WPF custom control reported an error: the default value type does not match the type of the "ischecked" property.

Question

To write a WPF custom control, add a checkbox in it, and then register a dependency to expose it to the outside world so as to obtain the ischecked status. Now an error is reported:
Error xdg0062 default value type does not match the type of 'ischecked' property

 public Nullable<Boolean> CheckBoxIsChecked
         {
             get { return (Nullable<Boolean>)GetValue(CheckBoxIsCheckedProperty); }
             set { SetValue(CheckBoxIsCheckedProperty, value); }
         }
         // Using a DependencyProperty as the backing store for ChcckBoxIsChecked.  This enables animation, styling, binding, etc...
         public static readonly DependencyProperty CheckBoxIsCheckedProperty =
             DependencyProperty.Register("IsChecked", typeof(Nullable<Boolean>), typeof(FileControl), new PropertyMetadata("CheckBox",new PropertyChangedCallback(onCheckBoxChanged)));
         static void onCheckBoxChanged(DependencyObject sender, DependencyPropertyChangedEventArgs args)
         {
             ((FileControl)sender).OnCheckBoxValueChanged(args);
         }
         private void OnCheckBoxValueChanged(DependencyPropertyChangedEventArgs e)
         {
             bool temp = bool.Parse(e.NewValue.ToString());
             this.checkBox.IsChecked = (Nullable < Boolean >) temp;
         }

Answer 1

Try this modification:

static readonly DependencyProperty CheckBoxIsCheckedProperty =
   DependencyProperty.Register( "CheckBoxIsChecked", typeof( bool? ), typeof( FileControl ), new PropertyMetadata( null, onCheckBoxChanged  ) );