WPF: Binding a page control to parent window's property

Question

Alright so I want to bind the Foreground property of a TextBlock control to a property in the codebehind of the MainWindow. The control is on a page which is loaded in a frame on the MainWindow. I have tried a few things, but the binding I would think should work, as demonstrated, does not.

The mainwindow and Page xaml:

<MainWindow>
    <Frame></Frame>
</MainWindow>

<Page>
    <TextBlock Foreground="{Binding RelativeSource={
        RelativeSource FindAncestor, AncestorType={
        x:Type local:MainWindow}}, Path=TextBrush}" />
    <!-- or Foreground="{Binding TextBrush, RelativeSource={
        RelativeSource FindAncestor, AncestorType={
        x:Type local:MainWindow}}}" /> -->
</Page>

And the mainwindow codebehind:

public partial class MainWindow : INotifyPropertyChanged {
private Brush _textBrush;
public Brush TextBrush
    {
        get => _textBrush;
        set
        {
            _textBrush = value;
            OnPropertyChanged("TextBrush");
        }
    }

public ICommand SwitchToDarkmodeCommand
    {
        get
        {
            return new DelegateCommand(() =>
            {
                TextBrush = Brushes.White;
                BackgroundBrush = (Brush)new BrushConverter().ConvertFromString("#D2D2D2");
            });
        }
    }

It seems way too simple. What am I doing wrong here?

Edit: the output. Good point.

System.Windows.Data Error: 4 : Cannot find source for binding with reference 
'RelativeSource FindAncestor, AncestorType='AddINDesignerWPF.MainWindow', AncestorLevel='1''. 
BindingExpression:Path=TextBrush; DataItem=null; target element is 'TextBlock' (Name=''); 
target property is 'Foreground' (type 'Brush')

You want `{RelativeSource AncestorType=Window}`. With RelativeSource, use the type of the element in the XAML: ``, ``, etc. — 15ee8f99-57ff-4f92-890c-b56153, Jan 05 '18 at 17:12

score 0 · Answer 1 · answered Jan 05 '18 at 14:41

0

try to use dependency property instead of normal property:

  public static readonly DependencyProperty TextBrushProperty =
  DependencyProperty.Register("TextBrush", typeof(Brush), typeof(MainWindow), new UIPropertyMetadata());

public Brush TextBrush
{
    get { return (Brush)this.GetValue(TextBrushProperty); }
    set { this.SetValue(TextBrushProperty, value); }
}

answered Jan 05 '18 at 14:41

Luke

58
6

It's true that OP should be using dependency properties rather than INPC properties on visual elements like windows, but INPC properties do work. This is not going to fix the issue. – 15ee8f99-57ff-4f92-890c-b56153 Jan 05 '18 at 17:15

score 0 · Accepted Answer · answered Jan 05 '18 at 21:15

I just spent some time looking through decompiled WPF code, the reason why the binding fails is that the visual tree somehow stops right before the frame when walking up from the TextBlock in the Page. Thus, the RelativeSource binding with FindAncestor cannot find the source property, no matter whether it is a dependency or INPC property, and no matter whether you use Window or MainWindow as Type.

The reason for this behavior probably is that the frame control isolates its content from the rest of the application, since it is meant for external content you might not trust. This SO post gives some more information, and also offers a solution how to pass the data context to the page in the frame.

That said, the solution for you might be not to use a Frame/Page at all, depending on what you want to achieve in the first place.

Wow. I read the post and that is not at all what I had expected, though it does make sense seeing as though the content in the frame could be webcontent. Thank you for your answer! — NullError, Jan 08 '18 at 12:39

WPF: Binding a page control to parent window's property

2 Answers2

Linked