在将（视图的）背后代码中的依赖属性绑定到其视图模型的属性时，我在做错什么？

Question

A couple of other posts seem to indicate that data can be shared between a View's code behind and viewmodel by binding the dependency property in the code behind and property in the viemodel. Also, I have read that the DP should be in the code behind when itself is being bound in a Main Window/User Control relationship.

The following is from the code behind (SetupUC)

 public static readonly DependencyProperty UC1Property =
     DependencyProperty.Register(
     "UC1", typeof(string), typeof(SetupUC),
     new FrameworkPropertyMetadata()
     {
        PropertyChangedCallback = OnUC1Changed,
        BindsTwoWayByDefault = true
     });

    public string UC1
    {
        get { return (string)GetValue(UC1Property); }
        set
        {
            SetValue(UC1Property, value);
        }
    }

public SetupUC()
    {
        InitializeComponent();
        SetupViewModel svm = new SetupViewModel();
        this.DataContext = svm;
        Binding binding = new Binding("ViewModelStringProperty") { Source = svm, Mode = BindingMode.TwoWay };
        BindingOperations.SetBinding(this, SetupUC.UC1Property, binding);
    }

and the viewmodel (SetupViewModel)

private string _viewModelStringProperty;
    public string ViewModelStringProperty
    {
        get { return _viewModelStringProperty; }
        set
        {
            _viewModelStringProperty = value;
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs("ViewModelStringProperty"));
            }
        }
    }

public event PropertyChangedEventHandler PropertyChanged;
    public void OnPropertyChanged(string propertyName)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(propertyName));
        }
    }

public event PropertyChangedEventHandler PropertyChanged;
    public void OnPropertyChanged(string propertyName)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(propertyName));
        }
    }

In debugging, UC1 seems to always be updated correctly from the Main Window as its changes are reflected in the user control view. However, in the viewmodel, ViewModelStringProperty does not ever seem to be updated - it's always null. Full disclosure(!), the following is how UC1 is bound in the user control XAML

<TextBox x:Name="tbx1" HorizontalAlignment="Left" Height="23" Margin="159,22,0,0" TextWrapping="Wrap" Text="{Binding UC1, ElementName=root}" VerticalAlignment="Top" Width="120" RenderTransformOrigin="0.017,0.304"/>

Again, this part seems to be fine, it's getting data to ViewModelStringProperty in the viewmodel that is not.

Answer 1

To ensure your changes in your TextBox are immediately pushed to the DependencyProperty and subsequently the view-model, ensure you have UpdateSourceTrigger=PropertyChanged on your TextBox.Text binding, ie:

Text="{Binding UC1, ElementName=root, UpdateSourceTrigger=PropertyChanged}"

With this, my example worked perfectly fine.

If this doesn't help, I suggest adding two additional TextBlocks , one bound to the UC1 and one bound to ViewModelStringProperty , this will make it easier to tell which are getting updated correctly, eg:

<StackPanel>
    <TextBox Text="{Binding UC1, ElementName=Root, UpdateSourceTrigger=PropertyChanged}" />
    <TextBlock Text="{Binding UC1, ElementName=Root}" />
    <TextBlock Text="{Binding ViewModelStringProperty}" />
</StackPanel>