Xamarin.Forms: MultiBinding IMultiValueConverter recieving Null values - xamarin

Question

Hello Guys i need help.
i am using a converter on a binding to set a background color depending on an object id.(stacklayout inside contentview)
<StackLayout.BackgroundColor>
<Binding Path="objectID" Converter="{StaticResource IntToColorConverter}"/>
</StackLayout.BackgroundColor>
This works.
Now i want to use the multiconverter (new in Xamarin 4.7) to return a different backgroundcolor depending on other object properties.(for context: object is a calendarentry and if its in the past it should be desaturated or something)
<StackLayout.BackgroundColor>
<MultiBinding Converter="{StaticResource MultiColorConverter}">
<Binding Path="objectID"/>
<Binding Path="value"/>
<Binding Path="value2"/>
</MultiBinding>
</StackLayout.BackgroundColor>
This does not work, since the values that are given to the converter are all NULL and the color becomes black (the return value if all vslues are NULL; so the converter is set up correctly also). It also shows this when i use a break point on the converter, that the array contains only NULL variables.
I don't know what i am missing here, the bindingcontext should be inheritated and does not change. Any hint would be appreciated.
The bindingcontext is set programatically on a contentpage on creation of the ContentView where i provide an object from a list of objects.
var evnt = new TimeTableEventView { BindingContext = Model.calenderevents[j] };

Answer 1

You need to return BindableProperty.UnsetValue to use the binding FallbackValue .
in xaml
<StackLayout.BackgroundColor>
<MultiBinding Converter="{StaticResource MultiColorConverter}">
<Binding Path="red"/>
<Binding Path="green"/>
<Binding Path="blue"/>
</MultiBinding>
</StackLayout.BackgroundColor>
in Converter
public class MultiColorConverter : IMultiValueConverter
{
public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
{
foreach (var value in values)
{
if (!(value is int b))
{
return Color.White;
// set a default value when unset
}
}
int red = (int)values[0];
int green = (int)values[1];
int blue = (int)values[2];
Color color = Color.FromRgb(red,green,blue);
return color;
}
public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
{
throw new NotImplementedException();
}
}
in code behind
public class MyViewModel
{
public int red { get; set; }
public int green { get; set; }
public int blue { get; set; }
public MyViewModel(int r, int g, int b)
{
red = r;
green = g;
blue = b;
}
}
BindingContext = new MyViewModel(120, 60, 180);