自定义UserControl中的WPF绑定

Question

我尝试创建自定义控件并遇到绑定问题。

这是我的财产：

public DateTime? SelectedDate
{
    get
    {
        return (DateTime?)GetValue(SelectedDateProperty); ;
    }
    set
    {
        SetValue(SelectedDateProperty, value);
    }
}
public static readonly DependencyProperty SelectedDateProperty = DependencyProperty.Register(
    "SelectedDate",
    typeof(DateTime?),
    typeof(ValidationDateSelector),
    new PropertyMetadata(null));

...

private void ChengeSelectedDate()
{
    if (_day != 0 && _month != 0 && _year != 0)
    {
        SelectedDate = new DateTime(_year, _month, _day);
        OnSelectedDateChenged();
    }
}

...

public event EventHandler<EventArgs> SelectedDateChanged;
private void OnSelectedDateChenged()
{
    if (SelectedDateChanged != null)
        SelectedDateChanged(this, EventArgs.Empty);
}

在XAML中绑定：

    <Ctrl:DateSelector x:Name="ValidationDateSelector"
SelectedDate="{Binding DateOfBirth, Mode=TwoWay}"
SelectedDateChanged="DatePicker_OnSelectedDateChanged"/>

所以Binding不起作用，但DateSelector.SelectedDate是prathent，我可以在代码中为DateOfBirth赋值。为什么？我的逻辑有什么不对？