在ResourceDictionary中为BindableProperty赋值自定义类实例遇空引用异常

自定义Colors类代码 #

namespace Foo {
  public class Colors {
    public Color Positive { get; set; }
    public Color Negative { get; set; }
    public Color Neutral { get; set; }

    public Colors() { }
  }
}

Bar控件（包含BindableProperty）代码 #

namespace Foo {
  public class Bar : ContentView {
    public Colors Colors {
      get => (Colors)GetValue(ColorsProperty);
      set => SetValue(ColorsProperty, value);
    }

   public static BindableProperty ColorsProperty = 
            BindableProperty.Create(nameof(Colors), typeof(Colors), 
            typeof(Bar), null, BindingMode.OneWay);
  }
}

styles.xaml代码 #

<?xml version="1.0" encoding="utf-8" ?>
<ResourceDictionary xmlns="http://xamarin.com/schemas/2014/forms"
                    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
                    xmlns:foo="clr-namespace:Foo;assembly=Foo">
  <foo:Colors
        x:Key="colors"
        Positive="Green"
        Negative="Red"
        Neutral="Blue"/>

  ...

  <Style TargetType="foo:Bar">
    <Setter Property="Colors" Value="{StaticResource colors}"/>
  </Style>
</ResourceDictionary>

异常现象与详情 #

调试时Colors类构造函数断点已触发，说明实例已创建，但加载styles.xaml时触发Mono.Android中的未处理NullReferenceException，移除该属性赋值则运行正常。异常发生在App.xaml.g.cs中global::Xamarin.Forms.Xaml.Extensions.LoadFromXaml(this, typeof(App));语句之后。

异常栈信息：

System.NullReferenceException: Object reference not set to an instance of an object.
  at Android.Runtime.JNINativeWrapper._unhandled_exception (System.Exception e) [0x0000e] in /Users/runner/work/1/s/xamarin-android/src/Mono.Android/Android.Runtime/JNINativeWrapper.g.cs:12 
  at Android.Runtime.JNINativeWrapper.Wrap_JniMarshal_PPL_V (_JniMarshal_PPL_V callback, System.IntPtr jnienv, System.IntPtr klazz, System.IntPtr p0) [0x0001d] in /Users/runner/work/1/s/xamarin-android/src/Mono.Android/Android.Runtime/JNINativeWrapper.g.cs:111 
  at (wrapper native-to-managed) Android.Runtime.JNINativeWrapper.Wrap_JniMarshal_PPL_V(intptr,intptr,intptr)

解决方案 #

• 将BindableProperty字段改为公共静态只读，并将默认值从null改为new Colors()，符合Xamarin.Forms规范同时避免初始空值问题：

  public static readonly BindableProperty ColorsProperty = 
          BindableProperty.Create(nameof(Colors), typeof(Colors), 
          typeof(Bar), new Colors(), BindingMode.OneWay);
  

• 检查Bar控件内部处理Colors属性的逻辑（如PropertyChanged回调、渲染逻辑），添加空值判断，避免空引用访问。
• 给Colors类的属性设置默认Color值，防止XAML初始化后属性仍为空：

  public class Colors {
    public Color Positive { get; set; } = Color.Green;
    public Color Negative { get; set; } = Color.Red;
    public Color Neutral { get; set; } = Color.Blue;
  
    public Colors() { }
  }
  

• 清理项目的bin/obj文件夹，重新生成解决方案，清除XAML编译缓存。
• 确认ResourceDictionary中colors资源的定义在Style之前，避免静态资源查找失败。

内容的提问来源于stack exchange，提问作者Björn Albowitz