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

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花落未央
花落未央 2020-12-08 06:11

In WPF, it seems to be impossible to select (with the mouse) a \"null\" value from a ComboBox. Edit To clarify, this is .NET 3.5 SP1.

Here\'s some c

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  • 天命终不由人
    天命终不由人 (楼主)
    2020-12-08 06:52

    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()
            .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;
    }
}

      3. 



      Example of use:
      


      

      


      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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