Question

I am having a very difficult time setting up a binding which I think should be easy. Help is greatly appreciated.

I have a resource dictionary named FormResource.xaml. In this dictionary contains a Style for the ScrollView that I redine the template for. The purpose is I want a wider vertical scrollbar on it.

<Style x:Key="LargeScrolling" TargetType="ScrollViewer">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="ScrollViewer">
                <Grid Background="{TemplateBinding Background}">
                    <Grid.RowDefinitions>
                        <RowDefinition Height="*"/>
                        <RowDefinition Height="Auto"/>
                    </Grid.RowDefinitions>
                    <ScrollContentPresenter x:Name="ScrollContentPresenter"
                            Margin="{TemplateBinding Padding}"
                            ContentTemplate="{TemplateBinding ContentTemplate}"/>
                    <ScrollBar x:Name="PART_VerticalScrollBar" 
                            Style="{StaticResource LargeVerticalScrollBar}" 
                            Width="{Binding ElementName=MDTForm, Path=ScrollBarWidth}"
                            IsTabStop="False"
                            Visibility="{TemplateBinding ComputedVerticalScrollBarVisibility}"
                            Grid.Column="1" Grid.Row="0" Orientation="Vertical"
                            ViewportSize="{TemplateBinding ViewportHeight}"
                            Maximum="{TemplateBinding ScrollableHeight}"
                            Minimum="0"
                            Value="{TemplateBinding VerticalOffset}"
                            Margin="0,-1,-1,-1"/>
                </Grid>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

I have a UserControl named FormControl.

public class FormControl : UserControl

I used to have this as a partial class with a XAML componenet, in which what I am trying to do worked, but I had to remove the XAML since I derive from this class in another assembly and WPF does not allow you to derive from a partial class in another assembly.

In FormControl I define a ScrollBarWidth property.

    public static readonly DependencyProperty ScrollBarWidthProperty = DependencyProperty.Register("ScrollBarWidth", typeof(double), typeof(FormControl));
    public double ScrollBarWidth
    {
        get { return (double)base.GetValue(ScrollBarWidthProperty); }
        set { base.SetValue(ScrollBarWidthProperty, value); }
    }

When I had this as a partial class in the main declaration I gave the FormControl class a Name of MDTForm, which is what I am using as the ElementName in my binding. I tried registering this name in FormClass.cs but no matter what I do the scrollbar is not picking up the property value.

Here is where I create my ScrollViewer in the FormControl class.

        _canvasScrollViewer = new ScrollViewer();
        _canvasScrollViewer.VerticalScrollBarVisibility = ScrollBarVisibility.Auto;
        _canvasScrollViewer.VerticalAlignment = VerticalAlignment.Top;
        _canvasScrollViewer.MaxHeight = Constants.ScrollViewMaxHeight;
        _canvasScrollViewer.Style = (Style)FindResource("LargeScrolling");

The only way that I got this to work was to bind to a static property. I used this for the binding.

Width="{Binding Source={x:Static form:FormControl.ScrollBarWidthP}}"

Then defined the property as such.

public static double ScrollBarWidth { get; set; }

However, I don’t want this as I can have multiple FormControl objects loaded at the same time and they may not all have the same scroll bar width property.

Answer 1

Use a RelativeSource Binding instead of ElementName:

{Binding RelativeSource={RelativeSource Mode=FindAncestor,
         AncestorType={x:Type controls:FormControl}}, Path=ScrollBarWidth}

This will walk up the visual tree at runtime to find the parent control containing the ScrollViewer, which solves both your scoping and multiple instance issues.