WPF: ComboBox with reset item

Question

I want to make a ComboBox in WPF that has one null item on the top, when this gets selected, the SelectedItem should be set to null (reset to default state). I\

Answer 1

I used the following solution for a similar problem. It makes use of the Converter property of the binding to go back and forth between the internal representation (that null is a reasonable value) and what I want to appear in the ComboBox. I like that there's no need to add an explicit list in a model or viewmodel, but I don't like the fragile connection between the string literal in the converter and that in the ComboBox.

    
        
            (none)
            
        
    

and then the converter looks like:

public class MyPropertySelectionConverter : IValueConverter
{
    public static MyPropertySelectionConverter Instance
    {
        get { return s_Instance; }
    }

    public const String NoneString = "(none)";

    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        Object retval = value as MyPropertyType;
        if (retval == null)
        {
            retval = NoneString;
        }
        return retval;
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        Object retval = null;
        if (value is MyPropertyType)
        {
            retval = value;
        }
        else if (String.Equals(NoneString, value as String, StringComparison.OrdinalIgnoreCase))
        {
            retval = null;
        }
        else
        {
            retval = DependencyProperty.UnsetValue;
        }
        return retval;
    }


    private static MyPropertySelectionConverter s_Instance = new MyPropertySelectionConverter();
}