.NET MAUI中Community Toolkit Expander抛出NullReferenceException问题

我基于.NET 7和.NET MAUI开发应用，需要在页面展示包含名称、地址及4个按钮的客户列表。为优化加载流程，使用Community Toolkit的Expander组件，将客户名称放在Expander.Header区域，其余内容放在主体部分。但使用ListView或CollectionView加载后均抛出NullReferenceException，换成VerticalStackLayout直接展示所有信息则无异常。该功能在之前的Xamarin.Forms版本中可正常运行，怀疑是.NET MAUI Community Toolkit的bug，但未找到同类问题案例。

相关XAML代码 #

xmlns:toolkit="http://schemas.microsoft.com/dotnet/2022/maui/toolkit"
...
<ListView ItemsSource="{Binding SearchResults}"
          HasUnevenRows="True"
          SeparatorVisibility="None">
    <ListView.ItemTemplate>
        <DataTemplate>
            <ViewCell x:DataType="m:ProjetoResumo">
                <toolkit:Expander>
                    <toolkit:Expander.Header>
                        <VerticalStackLayout BackgroundColor="{StaticResource WhiteColor}"
                                             Margin="7"
                                             Padding="5">
                            <Label Text="{Binding Nome}"
                                   TextColor="{StaticResource DarkBlueColor}"
                                   FontFamily="MontSemiBold"/>
                            <Image Source="expandarrow.png"
                                   HeightRequest="20"
                                   WidthRequest="20"
                                   HorizontalOptions="End">
                            </Image>
                        </VerticalStackLayout>
                    </toolkit:Expander.Header>
                    <VerticalStackLayout>
                        <Label Text="{Binding Endereco}"
                               FontSize="Caption"/>
                        <Grid ColumnDefinitions="*,*,*,*"
                              HeightRequest="32"
                              Margin="0, 5, 0, 0">
                            <ImageButton Source="info.png"
                                         Grid.Column="0"/>
                            <ImageButton Source="phone.png"
                                         Grid.Column="1"/>
                            <ImageButton Source="whatsapp.png"
                                         Grid.Column="2"/>
                            <ImageButton Source="location.png"
                                         Grid.Column="3"/>
                        </Grid>
                    </VerticalStackLayout>
                </toolkit:Expander>
            </ViewCell>
        </DataTemplate>
    </ListView.ItemTemplate>
</ListView>

异常调用栈 #

0xFFFFFFFFFFFFFFFF in Android.Runtime.JNIEnv.monodroid_debugger_unhandled_exception C#
0x1A in Android.Runtime.JNINativeWrapper._unhandled_exception at /Users/runner/work/1/s/xamarin-android/src/Mono.Android/Android.Runtime/JNINativeWrapper.g.cs:12,5 C#
0x1D in Android.Runtime.JNINativeWrapper.Wrap_JniMarshal_PP_V at /Users/runner/work/1/s/xamarin-android/src/Mono.Android/Android.Runtime/JNINativeWrapper.g.cs:23,26    C#
0xB in CommunityToolkit.Maui.Views.Expander. at /_/src/CommunityToolkit.Maui/Views/Expander/Expander.shared.cs:201,42   C#
0x6 in Microsoft.Maui.Dispatching.Dispatcher. at D:\a\_work\1\s\src\Core\src\Dispatching\Dispatcher.Android.cs:22,24    C#
0xE in Java.Lang.Thread.RunnableImplementor.Run at /Users/runner/work/1/s/xamarin-android/src/Mono.Android/Java.Lang/Thread.cs:36,6 C#
0x8 in Java.Lang.IRunnableInvoker.n_Run at /Users/runner/work/1/s/xamarin-android/src/Mono.Android/obj/Release/net7.0/android-33/mcw/Java.Lang.IRunnable.cs:84,4    C#
0x8 in Android.Runtime.JNINativeWrapper.Wrap_JniMarshal_PP_V at /Users/runner/work/1/s/xamarin-android/src/Mono.Android/Android.Runtime/JNINativeWrapper.g.cs:22,5  C#

• 升级Community Toolkit版本：异常指向Expander组件的201行，大概率是旧版本的已知bug。检查NuGet包CommunityToolkit.Maui的版本，升级到最新稳定版（如>=5.3.0），早期版本的Expander在ListView/CollectionView中存在视图初始化或上下文绑定的问题。
• 调整Expander的包裹结构：在ViewCell中给Expander套一层VerticalStackLayout，避免视图树初始化顺序导致的空引用：

  <ViewCell x:DataType="m:ProjetoResumo">
      <VerticalStackLayout>
          <toolkit:Expander>
              <!-- 原Expander内容不变 -->
          </toolkit:Expander>
      </VerticalStackLayout>
  </ViewCell>
  

• 添加绑定空值处理：确保ProjetoResumo对象的Nome、Endereco属性不会为null，或在绑定中添加空值 fallback：

  <Label Text="{Binding Nome, FallbackValue=''}" .../>
  <Label Text="{Binding Endereco, FallbackValue=''}" .../>
  

• 替换为CollectionView并移除ViewCell：CollectionView在MAUI中对复杂视图的兼容性更好，无需ViewCell包裹：

  <CollectionView ItemsSource="{Binding SearchResults}"
                  ItemsLayout="VerticalList">
      <CollectionView.ItemTemplate>
          <DataTemplate x:DataType="m:ProjetoResumo">
              <toolkit:Expander>
                  <!-- 原Expander内容不变 -->
              </toolkit:Expander>
          </DataTemplate>
      </CollectionView.ItemTemplate>
  </CollectionView>
  

• 显式设置绑定上下文：在Expander上直接指定绑定上下文，确保Header和主体部分能正确继承DataTemplate的上下文：

  <toolkit:Expander BindingContext="{Binding .}">
      <!-- 原内容 -->
  </toolkit:Expander>
  

内容的提问来源于stack exchange，提问作者Gabic