不使用XAML：

using System.Windows;
using System.Windows.Controls;
using System.Windows.Markup;
using System.Windows.Media;

namespace TrainingWpf1
{
    [ContentProperty(nameof(Children))]
    public partial class PropertyPanel : UserControl
    {

        public PropertyPanel()
        {
            //InitializeComponent();

            DockPanel panContent = new DockPanel();
            this.Children = panContent.Children;
            Border border = new Border()
            {
                Padding = new Thickness(10),
                BorderThickness = new Thickness(1),
                CornerRadius = new CornerRadius(6),
                Background = Brushes.CadetBlue,
                Child = panContent
            };

            DockPanel dockPanel = new DockPanel();
            TextBlock textBlock = new TextBlock() { Text = "Title" };
            textBlock.SetValue(DockPanel.DockProperty, Dock.Top);
            dockPanel.Children.Add(textBlock);
            dockPanel.Children.Add(border);
            Content = dockPanel;
        }

        public UIElementCollection Children
        {
            get => (UIElementCollection)GetValue(ChildrenProperty.DependencyProperty);
            private set => SetValue(ChildrenProperty, value);
        }

        public static readonly DependencyPropertyKey ChildrenProperty = DependencyProperty.RegisterReadOnly(
                nameof(Children),
                typeof(UIElementCollection),
                typeof(PropertyPanel),
                new PropertyMetadata());
    }

}

我犯了上面提到的错误。我在Studio创建的XAML中安装了模板。当您添加用户控件时，它会从两个文件创建代码：XAML和Code-Benind中的分部类。
对于默认元素，实现是不同的。C#中具有属性的单独类和具有此类模板的单独XAML主题。请查看自定义元素是如何实现的。
为了简化示例，我没有设置主题，而是在窗口资源中创建了一个默认样式。
使用者控件的C#

using System.Collections.ObjectModel;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Markup;

namespace TrainingWpf1
{
    [ContentProperty(nameof(Children))]
    public class PropertyPanel : UserControl
    {
        public PropertyPanel()
        {
            Children = new ObservableCollection<UIElement>();
        }

        public ObservableCollection<UIElement> Children
        {
            get => (ObservableCollection<UIElement>)GetValue(ChildrenProperty.DependencyProperty);
            private set => SetValue(ChildrenProperty, value);
        }

        public static readonly DependencyPropertyKey ChildrenProperty = DependencyProperty.RegisterReadOnly(
                nameof(Children),
                typeof(ObservableCollection<UIElement>),
                typeof(PropertyPanel),
                new PropertyMetadata(null));
    }

}

Windows的XAML

<Window x:Class="TrainingWpf1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:TrainingWpf1"
        mc:Ignorable="d"
        Title="MainWindow" Height="400" Width="400"
        >
    <Window.Resources>
        <Style TargetType="{x:Type local:PropertyPanel}">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate>
                        <DockPanel>
                            <TextBlock DockPanel.Dock="Top" Text="Title"/>
                            <Border Padding="10" BorderThickness="1" CornerRadius="6" Background="CadetBlue">
                                <ItemsControl ItemsSource="{Binding Children, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type local:PropertyPanel}}}">
                                    <ItemsControl.ItemsPanel>
                                        <ItemsPanelTemplate>
                                            <DockPanel/>
                                        </ItemsPanelTemplate>
                                    </ItemsControl.ItemsPanel>
                                </ItemsControl>
                            </Border>
                        </DockPanel>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </Window.Resources>
    <Grid>
        <local:PropertyPanel>
            <TextBlock x:Name="ThisProduceAnError" Text="Hello"/>
        </local:PropertyPanel>
    </Grid>
</Window>