wpf依赖项绑定找不到源

我在组件加载前对checkbox的isChecked属性进行了绑定

public FileControl()
        {
            RelativeSource relativeSource = new RelativeSource();
            relativeSource.Mode = RelativeSourceMode.FindAncestor;
            relativeSource.AncestorType = typeof(FileControl);
            Binding bind = new Binding() { RelativeSource=relativeSource };
            checkBox.SetBinding(FileIsCheckedIsCheckedProperty,bind);
            InitializeComponent();
            
        }

自定义依赖项：

        public Nullable<bool> FileIsChecked
        {
            get { return (Nullable<bool>)GetValue(FileIsCheckedIsCheckedProperty); }
            set { SetValue(FileIsCheckedIsCheckedProperty, value); }
        }

        // Using a DependencyProperty as the backing store for ChcckBoxIsChecked.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty FileIsCheckedIsCheckedProperty =
            DependencyProperty.Register("FileIsChecked", typeof(Nullable<bool>), typeof(FileControl), new FrameworkPropertyMetadata(false));

然后在mainwindow中运行出现错误：
CheckBox.FileIsChecked Nullable`1 找不到源: RelativeSource FindAncestor, AncestorType='FileOperator.FileControl', AncestorLevel='1'。

我在前台XAML绑定是没有问题的，可以正常访问：

<CheckBox Name="checkBox" IsChecked="{Binding IsChecked, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type local:FileControl}}}">
                </CheckBox>