Is there any way to convert the members of a collection used as an ItemsSource?

Question

In WPF you can use an IValueConverter or IMultiValueConverter to convert a data-bound value from say an int to a Color.

I have a collection of Model objects which I would like to convert to their ViewModel representations but in this scenario,

<ListBox ItemsSource="{Binding ModelItems, 
     Converter={StaticResource ModelToViewModelConverter}" />

the converter would be written to convert the whole collection ModelItems at once.

I wish to convert the items of the collection individually, is there a way to do that? I might want to use a CollectionViewSource and have some filtering logic so I don't want to have to iterate over the whole collection every time something changes.

score 26 · Accepted Answer · answered Sep 21 '11 at 19:50

You cannot set the converter on the collection itself, because it would get the collection as input. You have two choices:

Make sure your converter can also deal with collections (IEnumerable).
Use the converter within the item template.

If you want to use the second approach, then use something like this:

<ListBox ItemsSource="{Binding ModelItems}">
  <ListBox.ItemTemplate>
    <DataTemplate>
      <ContentPresenter Content="{Binding Converter={StaticResource ModelToViewModelConverter}}" 
                        ContentTemplate="{StaticResource MyOptionalDataTemplate}"/>
    </DataTemplate>
  </ListBox.ItemTemplate>
</ListBox>

If you don't need a custom datatemplate, then you can skip the ContentTemplate attribute.

score 2 · Answer 2 · answered Jan 23 '18 at 07:44

Here is another example. I'm using MVVM Caliburn Micro. MyObjects is a list of enums in my case.

<ListBox x:Name="MyObjects">
  <ListBox.ItemTemplate>
    <DataTemplate>
      <TextBlock Text="{Binding ., Converter={StaticResource MyConverter}}"/>
    </DataTemplate>
  </ListBox.ItemTemplate>
</ListBox>

score 2 · Answer 3 · edited Jul 18 '16 at 08:52

Yes you can. It is acting the same as with the IValueConverter. You simply treat the value parameter for the Convert method as a collection.

Here is an example of Converter for a collection:

public class DataConvert : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        ObservableCollection<int> convertible = null;
        var result = value as ObservableCollection<string>;

        if (result != null)
        {
            convertible = new ObservableCollection<int>();
            foreach (var item in result)
            {
                if (item == "first")
                {
                    convertible.Add(1);
                }
                else if (item == "second")
                {
                    convertible.Add(2);
                }
                else if (item == "third")
                {
                    convertible.Add(3);
                }
            }
        }

        return convertible;
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

In this case is just a proof of concept, but I think it should show the idea very well. The Converter converts from a simple collection of strings like this:

ModelItems = new ObservableCollection<string>();
        ModelItems.Add("third");
        ModelItems.Add("first");
        ModelItems.Add("second");

into a collection of integers corresponding to the string meaning.

And here is the corresponding XAML (loc is the reference of the current assembly where is the converter):

<Window.Resources>
    <loc:DataConvert x:Key="DataConverter"/>
</Window.Resources>
<Grid x:Name="MainGrid">
    <ListBox ItemsSource="{Binding ModelItems, Converter={StaticResource DataConverter}}"/>
</Grid>

If you want to make a two way binding, you have to implement also the convert back. From the experience of working with MVVM, i suggest to use something like the Factory Pattern to transform from Model in ViewModel and backwards.

Thanks louie, but this isn't really what I'm after, I might want to use a CollectionViewSource and have some filtering logic so I don't want to have to iterate over the whole collection every time something changes, hence the last sentence of my question. — Grokodile, Sep 21 '11 at 20:42

Is there any way to convert the members of a collection used as an ItemsSource?

3 Answers3

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