ComboBox(Previous DataContext) SelectedItem property set as null value in UWP?

问题

Initially, ComboBox DataContext is set with Profession1 and SelectedValue as Politician. At Runtime, i changed the Datacontext to Profession2. Doing this is changing the Profession1 to null.

Please refer the below code:

<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" />

Code Behind:

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

Model:

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;
        }
    }
}

I have used the below code, to check the previous datacontext (Profession1->Professon) value .

Code

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

Output is : null. Expected value : Politician

Please someone suggest on this.

回答1:

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

Output is : null. Expected value : Politician Please someone suggest on this.

The problem is that when you modify the DataContext of combobox, the DataContext is set null first and then turns to Profession2. So the Profession property of Profession1 will be set null. For your requirement, you could set the judgment condition to solve this issue.

public string Profession
{
    get
    {

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

来源：https://stackoverflow.com/questions/46112424/comboboxprevious-datacontext-selecteditem-property-set-as-null-value-in-uwp

标签

combobox

uwp