get { return (int)this.GetValue(SelectedYearProperty); }
set { this.SetValue(SelectedYearProperty, value); }
}
public static readonly DependencyProperty SelectedYearProperty =
DependencyProperty.Register("SelectedYear",
typeof(int),
typeof(CustomizableDatePicker));
#endregion