Why can’t I select a null value in a ComboBox?

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Well I recently ran into the same problem with null value for ComboBox.
I’ve solved it by using two converters:

  1. For ItemsSource property: it replaces null values in the collection by any value passed inside converter’s parameter:

    class EnumerableNullReplaceConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        var collection = (IEnumerable)value;

        return
            collection
            .Cast<object>()
            .Select(x => x ?? parameter)
            .ToArray();
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotSupportedException();
    }
}

  2. For SelectedValue property: this one does the same but for the single value and in two ways:

    class NullReplaceConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return value ?? parameter;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return value.Equals(parameter) ? null : value;
    }
}

Example of use:

<ComboBox 
    ItemsSource="{Binding MyValues, Converter={StaticResource EnumerableNullReplaceConverter}, ConverterParameter="(Empty)"}" 
    SelectedValue="{Binding SelectedMyValue, Converter={StaticResource NullReplaceConverter}, ConverterParameter="(Empty)"}"
    />

Result:

enter image description here

Note:
If you bind to ObservableCollection then you will lose change notifications. Also you don’t want to have more than one null value in the collection.

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