WinUI3中DoubleCollection类型Dependency Property致应用崩溃

问题情况 #

我在WinUI 3里做了个叫testControl的自定义控件，定义了LineStrokeDashArray这个依赖属性，代码是这样的：

public class testControl : Control
{
    public DoubleCollection LineStrokeDashArray
    {
        get { return (DoubleCollection)GetValue(LineStrokeDashArrayProperty); }
        set { SetValue(LineStrokeDashArrayProperty, value); }
    }

    public static readonly DependencyProperty LineStrokeDashArrayProperty =
        DependencyProperty.Register("LineStrokeDashArray", typeof(DoubleCollection), typeof(testControl), new PropertyMetadata(default(DoubleCollection)));
}

然后在控件的ControlTemplate里绑定了这个属性：

<Line 
       VerticalAlignment="Center"
       Stretch="Fill"
       Stroke="{TemplateBinding LineStroke}"
       StrokeDashArray="{TemplateBinding LineStrokeDashArray}"
       StrokeEndLineCap="Square"
       StrokeStartLineCap="Square"
       StrokeThickness="{TemplateBinding LineStrokeThickness}"
       X2="1" />

结果用<local:testControl LineStrokeDashArray="2,2" />调用控件时，直接崩了。我试过把属性元数据改成new PropertyMetadata(new DoubleCollection())，还是崩，而且WinUI 3没给详细报错信息，只显示崩溃界面。

问题原因 #

WinUI 3的XAML解析器没办法自动把字符串"2,2"转换成DoubleCollection类型，直接赋值会触发类型转换异常或者空引用，导致应用崩溃。另外如果在PropertyMetadata里直接初始化new DoubleCollection()，所有控件实例会共享同一个集合，也可能引发未知问题。

修复方案 #

1. 给属性加类型转换器 #

让XAML能正确解析字符串到DoubleCollection，修改依赖属性定义和属性标记：

[TypeConverter(typeof(DoubleCollectionConverter))]
public DoubleCollection LineStrokeDashArray
{
    get { return (DoubleCollection)GetValue(LineStrokeDashArrayProperty); }
    set { SetValue(LineStrokeDashArrayProperty, value); }
}

public static readonly DependencyProperty LineStrokeDashArrayProperty =
    DependencyProperty.Register(
        nameof(LineStrokeDashArray),
        typeof(DoubleCollection),
        typeof(testControl),
        new PropertyMetadata(null, null, CoerceLineStrokeDashArray));

// 强制值回调，确保空值时返回空集合而非null
private static object CoerceLineStrokeDashArray(DependencyObject d, object baseValue)
{
    return baseValue ?? new DoubleCollection();
}

2. 替换TemplateBinding为RelativeSource绑定（可选） #

如果TemplateBinding还是有问题，换成TemplatedParent绑定更稳妥：

<Line 
       VerticalAlignment="Center"
       Stretch="Fill"
       Stroke="{Binding LineStroke, RelativeSource={RelativeSource TemplatedParent}}"
       StrokeDashArray="{Binding LineStrokeDashArray, RelativeSource={RelativeSource TemplatedParent}}"
       StrokeEndLineCap="Square"
       StrokeStartLineCap="Square"
       StrokeThickness="{Binding LineStrokeThickness, RelativeSource={RelativeSource TemplatedParent}}"
       X2="1" />

3. 正确初始化默认值 #

别在PropertyMetadata里直接创建集合实例，改在构造函数里初始化：

public testControl()
{
    DefaultStyleKey = typeof(testControl);
    LineStrokeDashArray = LineStrokeDashArray ?? new DoubleCollection();
}

验证 #

改完之后用<local:testControl LineStrokeDashArray="2,2" />调用，应用不再崩溃，Line控件能正常显示虚线样式。

内容的提问来源于stack exchange，提问作者Power Code