为何ComboBox双向绑定源属性时初始SelectedItem为0？

我需要让ComboBox从ItemSource集合中自动选中一个类型匹配的初始项，目前遇到的问题是：

• 源属性已在ViewModel构造函数中设置，且显式触发了属性变更通知（已安装PropertyChanged.Fody 4.1.0，原本无需手动触发）
• 手动选择ComboBox选项时，源属性能正常更新，但初始状态下SelectedItem始终显示为0，ComboBox未选中任何项

初始实现代码 #

UxSettingKeyValue构造函数 #

SelectedItem = Setting.Available.FirstOrDefault(e => e.IsChecked);

// Setting.Available 类型为 IEnumerable<SettingState>

ViewModel构造函数 #

var setting = _provider.MakeByKey(ClimateCondition.Pm25.AsKey(), ClimateCondition.Pm25);
var caption = _languageService.GetResource(setting.Key);
    
Pm25 = new UxSettingKeyValue(caption, setting);
    
SelectedItem = Pm25.SelectedItem;
     
RaisePropertyChanged(nameof(SelectedItem)); // 因已安装PropertyChanged.Fody 4.1.0，本无需此操作

ViewModel的SelectedItem属性 #

internal SettingState SelectedItem
{
    get;set;
}

初始XAML代码 #

<ComboBox
    ItemsSource="{x:Bind ViewModel.Pm25.Setting.Available}"
    SelectedItem="{x:Bind ViewModel.SelectedItem, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
    DisplayMemberPath="Caption"/>

初始状态表现 #

• 初始时ComboBox未显示任何选中项
• 实时属性资源管理器显示SelectedItem值为0
• ItemSource已正确初始化，下拉展开后能正常看到所有SettingState选项
• 已将所有异步操作改为同步调试，问题依旧

按建议修改后的实现（改用Loaded事件初始化） #

根据建议，将初始化逻辑从ViewModel构造函数移到ContentDialog的Loaded事件中，但问题仍未解决：

ContentDialog XAML（注册Loaded事件） #

<ContentDialog 
    x:Class="Weather.History.Mvvm.Views.SettingsDialog"
    xmlns:prismmvvm="using:Prism.Mvvm"
    xmlns:mycnv="using:Weather.History.Mvvm.ValueConverter"
    prismmvvm:ViewModelLocator.AutowireViewModel="True"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:myctr="using:Weather.History.UC"
    xmlns:myux="using:Weather.History.Mvvm.Model"
    xmlns:wctk="using:CommunityToolkit.WinUI.Controls"
    Loaded="OnContentDialogLoaded"
    PrimaryButtonText="Close"
    PrimaryButtonClick="OnClose">

页面Loaded事件处理程序 #

private async void OnContentDialogLoaded(object sender, Microsoft.UI.Xaml.RoutedEventArgs e)
{
        await ViewModel.LoadSettingsAsync();
}

ViewModel的LoadSettingsAsync初始化方法 #

internal Task LoadSettingsAsync()
{
    var dispatcher = DispatcherQueue.GetForCurrentThread();

    return Task.Run(async () =>
    {
        var setting = _provider.MakeByKey(ClimateCondition.AirPressure.AsKey(), ClimateCondition.AirPressure);
        var caption = _languageService.GetResource(setting.Key);


        setting = _provider.MakeByKey(ClimateCondition.Pm25.AsKey(), ClimateCondition.Pm25);
        caption = _languageService.GetResource(setting.Key);

        dispatcher.EnqueueAsync(() => Pm25 = new UxSettingKeyValue(caption, setting), DispatcherQueuePriority.Normal);

        setting = _provider.MakeByKey(ClimateCondition.Pm10.AsKey(), ClimateCondition.Pm10);
        caption = _languageService.GetResource(setting.Key);

        dispatcher.EnqueueAsync(() => Pm10 = new UxSettingKeyValue(caption, setting), DispatcherQueuePriority.Normal);

        // ... 其他设置项初始化逻辑

        await SeekMyHomeAsync(dispatcher);
    });
}

调整后的ComboBox XAML（绑定到Pm25自身的SelectedItem） #

<wctk:SettingsCard Description="{x:Bind ViewModel.Pm25.Caption}"
                   Header="PM 25"
                   HeaderIcon="{winui3:FontIcon GlyphName=Accept}">
    <ComboBox ItemsSource="{x:Bind ViewModel.Pm25.Setting.Available, Mode=OneWay}"
              SelectedItem="{x:Bind ViewModel.Pm25.SelectedItem, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
              DisplayMemberPath="Caption"/>
</wctk:SettingsCard>

修改后问题依旧 #

ItemSource能正常加载并显示下拉选项，但SelectedItem始终显示为0，ComboBox无初始选中项，直到手动选择后才会更新。

内容的提问来源于stack exchange，提问作者Tommy