CODEWITHSUNDEEP
c#xamlbindingwindows-phone-8.1
Wizard
Wizard

Reputation: 1162

Binding using ValueConverter - ConvertBack Parameter always null

I have a control that is bound to a question, that question has many possible options. Each option can have a response.

Example:

Question: Will it fit in a shoe box?

Options: Yes, No, Maybe

Response: options[0].Response = new Response()

  • Meaning that the first option has a response, therefore it has been answered...

For this scenario, I have this represented this using a checkbox. If it is checked, it's answered, if not, response is null. I hope that makes my intention clear.

I am however having problems with the binding in that I can't get the response into the option. I thought I'd be able to use the Convert Parameter but this doesn't seem to work either..

This is how I'm binding to the check box:

<CheckBox  Content="{Binding Text}" 
           IsChecked="{Binding Response, 
                      Mode=TwoWay,  
                      Converter={StaticResource isQuestionResponseNullConverter}, 
                       ConverterParameter={Binding Response}}"
            />

So this is the CheckBox with my value converter, it works one way using the following: 

public object Convert(object value, Type targetType, object parameter, string language)
    {
        IResponse response = (IResponse )value;

        if (response== null)
        {
            false;
        }
        return true;
    }

However, I want to bind back using the ConvertBack method so that if that check box is selected, the binding for that question's response will be that of the CheckBox's binding context. I thought I would be able to do this using the Converter Parameter as shown in the XAML above, however it seems that the parameter is null, am I doing this wrong?

I have read up on this and some sources are saying a DependencyObject is required, I don't understand why however, surely that's what ConvertBack is for?

 public object ConvertBack(object value, Type targetType, object parameter, string language)
    {
        if ((bool)value == true)
        {
            return parameter; //<-- Always null.. 
        }
        else
        {
            return null;
        }
    }

Upvotes: 0

Views: 1761

Answers (1)

Juan Pablo Garcia Coello
Juan Pablo Garcia Coello

Reputation: 3232

The Converter should be:

public class isQuestionResponseNullConverter : DependencyObject, IValueConverter
{

    public object Parameter
    {
        get { return (object)GetValue(ParameterProperty); }
        set { SetValue(ParameterProperty, value); }
    }
    public static readonly DependencyProperty ParameterProperty =
        DependencyProperty.Register("Parameter", typeof(object), typeof(isQuestionResponseNullConverter), new PropertyMetadata(null));



    public object Convert(object value, Type targetType, object parameter, string language)
    {
        IResponse response = (IResponse)value;

        if (response == null)
        {
            return false;
        }
        return true;

    }

    public object ConvertBack(object value, Type targetType, object parameter, string language)
    {
        if ((bool)value == true)
        {
            return Parameter; //Get Parameter
        }
        else
        {
            return null;
        }

    }
}

And in the XAML

  <Page.Resources>
    <local:isQuestionResponseNullConverter x:Key="isQuestionResponseNullConverter" Parameter="{Binding Response}"/>
</Page.Resources>

<Grid x:Name="Grid_Master" Background="Red">

    <CheckBox VerticalAlignment="Top"  Content="{Binding Text}" 
       IsChecked="{Binding Response, Mode=TwoWay,  Converter={StaticResource isQuestionResponseNullConverter}}"/>

</Grid>

And now in the back method you get the Parameter !

I hope you find it easy to use and useful

Upvotes: 1

Related Questions