自定义控件定义一个Border的外部属性，要求类型为Border，外部Border传入时才展示Border public partial class CustomFont : ItemsControl, INotifyPropertyChanged { public CustomFont() { InitializeComponent(); } private string title2 = string.Empty; /// <summary> /// 展示文字 /// </summary> public string Title2 { get { return title2; } set { title2 = value; OnPropertyChanged("Title2"); } } private bool showUnderline = false; // ... public bool ShowUnderline { get { return showUnderline; } set { showUnderline = value; OnPropertyChanged("ShowUnderline"); } } private double characterSpacing = 0; /// <summary> /// 字符间距 /// </summary> public double CharacterSpacing { get { return characterSpacing; } set { characterSpacing = value; OnPropertyChanged("CharacterSpacing"); } } public event PropertyChangedEventHandler PropertyChanged; protected void OnPropertyChanged(string propertyName) { if (PropertyChanged != null) { PropertyChanged(this, new PropertyChangedEventArgs(propertyName)); } } }

如果你想在自定义控件中定义一个 Border 类型的属性，可以在自定义控件的类中添加一个名为 Border 的属性，并在属性的 Set 方法中进行相应的操作。具体实现代码如下：

public partial class CustomControl : Control
{
    public static readonly DependencyProperty BorderProperty =
        DependencyProperty.Register("Border", typeof(Border), typeof(CustomControl), new PropertyMetadata(null));

    public Border Border
    {
        get { return (Border)GetValue(BorderProperty); }
        set { SetValue(BorderProperty, value); }
    }

    protected override void OnRender(DrawingContext drawingContext)
    {
        base.OnRender(drawingContext);

        if (Border != null)
        {
            Border.Measure(new Size(double.PositiveInfinity, double.PositiveInfinity));
            Border.Arrange(new Rect(new Point(0, 0), Border.DesiredSize));
            drawingContext.DrawRectangle(Border.Background, new Pen(Border.BorderBrush, Border.BorderThickness), new Rect(0, 0, Border.DesiredSize.Width, Border.DesiredSize.Height));
        }
    }
}

在 XAML 中使用该自定义控件时，可以在外部定义一个 Border 控件，并将它赋值给 CustomControl 的 Border 属性，如下所示：

<Window ...>
    <Grid>
        <Border x:Name="myBorder" BorderThickness="2" BorderBrush="Red">
            <TextBlock Text="Hello, World!" />
        </Border>
        <local:CustomControl Border="{Binding ElementName=myBorder}" />
    </Grid>
</Window>

这样，在 CustomControl 中就可以显示外部定义的 Border 了。注意，这里使用了数据绑定，需要在 CustomControl 的代码中实现 INotifyPropertyChanged 接口。