ComboBox(以前的DataContext) SelectedItem属性在UWP中设置为空值？

Question

最初，唐纳德·特朗普( ComboBox DataContext )以Profession1和SelectedValue为政治家。在运行时，我将Datacontext更改为Profession2。这样做就是将Profession1更改为null。

请参阅以下代码：

<Page.Resources>
    <local:MainPageViewModel x:Key="datacontent"></local:MainPageViewModel>
</Page.Resources>

<ComboBox x:Name="comboBox"
    ItemsSource="{Binding Professions,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"
    SelectedItem="{Binding Profession, Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"
    Width="100"
    Height="100"                              
    VerticalAlignment="Center"
    HorizontalAlignment="Stretch" />

代码背后：

var datacontent = (this.Resources["datacontent"] as MainPageViewModel);
this.comboBox.DataContext = datacontent.Profession1;

型号：

public class MainPageViewModel
{       
    public MainPageViewModel()
    {
        Profession1 = new Person();        
        Profession2 = new Person();
    }
    private Person profession1;

    public Person Profession1
    {
        get { return profession1; }
        set { this.profession1 = value; }
    }

    private Person profession2;

    public Person Profession2
    {
        get { return profession2; }
        set { this.profession2 = value; }
    }
}

public class Person : INotifyPropertyChanged
{       
    public Person()
    {
        _professions = new List<string>();
        _professions.Add("Lawyer");
        _professions.Add("Politician");
        _professions.Add("Other");
    }

    private string _profession;
    public string Profession
    {
        get
        {
            if (string.IsNullOrWhiteSpace(_profession))
            {
                // _profession = _professions.LastOrDefault();
            }
            return _profession;
        }
        set
        {
            if (_profession != value)
            {
                _profession = value;
                NotifyPropertyChanged("Profession");
            }
        }
    }      

    private List<string> _professions;

    public List<string> Professions
    {
        get
        {
            return _professions;
        }
    }
}

我使用了下面的代码来检查以前的datacontext (Profession1 1->Professon)值。

码

((this.Resources["datacontent"] as MainPageViewModel).Profession1 as Person).Profession 

输出为: null。期望值:政治家

请有人建议一下。

Answer 1

((this.Resources["datacontent"] as MainPageViewModel).Profession1 as Person).Profession 输出为: null。期望值:政治家请有人提出这个建议。

问题是，当您修改DataContext of combobox时，首先将DataContext设置为null，然后转到Profession2。因此，Profession属性Profession1将被设置为null。根据您的要求，您可以设置解决此问题的判断条件。

public string Profession
{
    get
    {

        return _profession;
    }
    set
    {
        if (_profession != value && value != null)
        {
            _profession = value;
            OnPropertyChange();
        }
    }
}