视图模型不会更新到 DataContext

Question

I have a working program where the view model feed the data to the view. This is done in the constructor and also in a Refresh() method, as shown below. I usually put a private property referring to the view model because I want to omit casting when communicating with it.

public class TheView
{
  private ViewModel TheViewModel { get; set; }

  public TheView()
  {
    TheViewModel = new ViewModel();
    DataContext = TheViewModel;
  }

  public Refresh()
  {
    TheViewModel = new ViewModel();
    DataContext = TheViewModel;
  }
}

Then, I got gready and started chasing lines. Since the constructor connects DataContext to the property TheViewModel , I figured I could just assign to the latter and the former would get it's stuff updated by itself. To my disappointment, I discovered that it wasn't so. The following get me the correct list of objects but the DataContext stays unaffected.

public class TheView
{
  private ViewModel TheViewModel { get; set; }

  public TheView()
  {
    TheViewModel = new ViewModel();
    DataContext = TheViewModel;
  }

  public Refresh() { TheViewModel = new ViewModel(); }
}

The question is why it is so. Or rather, if it's supposed to be so. I'm thinking, in case it isn't supposed to behave like this, perhaps I have issues elsewhere in the code that poofs the flow...

Answer 1

I think you do need understand that DataContext is a dependency property.

Its definition looks like this:

[DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
[Localizability(LocalizationCategory.NeverLocalize)]
public object DataContext
{
  get
  {
    return this.GetValue(FrameworkElement.DataContextProperty);
  }
  set
  {
    this.SetValue(FrameworkElement.DataContextProperty, value);
  }
}

...

    public static readonly DependencyProperty DataContextProperty = 
DependencyProperty.Register("DataContext", typeof (object...

Dependency properties are more than normal .NET properties because they support

• change notification
• property value inheritance
• providers which determine the value

In your case it is the "change notification" topic which is important.

When you (re-) assign a value to a property which is a dependency property (like DataContext ) then WPF will pick up this change automatically. In this case the change notifcation is done inside the setter of the dependency property so to speak.

If you assign to a "normal" property or even a private field this is not the case (the setter of the DataContext property is not invoked) even if it points to a DataContext.

Answer 2

You are basically re-assigning a private property, not the DataContext.

1. you create instance A of TheViewModel
2. you make the DataContext point to instance A
3. you create instance B of TheViewModel
4. you make the TheViewModel private property point to instance B
5. DataContext is still pointing to instance A

--> there is no reason why the DataContext should be refreshed. you didn't touch it. it's still pointing to the initial instance A, which is still in memory.

EDIT :

That answer was assuming the binding was using DataContext as a source, and not TheViewModel . DataContext is a DependencyProperty, whereas TheViewModel isn't. Replacing the whole view model with TheViewModel as a binding source needs to notify the view either with a dependency property or NotifyPropertyChanged