c# - 附加属性 + 样式 -> ArgumentNullException

Question

我创建了一个非常简单的附加属性:

public static class ToolBarEx 
{
    public static readonly DependencyProperty FocusedExProperty =
        DependencyProperty.RegisterAttached(
            "FocusedEx", typeof(bool?), typeof(FrameworkElement),
            new FrameworkPropertyMetadata(false, FocusedExChanged));

    private static void FocusedExChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        if (d is ToolBar)
        {
            if (e.NewValue is bool)
            {
                if ((bool)e.NewValue)
                {
                    (d as ToolBar).Focus();
                }
            }
        }
    }

    public static bool? GetFocusedEx(DependencyObject obj)
    {
        return (bool)obj.GetValue(FocusedExProperty);
    }

    public static void SetFocusedEx(DependencyObject obj, bool? value)
    {
        obj.SetValue(FocusedExProperty, value);
    }
}

在 Xaml 中设置它非常好，但如果我尝试在样式中设置它:

我在运行时收到一个 ArguemntNullException(说:“值不能为空。参数名称:property").

我不知道这里出了什么问题。任何提示都适用!

Answer 1

注册附加依赖属性时常犯的一个错误是错误地指定了 ownerType 参数。这必须始终是注册类，ToolBarEx 这里:

public static readonly DependencyProperty FocusedExProperty =
    DependencyProperty.RegisterAttached(
        "FocusedEx", typeof(bool?), typeof(ToolBarEx),
        new FrameworkPropertyMetadata(false, FocusedExChanged));

为了避免属性更改处理程序中出现不必要的代码，您可以安全地将 NewValue 转换为 bool:

private static void FocusedExChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    var toolBar = d as ToolBar;
    if (toolBar != null && (bool)e.NewValue)
    {
        toolBar.Focus();
    }
}