FallBackValue in IMultiValueConverter is UnsetValue

Question

I am trying to use a single IMultiValueConverter for more than one controls in XAML.

I am using a simple string Literal to tell what value the IMultiValueConverter is supposed to return.

But I am getting DependencyProperty.UnsetValue in values[2],ie value of parametter named Command when it comes to convert function of ModifierCategoryEnableDisable.

Similar arrangment is working on similar controls on this XAML form within other IMultiValueConverters but not here. Please guide what am i missing?

NOTE:

1. CurrentRec is the currently selected object from the ViewModel
2. DM_CategoryData is a Class and Current_Selected_Category is a List<DM_CategoryData> in the ViewModel's current object,ie CurrenRec.

XAML:

<GroupBox Width="226" Height="117"  Margin="0" Canvas.Top="252" Header="Modifiers" Canvas.Left="55" >
         <GroupBox.IsEnabled>
             <MultiBinding Mode="TwoWay" UpdateSourceTrigger="PropertyChanged" Converter="{StaticResource MDNS}">
                     <Binding Path="SearchFound" />
                     <Binding Path="CurrentRec.Current_Selected_Category"/>
                     <Binding Path="Command" FallbackValue="1" />
              </MultiBinding>
        </GroupBox.IsEnabled>
 </GroupBox>

C#:

public class ModifierCategoryEnableDisable : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
        string Command = values[2].ToString();
        bool Retval1 = false;
        string Retval2 = "";
        switch(Command)
        {
            case "1":
                bool SearchFound = (bool)values[0];
                DM_CategoryData CurrentSelectedItemCategory = (DM_CategoryData)(values[1]);
                Retval1 = SearchFound && (CurrentSelectedItemCategory == null ? true : CurrentSelectedItemCategory.IsModifier.Equals("1") ? false : true);
                break;
            case "2":
                Retval2 = "0";
                break;
        }               

        if(Command.Equals("1"))
        {
            return Retval1;
        }
        else
        {
            return Retval2;
        }
   }
}

Answer 1

In order to provide additional static data to a multibinding converter, use a ConverterParameter:

<MultiBinding Mode="TwoWay" UpdateSourceTrigger="PropertyChanged" Converter="{StaticResource MDNS}" ConverterParameter="1">
    <Binding Path="SearchFound" />
    <Binding Path="CurrentRec.Current_Selected_Category"/>
</MultiBinding>

And check the parameter in the Convert method:

public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
{
    string Command = parameter as string;

    // ...
}