CODEWITHSUNDEEP
wpfbindinguser-controls
Avrohom Yisroel
Avrohom Yisroel

Reputation: 9472

Error binding a dependency property of a user control to a property on the parent's view model

I have a user control that contains three checkboxes and three date pickers. For example, one of the date pickers on the user control looks like this (irrelevant properties like Width, etc removed for clarity)...

<telerik:RadDatePicker DisplayFormat="Long"
                       SelectedValue="{Binding DepositPaidDate, Mode=TwoWay}"/>

The view model for the control has a public property called PaidDate that is of type PaidDate (yup, the property and the class have the same name), the top-level Grid on the control has its DataContext set to the PaidDate property, and the individual controls in the Grid are bound to properties on this PaidDate object.

When this control is used on a window, and the window's code behind sets the PaidDate property on the control's VM explicitly, it all works fine. For example, I created a test window, whose constructor looked like this...

public PaidDateWindow(PaidDate paidDate, string windowTitle) {
  InitializeComponent();
  ((PaidDateControlViewModel)PaidDateCtrl.DataContext).PaidDate = paidDate;
  Title = windowTitle;
}

...and this worked just fine. I could show the window, and the data was displayed correctly.

The problem comes when I try to set this via a dependency property on the control. The dependency property in the user control's code behind looks like this...

public static readonly DependencyProperty PaidDateProperty = DependencyProperty.Register("PaidDate", typeof(PaidDate), typeof(PaidDateControl), new FrameworkPropertyMetadata(SetPaidDateStatic));

private static void SetPaidDateStatic(DependencyObject d, DependencyPropertyChangedEventArgs e) {
  (d as PaidDateControl).SetPaidDate((PaidDate)e.NewValue);
}

private void SetPaidDate(PaidDate paidDate) {
  if (DataContext != null) {
    ((PaidDateControlViewModel)DataContext).PaidDate = paidDate;
  }
}

public PaidDate PaidDate {
  get {
    return (PaidDate)GetValue(PaidDateProperty);
  }
  set {
    SetValue(PaidDateProperty, value);
  }
}

As you can see, the dependency property just passes the PaidDate object through to the view model, which has the same effect as when I did this manually in the previous bit of code.

When I try to bind this dependency property to a property on the window's view model, I get a binding error. Here is the XAML in the parent window...

<vrtSystemsUserControls:PaidDateControl
  PaidDate="{Binding Path=VRTSystem.PaidDate, Mode=TwoWay}" />

The parent window's VM contains a property called VrtSystem, and plenty of other controls on the window are bound to properties on that. VrtSystem also contains a property called PaidDate, and that is what I want to pass to the user control.

However, when I run this, I get the following binding error...

System.Windows.Data Error: 40 : BindingExpression path error:
'VRTSystem' property not found on 'object' ''PaidDateControlViewModel' (HashCode=18319327)'.
BindingExpression:Path=VRTSystem.PaidDate; DataItem='PaidDateControlViewModel' (HashCode=18319327);
target element is 'PaidDateControl' (Name=''); target property is 'PaidDate' (type 'PaidDate')

Now it looks to me as though WPF is passing the actual binding information through to the user control, instead of the PaidDate object, as the error says it is trying to find a VrtSystem property on the user control's VM. I have no idea why it would be doing that, as I thought the idea of the binding was to resolve the binding at the window level, and then send the results (ie the PaidDate object) in to the dependency property, where it would be sent to the VM.

I hope I've explained this clearly. Anyone able to see what's gone wrong?

Thanks for any help.

Upvotes: 1

Views: 971

Answers (1)

Peter Hansen
Peter Hansen

Reputation: 8907

When your binding is being resolved, it is looking for the VRTSystem property on the DataContext of the control it is being applied to.

The 'DataContext' property is being inherited by child-controls so if you set a DataContext on a Window all of its children will have the same DataContext. If however one of the children itself has a different DataContext applied, all of its children will use that.

In your case, the Window has a DataContext, but so has the UserControl. So by default all bindings on the UserControl or it's chilren, will expect to find the VRTSystem property on the UserControls DataContext which is not what you want in this case.

So to explicitly target the DataContext of the Window, you have to tell the binding, by setting its RelativeSource property like this:

{Binding Path=DataContext.VRTSystem.PaidDate, Mode=TwoWay,
         RelativeSource={RelativeSource AncestorType={x:Type Window}}}

Upvotes: 2

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