具有通用所有者类的依赖项属性的定义是否不正确？

Question

我有一个自定义通用用户控件，我希望有一个名为AdditionalContent的内容属性。删除所有非错误特定代码的减少实现是。

[ContentProperty("AdditionalContent")]
public class WgReactiveUserControl<TViewModel> : UserControl
        where TViewModel : class
{
        public UIElement AdditionalContent
        {
            get { return ( UIElement ) GetValue(AdditionalContentProperty); }
            set { SetValue(AdditionalContentProperty, value); }
        }

        public static readonly DependencyProperty AdditionalContentProperty =
            DependencyProperty
              .Register("AdditionalContent", 
                          typeof(UIElement),                       
                          typeof(WgReactiveUserControl<TViewModel>),
              new PropertyMetadata(null));
}

我在设计器中遇到以下错误。

引用此代码。

纯文本代码是

<w:MeasuredViewportLayoutHelpPanelBase 
         x:Class="Weingartner.EyeshotExtensions.MeasuredViewportLayoutHelpPanel"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
         xmlns:w="clr-namespace:Weingartner.EyeshotExtensions"
         mc:Ignorable="d"
         d:DesignHeight="300" d:DesignWidth="300"
         d:DataContext="{d:DesignInstance w:MeasuredViewportLayoutHelpPanelModel, d:IsDesignTimeCreatable=false}">


    <Border BorderThickness="1" Background="#C0333333" BorderBrush="#C0D4D4D4" Margin="20">

    </Border>

</w:MeasuredViewportLayoutHelpPanelBase>

代码背后是

public class MeasuredViewportLayoutHelpPanelModel 
{ }

public class MeasuredViewportLayoutHelpPanelBase 
  : WgReactiveUserControl<MeasuredViewportLayoutHelpPanelModel>
{ }

public partial class MeasuredViewportLayoutHelpPanel 
  : MeasuredViewportLayoutHelpPanelBase
{
}

我无法弄清楚依赖属性AdditionalContent的声明可能会导致设计器发出此错误的错误。