Problems with DataTemplate, Listbox and Binding to SelectedItem

Question

Hi

I'm trying to define a DataTemplate for an enum class.
A Listbox containing RadioButton for every enum state appeared to be a common solution and worked so far.
The point were I'm getting into trouble is when I try to bind the SelectedItem of a Listbox (which is one of the enum states)  to the object which the datatemplate was applied to.
My XAML:
<DataTemplate DataType="{x:Type custScrewing:ScrewDriverState}">
<DataTemplate.Resources>
    <!-- This data provider provides an array of different ScrewDriverState enums -->
    <ObjectDataProvider MethodName="GetValues"
                                            ObjectType="{x:Type sys:Enum}"
                                            x:Key="allScrewDriverStates">
        <ObjectDataProvider.MethodParameters>
            <x:Type TypeName="custScrewing:ScrewDriverState"/>
        </ObjectDataProvider.MethodParameters>
    </ObjectDataProvider>
    <!-- A value converter which returns the description found in an enum value's
DescriptionAttribute, or the value's name if the attribute is not present. -->
    <custCommon:EnumValueDescriptionConverter x:Key="enumValueDescriptionConverter" />
</DataTemplate.Resources>
    <ListBox Name="lbScrewDriverStates"
                     SelectedItem="{Binding}"                                
                     ItemsSource="{Binding Source={StaticResource allScrewDriverStates}}"
                     BorderThickness="0"
                     ScrollViewer.VerticalScrollBarVisibility="Auto"
                     IsSynchronizedWithCurrentItem="True"
                 >
        <ListBox.ItemTemplate>
            <DataTemplate>
                <RadioButton Content="{Binding Converter={StaticResource enumValueDescriptionConverter}}"
                                         IsChecked="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ListBoxItem}},Path=IsSelected}"/>
            </DataTemplate>
        </ListBox.ItemTemplate>
    </ListBox>               
</DataTemplate>

If I start my application, an System.InvalidOperationException occurs saying "Two-way binding requires Path or XPath.".
The declaration SelectedItem="{Binding}" causes this problem and this is actually pretty strange since usually if Path is not provided then the the binding should be applied to the object (binding source) itself. I tried many things but I can't get it to work.

Can somebody help me out?

Thanks,
Philipp

Answer 1

No ideas?
Or is maybe my explanation a little bit weird? In such a case just let me know and I'll provide any details on it.

I took the main ideas for creating such a data template for my enum class from:
- http://www.infusionblogs.com/blogs/jsmith/archive/2006/08/29/835.aspx
- http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=893790&SiteID=1

Please take a look at this problem. You'll help me a lot.
Philipp

For the case somebody wants to reproduce this problem, here are some sources:
public enum ScrewDriverState
{
[Description("ScrewDriver IDLE")]
IDLE,
[Description("ScrewDriver SCREWING")]
SCREWING,
[Description("ScrewDriver UNSCREWING")]
UNSCREWING
}

[ValueConversion(typeof(Enum), typeof(String))]
public class EnumValueDescriptionConverter : IValueConverter
{
object IValueConverter.Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
if (value is Enum == false)
throw new ArgumentException("'value' must be an enum.");

// Get the field in the enum type which represents the argument value.
FieldInfo field = value.GetType().GetField(value.ToString());

// Get the DescriptionAttribute on that field, if it exists.
object[] attributes = field.GetCustomAttributes(typeof(DescriptionAttribute), false);

// The enum value is not decorated with the attribute, so just return the value itself.
if (attributes.Length == 0)
return value.ToString();

// Return the description applied to the enum value.
DescriptionAttribute descriptionAttribute = attributes[0] as DescriptionAttribute;
return descriptionAttribute.Description;
}

object IValueConverter.ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
throw new NotSupportedException("ConvertBack not supported.");
}
}

Answer 2

Philipp Konradi wrote:

If I start my application, an System.InvalidOperationException occurs saying "Two-way binding requires Path or XPath.". The declaration SelectedItem="{Binding}" causes this problem and this is actually pretty strange since usually if Path is not provided then the the binding should be applied to the object (binding source) itself. I tried many things but I can't get it to work.

The problem is as you have described, you need a path, a binding without a path binds to the current datacontext. But in this situation, even if you could have a two-way binding, updating the source could only replace the datacontext - it couldn't actually set the value somewhere else because it doesn't know where it came from.

Usually a datatemplate would provide the ability to edit the properties of an item, not replace the item itself. Unfortunately I think the only way to get around this, is to rearchitect what you are doing.

Answer 3

try this for listbox

<ListBox SelectedValue="{Binding ElementName=win1, Path=State}" ItemsSource="{Binding Source={StaticResource odp}}" Name="q1">

                     <ListBox.Resources>

                           <Style x:Key="{x:Type ListBoxItem}" TargetType="{x:Type ListBoxItem}" >

                                  <Setter Property="Template">

                                         <Setter.Value>

                                                <ControlTemplate>

                                                       <Border Background="Transparent">

                                                              <RadioButton Content="{TemplateBinding ContentControl.Content}" 

                                                                        IsChecked="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ListBoxItem}},Path=IsSelected}"/>

                                                       </Border>

                                                </ControlTemplate>

                                         </Setter.Value>

                                  </Setter>

                           </Style>

                     </ListBox.Resources>

              </ListBox>

 

<ObjectDataProvider MethodName="GetValues" ObjectType="{x:Type sys:Enum}" x:Key="odp">

                     <ObjectDataProvider.MethodParameters>

                           <x:Type TypeName="l:ScrewDriverState"/>

                     </ObjectDataProvider.MethodParameters>

              </ObjectDataProvider>

 

Answer 4

Douglas and lee d, thank you both a lot!
Now I see my misconception.

Previously I saw already that Styles were used with the Enum Listbox. But the concept of DataTemplates  appeared to me more powerfull since in case of Styles I fix myself on using a Listbox for visualization, but with DataTemplates I saw the possibility to have full control of data visualization, e.g. use a ComboBox instead of the ListBox. As it turns out now I didn't considered the drawbacks for this more on flexibility and easy to use (I could just write GroupBox.Content = <myData>). But thanks to Douglas, it became now clear to me that inside a data template one does not have a (good) connection to the outer data and following can't bind the templated object to it. So I think I have to return to Styles again :-)

Thanks again,
Philipp