Question

Control template:

    <ControlTemplate x:Key="BasicShape2">
    <StackPanel Name="sp">
        <Border Name="bd" CornerRadius="3.5" BorderThickness="1" BorderBrush="{Binding RelativeSource={RelativeSource TemplatedParent},Path=DataContext.NodeType, Converter={StaticResource NodeTypeColorConverter}, Mode=OneWay}" Height="32" Padding="1">
            <TextBlock Name="tbName" Grid.Column="1" Text="" HorizontalAlignment="Center" VerticalAlignment="Bottom" FontSize="16" />
        </Border>
    </StackPanel>
</ControlTemplate>

a class which this template will apply to:

    public class MyThumbEx : Thumb
{
    public static readonly DependencyProperty MemberInfoProperty = DependencyProperty.Register("MemberInfo", typeof(FamilyMemberInfo), typeof(MyThumbEx));
    public FamilyMemberInfo MemberInfo
    {
        get { return (FamilyMemberInfo)GetValue(MemberInfoProperty); }
        set { SetValue(MemberInfoProperty, value); }
    }

    public MyThumbEx(ControlTemplate template, FamilyMemberInfo info, Point position)
    {
        this.MemberInfo = info;
        this.DataContext = this.MemberInfo;
        this.Template = template;
    }

    public override void OnApplyTemplate()
    {
        base.OnApplyTemplate();
        this.ApplyTextContent();
    }

    public void ApplyTextContent()
    {
        TextBlock tbName = this.Template.FindName("tbName", this) as TextBlock;
        if (tbName != null)
        {
            tbName.Text = this.MemberInfo.Name;
        }
    }
}

initialize and display it on a canvas:

        public MainWindow()
    {
        InitializeComponent();
        //
        FamilyMemberInfo mi = new FamilyMemberInfo();
        mi.Name = "someone";
        mi.ID = "id1";
        MyThumbEx te = new MyThumbEx(Application.Current.Resources["BasicShape2"] as ControlTemplate, mi, new Point(0, 0));
        //
        this.cvMain.Children.Add(te);
    }

These codes work fine, but be noticed that in the control template, I have to set Path=DataContext.NodeType, not just Path=NodeType. I’m new to WPF, and I found that normally, when I did binding without using this template stuff, I didn’t need to specify the predicate ‘DataContext’, right? Why we need here?

Another thing I found is, I can comment out this.DataContext = this.MemberInfo, and change binding path to Path=MemberInfo.NodeType, the code still works fine. Could anyone explain that for me?

Thanks in advance!

Answer 1

If you dont change the DataContext manuelly, every child automatically has the DataContext of its Parent. So if your Window has f.e. the ViewModel as DataContext all of its Controls have access to the ViewModels Properties through {Binding Path=Property}.

But in case of a ControlTemplate the usual typical flow where DataContext just cascades through from the parent to child doesn’t apply here. So you have to set the DataContext first, either through Property="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=DataContext.Property}" or DataContext="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=DataContext}" Property="{Binding Path=Property}".

To your second point: It could be, that the ControlTemplate automatically uses the code-behind of its containing Element as DataContext, so you can use the code-behinds properties without setting the DataContext, but I am not 100% sure about this.