How can I set my data binding in a WPF ItemsControl child to use a property on the parent data context?

Question

I have a WPF ListView with a collection of RadioButtons. I want to set the GroupName of the child controls to be bound to a property on the parent data context. At the moment I am doing this by duplicating the property in each of the children's data context but that can't be right.

My XAML:

 <ListView ItemsSource="{Binding OptionItems}" >
     <ListView.ItemTemplate>
         <DataTemplate DataType="{x:Type Logging:FilterOptionsRadioListViewModel}">
            <RadioButton GroupName="{Binding GroupName}" Content="{Binding Option.Value}" Tag="{Binding Option.Key}" IsChecked="{Binding IsChecked}" Command="Logging:FilterOptionsRadioListViewModel.CheckedChangedCommand" />
         </DataTemplate>
     </ListView.ItemTemplate>
 </ListView>

GroupName is a property on the parent View Model. I currently pass this onto the child View Model (where it is also a property) in the child constructor:

var item = new FilterOptionsRadioListItemViewModel(option, this.GroupName);

What is the correct way of doing this?

Answer 1

You certainly can do that, but if you don't want to have to copy the GroupName to every child Option instance, you could instead do something like the following

// I'm imagining your viewmodels look something like this
public class ParentViewModel
{
    private ObservableCollection<Option> m_OptionItems;
    public ObservableCollection<Option> OptionItems
    {
        get { return m_OptionItems; }
        set { m_OptionItems = value; }
    }

    private string m_ParentGroupName;
    public string ParentGroupName
    {
        get { return m_ParentGroupName; }
        set
        {
            m_ParentGroupName = value;
        }
    }


}

public class Option
{
    private string m_Value;
    public string Value
    {
        get { return m_Value; }
        set
        {
            m_Value = value;
        }
    }



}

Then you can use a relative binding so you can look up the control tree to find the right datacontext, so you don't have to copy the ParentGroupName to each child Option instance.

Answer 2

If you name your window, you can traverse the root DataContext.

<Window x:Class="MyNamespace.MainWindow"
        Name="Window"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
        Title="MainWindow" Height="500" Width="725">

<ListView ItemsSource="{Binding OptionItems}" >
     <ListView.ItemTemplate>
         <DataTemplate DataType="{x:Type Logging:FilterOptionsRadioListViewModel}">
            <RadioButton GroupName="{Binding ElementName=Window, Path=DataContext.ParentGroupName}" Content="{Binding Option.Value}" Tag="{Binding Option.Key}" IsChecked="{Binding IsChecked}" Command="Logging:FilterOptionsRadioListViewModel.CheckedChangedCommand" />
         </DataTemplate>
     </ListView.ItemTemplate>
 </ListView>

Answer 3

 <ListView ItemsSource="{Binding OptionItems}" >
     <ListView.ItemTemplate>
         <DataTemplate DataType="{x:Type Logging:FilterOptionsRadioListViewModel}">
            <RadioButton GroupName="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type ListView}}, Path=DataContext.GroupName}" Content="{Binding Option.Value}" Tag="{Binding Option.Key}" IsChecked="{Binding IsChecked}" Command="Logging:FilterOptionsRadioListViewModel.CheckedChangedCommand" />
         </DataTemplate>
     </ListView.ItemTemplate>
 </ListView>